Grégoire Richard
email: gregoire.richard@inria.fr

Thomas Pietrzak

Ferran Argelaguet

Anatole Lécuyer

Géry Casiez

MultiVibes: What if your VR Controller had 10 Times more Vibrotactile Actuators?

Keywords:

Figure 1: (a) MultiVibes prototype comprising 10 actuators in contact with the skin that can be controlled individually in amplitude and frequency. (b) Interaction with the edge of a virtual cube. (c) As the user slides the controller along the edge of the cube, they feels a phantom vibration point moving inside their hand according to the point of contact. The vibration point location is determined by the three closest actuators, controlled using our funneling model.

INTRODUCTION

Haptic integration is a crucial aspect of immersive experiences in VR [START_REF] Aspell | Keeping in touch with one's self: multisensory mechanisms of self-consciousness[END_REF][START_REF] Kilteni | The sense of embodiment in virtual reality[END_REF][START_REF] Marucci | The impact of multisensory integration and perceptual load in virtual reality settings on performance, workload and presence[END_REF][START_REF] Slater | Immersion, presence and performance in virtual environments: An experiment with tridimensional chess[END_REF], as haptic feedback can improve performance [START_REF] Brasen | Effects of vibrotactile feedback in commercial virtual reality systems[END_REF], presence [START_REF] Dinh | Evaluating the importance of multi-sensory input on memory and the sense of presence in virtual environments[END_REF][START_REF] Kaul | Increasing presence in virtual reality with a vibrotactile grid around the head[END_REF] and embodiment [START_REF] Fröhner | Can wearable haptic devices foster the embodiment of virtual limbs[END_REF][START_REF] Richard | Studying the role of haptic feedback on virtual embodiment in a drawing task[END_REF]. There are many ways to integrate haptic feedback for improved touch and interactions in immersive virtual environments (IVEs), especially with handheld haptic devices. Compared to grounded devices, handheld devices offer a much larger workspace to interact with the environment. They can also be operated with the hand as opposed to non-handheld wearable devices [START_REF] García-Valle | Evaluation of presence in virtual environments: Haptic vest and user's haptic skills[END_REF][START_REF] Krogmeier | Human-virtual character interaction: Toward understanding the influence of haptic feedback[END_REF].

Several research projects introduce rich haptic feedback, either kinesthetic [START_REF] Choi | Claw: A multifunctional handheld haptic controller for grasping, touching, and triggering in virtual reality[END_REF][START_REF] Gonzalez | X-rings: A hand-mounted 360 shape display for grasping in virtual reality[END_REF][START_REF] Zenner | Drag:on: A virtual reality controller providing haptic feedback based on drag and weight shift[END_REF] or tactile [START_REF] Benko | Normaltouch and texturetouch: High-fidelity 3d haptic shape rendering on handheld virtual reality controllers[END_REF][START_REF] Whitmire | Haptic revolver: Touch, shear, texture, and shape rendering on a reconfigurable virtual reality controller[END_REF], for specific interactions that may not easily transfer to consumer-grade VR devices. However, consumer-grade VR controllers usually integrate haptic feedback, in the form of vibrotactile feedback, mostly rendered with one actuator making the frame resonate, allowing for basic effects. Yet, tactile feedback can be enhanced using multiple actuators to provide spatial feedback and leverage haptic illusions, such as saltation or funneling effects. Several works have started to explore such solutions [START_REF] Cabaret | Does multi-actuator vibrotactile feedback within tangible objects enrich vr manipulation[END_REF][START_REF] Cabaret | Perception of spatialized vibrotactile impacts in a hand-held tangible for virtual reality[END_REF][START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF][START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF][START_REF] Lacôte | Investigating the haptic perception of directional information within a handle[END_REF]. Some have kept the shape of controllers with cylindrical prototypes [START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF][START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF][START_REF] Lacôte | Investigating the haptic perception of directional information within a handle[END_REF] but none implemented spatialized models that allow active interaction with VR environments. As Cabaret et al. [START_REF] Cabaret | Perception of spatialized vibrotactile impacts in a hand-held tangible for virtual reality[END_REF] put it, current VR controllers offer "monolithic vibration of the entire hand-held device", while some research prototypes can be technologically complex for commercial purposes in the short term. Thus, there is room for improving existing VR controllers by enriching simple technologies, like vibrotactile feedback, using multiple actuators and making use of haptic illusions.

In this work, we present the design of MultiVibes, a haptic VR controller, featuring multiple actuators that allow spatialized rendering of vibrotactile feedback on the controller surface. With a simple design and leveraging the funneling effect, MultiVibes can render moving vibration patterns and enrich interactions in VR. We first discuss the related work and theory behind spatialized vibrations before presenting the design of the controller. Finally, we introduce two subsequent experiments that evaluate the capacities of MultiVibes in terms of perception and user experience. Our results show that spatialized vibrations enrich interactions in IVEs, and that the design of MultiVibes is convincing enough to not show significant differences when compared with recent consumer-grade VR controllers such as the Valve Index controller . With MultiVibes, unlike other consumer-grade VR controllers, it is possible to render information regarding contacts with the environment, either a timely contact like hitting the ground with a pickaxe or prolonged contacts such as petting an animal, or even more symbolic information related to direction and orientation, or moving patterns to signify a progress bar.

RELATED WORK

An interaction technique can be considered as the coupling of a physical device -the haptic device -and an interaction language -designing and computing haptic stimuli [START_REF] Nigay | A generic platform for addressing the multimodal challenge[END_REF]. In this section, we consider haptic interactions through this prism and review the literature regarding haptic solutions, particularly graspable and handheld devices, and the scope of spatialized tactile experiences. We limit the scope of our review to ungrounded devices and focus on handheld haptic devices. We refer to more extensive reviews on haptic devices: haptic gloves [START_REF] Perret | Touching virtual reality: A review of haptic gloves[END_REF], and wearable haptic systems [START_REF] Pacchierotti | Wearable haptic systems for the fingertip and the hand: taxonomy, review, and perspectives[END_REF].

Handheld haptic devices

Most consumer-grade VR gears already include one or two handheld controllers to interact with the IVEs. These controllers usually include some vibrotactile feedback, always limited to one actuator inside the frame to make it resonate [START_REF] Valvesoftware | Valve index controllers[END_REF]. This configuration, although cost-effective, provides a limited range of haptic sensations.

Notable work has been conducted to extend the haptic sensations that such devices can deliver. We discuss a few such studies with approaches relevant to our own, whether in terms of shape, rendering methods, and actuators' position. For example, some works explored how the shape of the device can be altered [START_REF] Gonzalez | X-rings: A hand-mounted 360 shape display for grasping in virtual reality[END_REF][START_REF] Yoshida | Pocopo: Handheld pin-based shape display for haptic rendering in virtual reality[END_REF]. Devices for grasping interaction also make up a significant number of haptic peripherals proposed by research studies, especially for wearable devices [START_REF] Bouzit | The rutgers master ii-new design force-feedback glove[END_REF][START_REF] Ma | Rml glove-an exoskeleton glove mechanism with haptics feedback[END_REF], but also handheld ones [START_REF] Achibet | The virtual mitten: A novel interaction paradigm for visuo-haptic manipulation of objects using grip force[END_REF][START_REF] Choi | Claw: A multifunctional handheld haptic controller for grasping, touching, and triggering in virtual reality[END_REF].

Researchers have proposed specific handheld devices that incorporate cylindrical peripherals, integrating vibrotactile feedback [START_REF] Cabaret | Perception of spatialized vibrotactile impacts in a hand-held tangible for virtual reality[END_REF][START_REF] Lacôte | Investigating the haptic perception of directional information within a handle[END_REF], electrotactile feedback [START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF] or cutaneous feedback [START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF]. Kajimoto [START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF] proposed a cylindrical handheld device that integrates both capacitive sensing and tactile feedback, through electrodes, covering the entire surface of the cylinder. To our knowledge, it is the first controller to combine tactile feedback and capacitive sensing before it was used in consumer grade gear [START_REF] Valvesoftware | Valve index controllers[END_REF]. Their experiment showed that circumferential patterns on the device allowed participants to recognize the direction of the stimuli. Although this prototype included a high density of electrodes, it was limited to electrotactile stimulations, which can be less accepted than vibrotactile feedback [START_REF] Stanke | Tactilewear: A comparison of electrotactile and vibrotactile feedback on the wrist and ring finger[END_REF], and it has not been used in interactive context.

Chen et al. [START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF] proposed two controllers that embed tactile pins inside the frame, creating pin arrays made up of 15 solenoids in each controller. The pin arrays were arranged in eight columns facing the eight cardinal directions. Using both hands, their prototypes allowed the recognition of impacts through cardinal directions and were shown to enhance immersive properties of virtual environments in two use cases. Chen et al. opted for pressure vectors instead of vibrotactile feedback as they hypothesize it is not possible to create spatialized vibrations on a controller without making the whole frame resonate. Cabaret et al. [START_REF] Cabaret | Perception of spatialized vibrotactile impacts in a hand-held tangible for virtual reality[END_REF] used vibrotactile feedback to show that two actuators, located at the top and bottom of a handle-shaped prototype, could provide good sense of direction and approximate position, using phantom vibrations (see Section 2.2.2). Although mostly successful in perceiving direction, the participants of this study had trouble correctly identifying the stimuli's position and judged most vibration models as moderately realistic. The authors hypothesize that a more spatialized vibrotactile feedback, using more than 2 actuators, in 3D spatialization, might be an interesting approach for VR. They proposed another approach [START_REF] Cabaret | Does multi-actuator vibrotactile feedback within tangible objects enrich vr manipulation[END_REF] with a spherical prototype for VR interaction, that includes five actuators, one for each fingertip, fitted inside the frame of the prototype. They focused on the design of different haptic sensations like impacts and textures on fingertips, and also implemented a 1D funneling effect based on a linear model (see Section 2.2.2) which is different from our approach of rendering haptic feedback on the palm and fingers and using a 2-dimensional funneling effect. Finally, proposing a device relatively similar to Kajimoto in terms of shape, Lacote et al. [START_REF] Lacôte | Investigating the haptic perception of directional information within a handle[END_REF] showed that it was possible to provide spatial feedback with 5 ac-tuators fitted inside a 3D printed handle and that vibrations were easier to follow than taps. They based the vibrations on Stimulus Onset Asynchrony (SOA) (see Section 2.2.1) and did not explore their handle as a mean of interaction.

Although there has been a lot of research proposing novel handheld haptic devices, especially for VR uses, the gap between devices made for research purposes and consumer-grade devices does not seem to narrow. Consumer-grade VR controllers offer limited haptic feedback, while some research prototypes offer novel haptic interactions that can be too complex for home use. Some research prototypes explored shapes close to actual VR controllers [START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF][START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF][START_REF] Lacôte | Investigating the haptic perception of directional information within a handle[END_REF], but they did not design the model of the feedback to allow for active interaction with a VR environment. The prototype and models by Cabaret et al. [START_REF] Cabaret | Does multi-actuator vibrotactile feedback within tangible objects enrich vr manipulation[END_REF] differ from the usual controller shape and they do not provide a funneling model to spatialize the feedback over the surface of their prototype. In the second subsection of our review, we cover the different modalities of vibrotactile feedback.

Spatialized vibrotactile feedback

Vibrations are the most common haptic feedback available for handheld devices such as controllers. They can be used to convey information about contact [START_REF] Krogmeier | Human-virtual character interaction: Toward understanding the influence of haptic feedback[END_REF], texture [START_REF] Ahmaniemi | Design of dynamic vibrotactile textures[END_REF][START_REF] Okamura | Vibration feedback models for virtual environments[END_REF], or other properties such as directions [START_REF] Cabaret | Perception of spatialized vibrotactile impacts in a hand-held tangible for virtual reality[END_REF][START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF] in a virtual environment. The accuracy of the spatial perception of the vibrations is important to simulate properties such as directions and position. It can be improved using more actuators or leveraging the limitations of human perception. The number of actuators fitted in a controller is often limited by the size of the actuators and their possible resonance with the frame. Leveraging the limitations of human perception with vibrotactile feedback can be achieved using the saltation effect [START_REF] Tan | Tactual displays for wearable computing[END_REF], the apparent tactile motions [START_REF] Israr | Tactile brush: drawing on skin with a tactile grid display[END_REF] or the funneling effect [START_REF] Alles | Information transmission by phantom sensations[END_REF][START_REF] Barghout | Spatial resolution of vibrotactile perception on the human forearm when exploiting funneling illusion[END_REF].

Saltation and apparent tactile motions

The saltation effect, also known as the cutaneous rabbit illusion, is a tactile illusion evoked by tapping two or more separate skin regions in rapid succession [START_REF] Geldard | Sensory saltation: Metastability in the perceptual world[END_REF][START_REF] Tan | Tactual displays for wearable computing[END_REF]. In this illusion, the different taps are felt as a train of discontinuous and discrete taps that move from one spot of tapping to the other. The saltation effect was shown to also work in 2 dimensions [START_REF] Kim | Extending" out of the body" saltation to 2d mobile tactile interaction[END_REF]. Apparent tactile motions are another kind of tactile illusion that relies on overlapping actuation times of close actuators to evoke the feeling of movement [START_REF] Israr | Control space of apparent haptic motion[END_REF]. This illusion depends on what is called Stimulus Onset Asynchrony (SOA), which is the time between two consecutive actuations. Both saltation and apparent tactile motions can recreate the illusion of movement for a vibrotactile feedback with a spatiotemporal mapping, such as pre-recorded patterns. However, these effects are not suitable for active real-time VR exploration, as users' movements are free and unconstrained.

Funneling

When using multiple actuators, it becomes possible to create phantom vibrations that are felt between actual vibrations [START_REF] Alles | Information transmission by phantom sensations[END_REF][START_REF] Barghout | Spatial resolution of vibrotactile perception on the human forearm when exploiting funneling illusion[END_REF]. These phantom sensations are referred to as the funneling effect. The funneling effect has been used in a wide range of works. The funneling effect has been shown to work for different parts of the body [START_REF] Barghout | Spatial resolution of vibrotactile perception on the human forearm when exploiting funneling illusion[END_REF][START_REF] Kim | Body-penetrating tactile phantom sensations[END_REF], including the hand. For further reading on the funneling effect, we refer to the taxonomy of phantom sensations by Park et al. [START_REF] Park | Tactile information transmission by 2d stationary phantom sensations[END_REF].

Dimensionality in funneling

The funneling effect corresponds to the phantom vibration perceived between two actuators when they vibrate at the same frequency. Although the funneling effect was historically characterized in one dimension, Lipari [START_REF] Lipari | Improved 2D Interpolation for Intensity and Position Control in 2D Vibrotactile Meshes[END_REF] generalized it to two dimensions, using 3 or 4 actuators to simulate phantom vibrations at any 2D point within the convex hull defined by all actuators. In the triangular cell model, with actuators located at each vertex of a triangle, the 2D model becomes a combination of two 1D funneling models by creating an intermediate phantom vibration, shown in green in Figure 2. The square cell model proposed by Lipari relies on the same theory, but requires a regular matrix of actuators [START_REF] Lipari | Improved 2D Interpolation for Intensity and Position Control in 2D Vibrotactile Meshes[END_REF]. Controlling actuators' amplitude The Energy Summation Model (ESM) in the Pacinian channels [START_REF] Israr | Tactile brush: drawing on skin with a tactile grid display[END_REF][START_REF] Makous | A critical band filter in touch[END_REF], defines the amplitude of the phantom vibration as the square root of the sum of the squared intensities of the two actuators:

A 3 A 1 A 2 V 1 V 2 β 1 -β α 1 -α
I 2 V 1 = I 2 A 1 + I 2 A 2 (1)
where I A 1 and I A 2 are the intensities of the physical actuators, and I V 1 is the amplitude of the resulting phantom vibration (Figure 2). This model allows better control of both the amplitude and perceived location of the phantom vibration than the linear or logarithmic models proposed by previous work [START_REF] Alles | Information transmission by phantom sensations[END_REF][START_REF] Israr | Tactile brush: drawing on skin with a tactile grid display[END_REF][START_REF] Park | Rendering moving tactile stroke on the palm using a sparse 2d array[END_REF]. Israr et al. [START_REF] Israr | Control space of apparent haptic motion[END_REF] initially proposed the ESM for 1D funneling, coupled with SOA to render a second dimension. Lipari et al. [START_REF] Lipari | Study of 2d vibration summing for improved intensity control in vibrotactile array rendering[END_REF] proposed their own funneling model, and compared it to the ESM for one dimensional funneling, but they did not find significant differences betweeen the two models. The ESM defines the following relationship between the intensities of the vibrations:

β I 2 A 1 = (1 -β ) I 2 A 2 (2)
where β corresponds to the ratio of the distance between A 1 and V 1 , and the distance between A 1 and A 2 . The intensities of A 1 and A 2 in relation to the one of V 1 and β is defined as:

I A 1 = I V 1 1 -β I A 2 = I V 1 β (3)
To our knowledge, no study implemented the energy summation model in two dimensions with 3 actuactors.

Summary

There are many ways to enrich vibrotactile feedback to convey more comprehensive information about directions and guidance, or contacts. Using multiple actuators, it is possible to create effects that are helpful for users in immersive settings. Although a few studies propose devices that explore those possibilities, none, to our knowledge, used multiple actuators to render sptatial vibrations on the surface of a VR controller. In this work, we propose the use of multiple voice coils to spatialize vibrations in 2D for VR interactions.

DESIGN AND IMPLEMENTATION

The overall aim of the design of MultiVibes is to enhance current consumer-grade VR controllers by increasing the spatial resolution of vibrotactile feedback, by increasing the number of actuators and leveraging the funneling effect. We provide here the link to the project webpage, including the different models and a demo of the 2D funneling effect. Figure 3: Views of the prototype. On the left, the frame of the controller, made of 3 modules, the header, and the Vive Tracker, fitted with the 10 actuators. In the center right, the actual prototype and the electronic board.

Design Goals

This work aims to design a prototype, including a matrix of actuators, that could be easily replicated and used as a means of interaction to serve as a VR controller. The actuator matrix, where each actuator can controlled in amplitude and frequency, can be used to generate funneling-based vibrations on the surface covered by the matrix. Inspired by Chen's prototype [START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF], we wanted to reproduce a similar actuator matrix scheme, with two actuators per finger, for a total of 10 actuators. The design of such a controller with spatialized vibrotactile feedback raises several research questions, mainly in terms of user perception and user experience. The first question is related to the level of spatialized information that users can perceive, and the second question is to what extent spatialized feedback coupled with the controller will enhance the user experience within IVEs. Before describing the different experiments set up to answer those questions, we describe the hardware of the prototype.

Hardware and Electronics

We aimed to keep the dimensions of our prototype close to those of existing controllers. This is a strong constraint in terms of volume/surface available. The actuators used were HiWave Haptic Exciter (HIHX9C005-8 Audio & Haptic Exciter), 26 mm long and 12 mm wide. The actuators, ten in total, were fitted directly on the surface of the frame, inside holes designed so that the mobile part of each actuator was not in contact directly with the frame, to prevent it from resonating with the actuators, in a way that the moving part would also be in contact with the users' hand. The resonance of the frame was a key design constraint as having the frame resonating would prevent any funneling effect. The controller frame was 3D printed, having a cylindrical shape, with a circular section with a diameter of 40 mm (see Figure 3). The actuators are connected to an Teensy 4.1 board through a custom-made electronic board equipped with a CMOS AND gate to modulate the frequency and amplitude signals to generate the tactile signal. The tactile feedback consisted of a square shape signal with a controllable frequency up to 1000 Hz [START_REF] Goff | Differential discrimination of frequency of cutaneous mechanical vibration[END_REF][START_REF] Gupta | Direct manipulation in tactile displays[END_REF], modulated with a 31 kHz square signal with a variable duty cycle to control the vibration amplitude. 31 kHz is the fastest clock-type signal with a 16 MHz microcontroller and 256 levels of precision duty cycle. The tactile information was transmitted to the Teensy 4.1 board using raw HID at 1000 Hz to minimize communication delays [START_REF] Casiez | Characterizing latency in touch and button-equipped interactive systems[END_REF]. The 10 actuators were placed on the frame as described in the following subsection.

Actuators' position

We positioned the actuators in the frame similarly to HapticVec [START_REF] Chen | Haptivec: Presenting haptic feedback vectors in handheld controllers using embedded tactile pin arrays[END_REF], with a matrix of actuators, that can be seen on Figure 3 and as illustrated in Figure 4. The size of the actuators significantly influenced the overall design of the frame and the positioning of the actuators.

A 1 A 2 A 3 A 4 A 5 A 6 A 7 A 8 A 9 A 10
Figure 4: Actuators' matrix projected on a hand with approximative positions (positions may vary with hand morphology).

We first designed smaller modular cylinders that could fit five actuators each. These modular cylinders have a 40 mm diameter, and a 30 mm height. Thus, the actuators are approximately 25 mm apart from each other. The modules were glued to align the position of the actuators. The frame of the controller consists of three of those modular cylinders with a 3D-printed cap on top to attach the Vive tracker. Our design could integrate up to fifteen actuators (see Figure 3). However, actuators were positioned considering how users grasped the device, and the size constraint of the actuators. The size of the actuators did not allow each fingertip to be in contact with exactly one actuator. Thus, we decided to position actuators in three columns, one on the fingertips and one on the metacarpal bones, as both are among the parts of the hands most sensitive to vibrations [START_REF] Schacher | Sensory and physiological issues[END_REF][START_REF] Verrillo | Vibrotactile sensitivity and the frequency response of the pacinian corpuscle[END_REF][START_REF] Verrillo | Vibration sensation in humans[END_REF]. The third column was placed at the bottom of the palm and the tenth actuator was used for the thumb.

Vibration Model

We based our vibration model on the funneling effect, and more precisely, the Lipari's triangular cell model [START_REF] Lipari | Improved 2D Interpolation for Intensity and Position Control in 2D Vibrotactile Meshes[END_REF], as the matrix we proposed is not completely regular (see Fig . 3), and an extension of the energy summation model [START_REF] Israr | Control space of apparent haptic motion[END_REF][START_REF] Park | Rendering moving tactile stroke on the palm using a sparse 2d array[END_REF] to 2D. If we go back to Figure 2, the amplitude of A 1 and A 2 is based on the position of the phantom vibration V 1 , which amplitude is also based on the position of the resulting phantom vibration V 2 . The amplitude of V 1 and A 3 are based on a 1D funneling problem, varying with the value of α, in the same way as Equation 3 from Sec. 2.2.2. Finally, by expressing all amplitudes in terms of the resulting phantom vibration V2, we obtain:

       I A 1 = I V 2 α (1 -β ) I A 2 = I V 2 α β I A 3 = I V 2 √ 1 -α (4)
This model creates a bijective relationship between the surface covered by the actuators and the amplitude of each actuator for each point belonging to the surface.

The frequency of the vibration for all actuators was kept identical, at the resonance frequency of the HiWave Haptic Exciter, which is 250 Hz.

Assessing the prototype

We designed two experiments to validate the design of our controller and spatialized feedback and evaluate its ability to enhance VR interaction and user experience. The first research question deals with vibrotactile rendering and the perception of feedback by participants, while the second research question deals more directly with the contribution of spatialized feedback to haptic experience.

The first experiment evaluates the ability of the controller to spatialize vibrotactile information at the level of a user's hand, when the user perceives vibration patterns which move over time passively, without active movement. The second experiment looks at the contribution of spatialized feedback to the participants' haptic experience in an interactive context with active movement. Finally, we briefly introduce different scenarios that showcase the implementation of the spatialized feedback with different VR interactions.

SPATIO-TEMPORAL DISCRIMINATION EXPERIMENT

This section presents a perceptual validation experiment that assesses the user's ability to recognize spatio-temporal vibratory patterns. The work of Kajimoto et al. [START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF] partially inspired the experiment. In their paper, they proposed an experimental protocol in which participants had to recognize spatio-temporal patterns rendered with an electrotactile feedback device. They had to recognize the direction and general movement of the rendered patterns. Using our prototype with the funneling effect, we propose a revisited version of this experimental protocol in a VR context.

This experiment was approved by the local ethics committee.

Objectives

We want to assess users' ability to perceive and keep track of a vibrating point moving in simple patterns, such as circles, lines, or more complex patterns, provided only through haptic means (i.e. without associated visual feedback). This experiment is related to our first research question concerning haptic rendering and vibration perception.

Participants

Twenty-one participants participated in the experiment (M = 32.2, SD = 7.6) and were recruited by word of mouth. We recruited righthanded (n = 16) and left-handed (n = 5) participants, assuming that laterality would not affect performance. The only inclusion criterion for participants was based on their ability to hold the frame while in contact with all actuators. Therefore, they were selected on the basis of the size of their hand. This led to all participants being male.

Apparatus

Participants were immersed in the IVE wearing a Valve Index headset, which provided visual feedback and a 6-DOF head tracking.

Participants held a Valve Index controller in their left hand and the prototype controller in their right hand. The virtual environment was implemented using Unity3D (2019.2.12f1) and the Unity Steam VR plugin. The experiment ran on an Intel (R) Core (TM) i7-9750H 2.60 GHz CPU with 32 GB of RAM and an 8 GB 2070 RTX GeForce Nvidia GPU. There was no virtual representation of the participants inside the IVE besides the left and right controllers. Furthermore, since vibrations are noisy, participants wore a Peltor 33 dB noise reduction headset to avoid bias when answering.

Experimental protocol

Generic instructions As the controller targets VR contexts, we opted to implement the experiment in VR, to collect data from the controller and feedback in the intended context of use, and also to avoid participants using physical-world information to locate the position of the vibrations. Each participant was first informed about the experiment, its general objective, and the tasks they would have to perform. They were told they would be feeling vibration patterns that they needed to identify, with no further details about the patterns. Then they were asked to fill out a consent form and we collected general information (age, gender, and hand dominance). The participants then sat on a chair and were equipped with the experimental apparatus.

Grasping instructions and calibration Specific instructions were given to the participants on how to hold the prototype so that each of the actuators was in contact with the skin. The experimenter guided the participant to correctly position their hand on the prototype. The experiment started with a calibration phase, where all actuators were triggered individually for 2 s each. This calibration phase had two objectives: first, to ensure that the prototype and each actuator were working properly; second, to check whether their grip on the prototype allowed them to feel the vibrations of each actuator and adjust it if necessary. Participants were instructed to report if they stopped feeling vibrations at any point during this phase.

Main task The main experimental phase was divided into several trials. Each trial proceeded in the same way: participants started a trial by pressing the trigger on the left controller, and half a second later, a vibration pattern was played on the prototype, while an interface displayed 12 choices of patterns (see Figure 5). The participants then selected with their left hand the pattern they thought was the one that was played. They had up to 30 s for each trial to choose a pattern while the pattern was played in a loop. If they did not answer within the 30 s, the trial were considered as missed. The trial ended when the participant chose the pattern by pressing the left trigger. We proposed 12 different vibration patterns for this experiment, which were repeated in 5 blocks, giving a total of 60 trials. These patterns are described in Figure 5.

Vibration patterns

As the aim of the experiment was to evaluate the ability of the controller to produce dynamic vibratory effects that users could understand, we wanted to propose a range of patterns from simple ones (circles around the circumference of the controller, or vertical lines) to more complex ones (spirals, thus combining the simple patterns to varying degrees). In total, 4 patterns of circles, 4 lines, and 4 spirals were proposed.

The vibratory patterns were played at a constant speed in the experiment. The speed and intensity used in the experiment were calibrated with a pilot experiment (n = 5). Playing the pattern at 8 cm/s and setting the vibration intensity of 80 % of the maximum amplitude allowed for a continuous and good feeling of the patterns. Frequency was kept constant at 250 Hz. The participants involved in the pilot experiment did not participate in the main experiment.

Measures

For each trial, we recorded the response times and the selected pattern. In addition, at the end of the experiment, participants had to answer two questions: one on the continuity of the vibrotactile patterns presented during the experiment and another one on the comfort of the vibrations ("How continuous/comfortable did the vibrations feel during the experiment?"). To not overburden participants, these questions were kept general (covering all patterns), but participants were allowed to elaborate on their responses using a text. Both questions were assessed using a 7-point Likert scale, with choices ranging from 1 -Not at all continuous/comfortable to 7 -Perfectly continuous/comfortable.

Hypotheses

The first hypothesis for this experiment concerns the ability of participants to keep track of a dynamic vibratory pattern. We hypothesize that the participants are able to recognize different patterns (H1). This first hypothesis can be broken down into two sub-hypotheses, the first concerning the participants' ability to recognize the general direction of the vibratory pattern (H1.1), and the second concerning their ability to position the pattern precisely in relation to their hand, and in relation to the controller (H1.2).

The second hypothesis is related to the quality of the vibratory effects and the perception of the continuity of the vibrations by the participants. Theoretically, the vibratory model described above (3.4) is bijective: every point in the actuator's matrix corresponds to a unique combination of actuators' at a certain amplitude. There should be no perceivable discontinuity in the intensity of the phantom vibration, either at the vertices or the edges of the triangular cells. Therefore, we hypothesized that the participants would perceive the proposed vibration patterns as continuous (H2).

Results

Regarding the recognition rates, Table 1 reports the recognition rates through the global confusion matrix. In this matrix, we can see the success percentages for each trial, and the confusion between trials. It is also possible to calculate the average percentage of success of the participants during the experiment. The results show an average success percentage of 58 % with a standard deviation of 13 %.

In addition, we computed the means and standard deviations for the time taken to answer each trial. On average, spirals took 16.6 s, lines took 14.0 s and circles took 12.1 s. The results are presented in Table 2. Participants reported their perception of the vibrations through two 7-Likert scale questions, one addressing the comfort (Mdn=5, SD = 1.3) and the other the continuity (Mdn=5, SD = 1.1) of the vibrations.

Table 1: Confusion matrix. Rows correspond to the tested patterns and columns to the recognize ones. Values are reported as percent of total number for each trials. Color transparency was linearly scaled from 0 to 100 %. Miss column represents the percentage of trials where participants did not answer before time ran out.

Discussion

There were two main hypotheses for this experiment: H1 related to the recognition of patterns, in terms of patterns' groups (H1.1) and position (H1.2), and H2 related to the quality and continuity of the perceived vibration. We first consider the results regarding comfort and continuity. On average, participants rated both questions 5 -Rather continuous/comfortable. These assessments are positive but limited to a general evaluation of all patterns and sensations. However, only one participant verbally reported not perceiving the entirety of a pattern. In general, these results suggest that the continuity of the patterns was overall good and lead us to partially validate H2.

Patterns' recognition

On average, the participants correctly identified the pattern played among the 12 patterns 58 % of the time. It is important to note that the participants perceived the pattern essentially as haptic (tactile, through the vibratory stimulation of the controller, and proprioceptive, through the position of their hand and fingers) without any visual indication, although sight remains the main sense in humans in the elaboration of a sensory judgment [START_REF] Boban | Changing finger movement perception: Influence of active haptics on visual dominance[END_REF]. Despite the lack of visual information, the participants were successful 58 % of the time compared to 8 % for random choices in identifying the correct pattern. These results show that the participants perceived and partially identified patterns solely based on tactile information.

Recognition between groups Here we discuss the recognition of patterns as pattern groups, i.e. if participants recognized lines as lines, spirals as spirals, or if there was confusion between groups. In addition to patterns C2 and L1, participants correctly identified the pattern group at least 70 % of the time, with a mean success rate of 78 %. This tends to validate H1.1. We discuss the results of specific patterns with respect to their position in the following paragraph. If we look closer at the average time to answer trials, we see differences between the groups. We suspected that participants might be using different strategies to identify different patterns. Participant 4003's comment was particularly revealing: "If it turned at a constant level, it was a if it involved all the fingers or the whole palm without turning, it was a line; otherwise it was a spiral". We directed informal questions in post-experimental exchanges to determine the strategy employed by participants, and it turns out that of them had similar reasoning.

Patterns' position Patterns vary greatly in terms of success, with C2 and L1 having the lowest recognition rate. They also have the highest average response time among patterns, which shows that participants had trouble identifying them. Pattern L1 started from the thumb, with the actuator located on A10 (see Figure 4), and propagated downwards between A8 and A2 then A7 and A1. The phantom vibration was located in a zone not covered by the user's hand under the thumb and between the bottom of the palm and the fingertips. As such, the low recognition rate for L1 can be explained by its position, in a zone that is difficult for participants to interpret. C2 on the other hand, was mistaken with patterns that were closer and more consistent than L1. C2 activated all actuators from the 2 top rows, which were common with other patterns such as C1, C3, and S4. More generally, we observe that patterns located close to the fingertips and the palm side show a high success rate, but with the exception of C2 and L1, the patterns show a success rate of at least 50 %. In addition to the specific zones of the frame, it seems that the participants successfully located the different patterns and vibrations, validating H1.2. Taken together, the results lead us to validate H1 partially.

Since active movement and exploratory procedures are crucial for interaction and touch [START_REF] Gibson | Observations on active touch[END_REF], we present in the next section a second experiment that focuses on interaction with active movements.

EXPERIMENT ON HAPTIC EXPERIENCE

In the first experiment, participants remained passive while feeling the vibrations. With this experiment, we explored the effect of spatialized vibrotactile feedback on user experience during active exploration. We compared the haptic experience of the participants according to 3 distinct conditions: control condition with a Valve Index controller and its integrated haptic feedback, our prototype controller with non-spatialized haptic feedback, and our prototype controller with spatialization.

The experiment addresses the research question related to the contribution of spatialized haptic feedback to user experience. To what extent does the spatialization of vibrotactile feedback improve the user experience, particularly the haptic experience, with respect to realism, immersion, and diversity? Our goal is to be able to compare our haptic solution (the prototype and the spatialized vibration model) with a consumer controller in an interactive VR context. To this end, we propose an experiment in an interactive virtual reality context in which we compare the haptic experience of the participants after interacting with the environment through different conditions, including a Valve Index controller alongside with our controller with and without the spatialized feedback. The local ethics committee approved this experiment.

Participants

18 participants took part in this experiment (M = 29.8, SD = 7.9), among whom 4 were left-handed. Participants were recruited based on their size of hand, as in the previous experiment, and again all participants were male. The participants were recruited by word of mouth.

Experimental conditions

We wanted to compare our prototype with recent consumer-grade VR controllers. Therefore, we proposed a baseline condition with a Valve Index controller, with its built-in vibrotactile feedback, CON-TROL. We compared this baseline condition with our prototype, under two different conditions. In one condition the prototype is used with spatialized vibrations (VIBSPATIAL), and the second one the prototype is used with non-spatialized vibrotactile feedback (VIBUNIFORM). CONTROL was based on the existing haptic feedback proposed by the controller. It was implemented through the SteamVR plugin [START_REF] Steamvr | Interface isteamvr_action_vibration[END_REF], with vibrations at 50 Hz, and a 100 % amplitude, which are the default values for vibrations signifying contacts. For VIBUNIFORM, the feedback was not based on a spatialized model. Instead, all actuators vibrated simultaneously at the same frequency and amplitude. The amplitude was set to 50 % of the maximum output for each of the ten actuators, and the frequency was set at 250 Hz. The vibrotactile feedback for VIBSPATIAL is implemented with the vibration model described in section 3.4 with the same amplitude and frequency as the first experiment (80%). In all conditions, the vibrations were only activated when the controller was in contact with the cube.

Experimental protocol

The experiment consisted of a simple task of active movement, where participants had to stay in contact with the edges of a virtual cube, and felt the movement of their hand/of the controller with vibrations co-localized with the point of contact of the controller with the virtual cube(see Figure 1). The task, which required active movements from users, was designed in such a way as to elicit congruent visuo-haptic stimulation and leverage the sensorimotor loop, as it is a crucial aspect when studying perception [START_REF] Gibson | Observations on active touch[END_REF].

Participants were first introduced to the experiment and then asked to read and sign a consent form and answer basic demographic questions (age and gender), before starting the experiment. The experiment followed a within-subjects protocol with Latin-square counterbalancing. Within the IVE, participants had an avatar limited to the controllers(see Video in the Supplementary Material). The right hand and controller were represented by a cylinder of the same size as the frame of the prototype.

Each condition went the same way as described below. After being introduced to the modalities of the experiment, the experimenter explained to the participants how to properly hold the controller, with the same instructions as in experiment 1, and helped them get the apparatus on. The apparatus was the same as in experiment 1. After gearing up, the participants were immersed in the IVE. A virtual hand-animated demonstration in the IVE showed participants how to complete the different movements. There were four different kinds of movement: a downward motion, an upward one, a circular motion, and a lateral motion. Each motion was repeated ten times, resulting in forty movements. After completing the task, the participants were asked to remove the gear (HMD and controllers) and answer the questionnaire. The protocol was repeated for each of the three conditions. For CONTROL and VIBUNIFORM, the vibrations triggered by contact with the cube were always the same. For VIBSPATIAL, the vibration point moved with the contact point between the controller and the cube in the IVE, creating a congruence between what the participants would see and feel.

Measures

The focus of this experiment was the added value of spatialized vibrotactile feedback on the user experience. As such, we decided to use the Haptic Experience Questionnaire, proposed by Sathyamurthi et al. [START_REF] Sathiyamurthy | Measuring haptic experience: Elaborating the hx model with scale development[END_REF], to assess the subjective touch experience for each condition (see 3). We implemented all 20 items and asked participants to rate each item on a 7-point Likert scale. We also asked participants to rate each condition on a 1-10 scale (1 being "terrible controller overall and 10 "great controller overall"), which we call the global likability of the condition, and explain what motivated their score on a free text. This was done to determine what factors made them like or dislike each condition. Table 3: Haptic experience questionnaire as proposed by Sathiyamurthy et al. [START_REF] Sathiyamurthy | Measuring haptic experience: Elaborating the hx model with scale development[END_REF]. Order of items is as proposed by the original authors.

ID Questions Autotelism H-1 I like having the haptic feedback as part of the experience. A-2 I like how the haptic feedback itself feels, regardless of its role in the system. A-3 I disliked the haptic feedback. A-4 I would prefer the system without the haptic feedback. A-1 The haptic feedback felt satisfying. Expressivity E-2 I felt adequate variations in the haptic feedback. E-4 The haptic feedback changes depending on how things change in the system. E-5 The haptic feedback reflects varying inputs and events. E-1 The haptic feedback all felt the same. Immersion I-4 The haptic feedback increased my involvement in the task. I-3 The haptic feedback helped me focus on the task. E-3 The haptic feedback helped me distinguish what was going on. I-2 I felt engaged with the system due to the haptic feedback. Realism R-1 The haptic feedback was realistic. R-2 The haptic feedback was believable. R-3 The haptic feedback was convincing. Harmony H-3 The haptic feedback felt disconnected from the rest of the experience. H-5 The haptic feedback felt out of place. I-1 The haptic feedback distracted me from the task. H-4 The haptic feedback felt appropriate when and where I felt it.

Hypotheses

We aim to assess the benefits of spatialized vibrotactile feedback for the user's haptic experience. This spatialization should allow to increase the richness of the haptic feedback during interactions with the environment. We therefore hypothesize that the VIBSPATIAL condition is significantly superior to the CONTROL and VIBUNI-FORM conditions in terms of overall haptic experience (H3), and particularly in terms of expressiveness (H3.1), immersion (H3.2) and realism (H3.3).

VIBUNIFORM proposes limited haptic feedback, without variations. We hypothesize that this feedback would be considered unrealistic, but also judged as not harmonious by the participants. Thus, we make the hypothesis that VIBUNIFORM is significantly less harmonious and realistic than VIBSPATIAL and CONTROL (H4).

Results

Because the data collected through the questionnaires were ordinal and did not follow normal distributions, each item was analyzed with nonparametric tests using Friedman analysis and Wilcoxon post hoc pairwise tests with Bonferroni correction.

Global likability

A Friedman analysis on global likability scores revealed significative differences between conditions (χ 2 (3) = 17.1, p < 0.0001). Post-hoc Wilcoxon analysis revealed significant differences between VIB-SPATIAL (Mdn = 8) and both CONTROL (Mdn = 6.5, p < 0.003) and VIBUNIFORM (Mdn = 6.5, p < 0.0001) (see Figure 6).

Control

VibUniform VibSpatial

Haptic experience

The questionnaire recently proposed by Sathiyamurthy et al. [START_REF] Sathiyamurthy | Measuring haptic experience: Elaborating the hx model with scale development[END_REF] has not yet been validated by other experiments. To provide additional data, we reproduced the dimension reduction approach to verify that we obtained a distribution of the different items close to that proposed by the authors of the questionnaire. These results are available in the supplementary material section of the article. As we were unable to reproduce all the initial groups found in Sathiyamurthy et al. study, we decided to test the items individually. We performed Friedman analyzes on all items of the haptic experience questionnaire. For items with significant tests, we performed Wilcoxon post hoc analyzes. These analyzes revealed significant differences for items A3, A4, A1, E2, E4, E5, E1, I4, E3, R1, R2, R3, H3, H5 and H4 (p < 0.05). The set of responses to the haptic experience questionnaire is summarized in Figure 7.

Discussion

In this experiment, we aimed to evaluate the contribution of our prototype to haptic experience. Taken together, our results show that spatialized vibrotactile feedback is superior to the baseline conditions (CONTROL, VIBUNIFORM). To be precise, the user's haptic experience and the global appreciation of the participants were significantly higher.

With H3, we assumed that the condition VIBSPATIAL would be superior to the other two conditions in terms of haptic experience. Of the 20 questions that made up the questionnaire, 8 of them showed that VIBSPATIAL obtained significantly higher ratings than the other two conditions. Thus, we can validate H3. We also hypothesized that VIBSPATIAL would be superior in terms of expressiveness (H3.1), immersion (H3.2) and realism (H3.3). Regarding expressiveness, composed of items E2, E4, E5 and E1, we can confirm the hypothesis: while the VIBSPATIAL ratings for E5 were only significantly higher compared to CONTROL, for the others, the VIBSPATIAL ratings were significantly higher than for the other conditions. For realism, made up of items R1, R2, R3, the results were similar: R1

Expressivity Immersion

Realism Harmony Autotelism Figure 7: Boxplot of haptic experience scores, for each items. Red markers above the boxes represent significant differences. and R2 items showed the significant superiority of VIBSPATIAL, while R3, which assessed the convincing aspect of the feedback, differed slightly. To our surprise, CONTROL was significantly lower than the other two conditions. We interpret this result as an expression of a higher expectation of the haptic feedback proposed by the controller Valve Index, which seemed, following the feedback of several participants, "more ergonomic". With these results, we can validate H3.3. The results are more nuanced with respect to immersion (Items I4, I3, E3, I2). Items I4 and E3 showed a significant superiority of VIBSPATIAL, which is encouraging. However, statistical analyzes did not reveal any differences for items I3 and I2. I3 dealt with helping the participants concentrate on completing the task. We consider that, given the experimental protocol and the task at hand, this was not surprising: the task did not have any particular goal as participants only had to focus on the generated haptic sensations. Thus, it seems reasonable to us that the different haptic modalities did not contribute differently to concentration during the task. We can also note that the scores on this question were high for all conditions. For item I2, which deals with engagement, we believe the task and interactions were not varied enough to show differences. It would be interesting to propose a richer interactive context to test this new hypothesis. However, we can, with two items among four, partially validate H3.2.

Finally, we hypothesized that the realism and harmony of CON-TROL would be superior to VIBUNIFORM (H4). Our results do not allow us to confirm these assumptions. These results are positive: the design of the controller seems to be good enough for participants to rate it as a suitable controller, even when compared to the Valve Index controller. The overall rating scores are consistent with this, since VIBUNIFORM and CONTROL have the same median. The scores for VIBUNIFORM still have a larger variance.

This second experiment evaluated the use of MultiVibes in the context of active movements and interaction with virtual objects. The results show that the users preferred MultiVibes and spatialized feedback over consumer-grade controllers. In the following section, we discuss the limitations of the prototype and our findings.

DISCUSSION

We presented two experiments assessing MultiVibes. Results of the first experiment show that participants can recognize the vibration patterns ∼60 % of the time among 12 different patterns. Compared to Kajimoto et al. [START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF], who only presented one pattern to participants, and Lacote et al. [START_REF] Lacôte | Investigating the haptic perception of directional information within a handle[END_REF], who presented 4 patterns (2 lines and 2 circles) with a recognition rate at ∼75 %, our results with MultiVibes, with 3 times the number of patterns, and more complex patterns, are encouraging. Results from the second experiment show that Mul-tiVibes is better in terms of haptic experience compared to current commercial VR controllers. We used a 2-dimensional funneling effect as vibration model, which allows for active interaction with environments as we did in the second experiment, which is not possible with vibration models like the one used by Lacote et al., based on SOA. The haptic experience data from the second experiment can also be used as an assessment of the validity of the questionnaire introduced by Sathiyamurthy et al. [START_REF] Sathiyamurthy | Measuring haptic experience: Elaborating the hx model with scale development[END_REF]. We provide the PCA analysis of the questionnaire items in the supplementary material.

Limitations The experiments described in the previous sections highlighted certain prototype limitations and the perception of vibrations. The first limitations are related to the design of the controller itself. Due to the radius of the frame, people with smaller hands could not hold the controller properly while touching all actuators. As such, the pools of participants for both experiments were biased towards one gender. Although there is no consensus on gender differences in vibration perception [START_REF] Neely | Gender differences in subjective responses to hand-arm vibration[END_REF], we believe that making the prototype usable by any hand size is important. It could be possible to either reduce the radius of the frame or implement some capacitive sensing to determine hand and finger position. We develop this idea below. Another limitation is that for the funneling effect to work properly, vibrations from actuators must be localized. To do that, the actuators were in direct contact with the skin. Yet, if gripped too tightly, the actuators' moving part cannot vibrate freely, thus changing the perception of the intensity of the phantom vibration. Moreover, if users change their grip during immersion, the hand might lose contact with one or multiple actuators, changing the perception of the resulting vibration. For the two experiments previously described, the experimenter made sure participants' hand was correctly positioned, but it could be an issue for non-controlled setups.

Future work

The results of the two experiments show that the design of MultiVibes can serve as a basis for controllers with enriched interactions in VR. Still, there are several considerations for future work to showcase the prototype and enhance its design.

We designed several scenarios that showcase multiple interactions based on the same 2D funneling model. These scenarios include toolmediated interaction, with vibrations symbolizing hits between the tool and the environment, interactions with animals with prolonged contacts, and other abstract renderings of directions, positions, or even progress bars. Those scenarios showcased in the accompanying video (see Supplementary material) have not yet been tested in a controlled setup. We also remarked informally that the perception of textures, through hardness/softness, during contact, is different depending on the steepness of the signal at the start of the vibration.

We have also considered improving the prototype by adding capacitive sensing in the frame, such as the Valve Index controller or Kajimoto et al. [START_REF] Kajimoto | Design of cylindrical whole-hand haptic interface using electrocutaneous display[END_REF] prototype. Capacitive sensing would allow for better control of the actuators: there is no need to activate any actuator not in direct contact with the fingers or palm. It would also allow for a regular actuator matrix that could fit all sizes of the hands. Capacitive sensing has already been implemented in consumer-grade controllers such as the Valve Index. Including multiple actuators, as we proposed with MultiVibes would further enhance haptic feedback inexpensively. Considering that adding actuators is a low-tech solution, unlike many other research haptic devices, we firmly believe it could be added to a new commercial product in the short to midterms.

CONCLUSION

In this paper, we presented a prototype of a VR controller that can spatialize vibrotactile feedback, MultiVibes. MultiVibes is based on the funneling effect, using multiple actuators to enrich haptic feedback for interactions in IVEs. The design of the prototype frame, based on previous designs, can be easily replicated by 3D printing. To validate MultiVibes, we designed two experiments to assess it: one based on the perception of vibrations and the ability of users to recognize dynamic vibration patterns, and the other based on the positive effect of spatialized feedback on the haptic experience. Taken together, our results show that the design of the controller is convincing as a VR controller, even when compared with consumer-grade VR controller like the Valve Index. Moreover, the spatialized vibrations show great promise in terms of transmission of information and increased haptic and user experience.

Figure 2 :

 2 Figure 2: Funneling model in 2 dimensions with 3 actuators.

Figure 5 :

 5 Figure 5: Matrix of the vibration patterns. On top, the circle patterns (C1 to C4), in the middle, the line patterns (L1 to L4), and at the bottom, the spiral patterns (S1 to S4).

Figure 6 :

 6 Figure 6: Aggregated scores for global likeability for each condition, ranging from 1 to 10.

Table 2 :

 2 Means and standard deviations for the time taken to answer each pattern.

		Circles	Lines	Spirals
		C1 C2 C3 C4 L1 L2 L3 L4 S1 S2 S3 S4
	Mean time (s) 9.1 18.4 12.2 8.8 18.6 10.6 14.1 12.9 17.0 14.7 17.0 16.1
	SD (s)	6.2 8.1 6.9 5.7 8.8 6.7 7.6 8.2 7.6 7.4 7.7 8.3

ACKNOWLEDGMENTS

This work was funded by the Inria Avatar project. The authors wish to thank Danny Kieken and Suliac Lavenant for their help with the demonstrators during the project.


Catherine Kapota

Philippe Maıtre

Joël Lemaire

Gilles Ohanessian

Gilles Ohanessian Spectres

come

Spectres IR de complexes acides aminés/Na+ produits par MALDI dans un spectromètre de masse FT-ICR

Introduction

• En phase condensée : les acides aminés (AA) sont sous forme zwitterionique dans une large gamme de pH • En phase gazeuse :

-les AA sont sous forme neutre (sans charge partielle) -certains AA coordinés à Na + peuvent présenter une forme zwitterionique SB (Ex : Pro [1]) ou une forme neutre CS1 (Ex: Gly [2]) comme isomère de plus basse énergie

2 -

 2 , a P. Maître, b J. Lemaire, b,c G. Ohanessian a kapota@dcmr.polytechnique.fr, a Laboratoire des Mécanismes Réactionnels, DCMR. Ecole Polytechnique. 91128 Palaiseau Cedex. France. b Laboratoire de Chimie Physique, Université de Paris-Sud, 91405 Orsay CEDEX, France , c Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, Université de Paris-Sud, CLIO-BP34, 91405 Orsay CEDEX, France. . Ab initio MP2 Programme Gaussian 98 I-Obtention du spectre IR 1-formation et piégeage des ions: Les ions sont formés par MALDI en focalisant un Laser YAG triplé sur l'échantillon MALDI puis piégés dans la cellule du FT-ICR irradiation par le LEL Impossible de faire une mesure directe de l'absorption infrarouge car la densité d'ions est trop faible  On induit une réaction chimique qui va être la signature de l'absorption infrarouge par IRMPD (Infra-Red Multi Photon Dissociation) en focalisant le LEL (laser IR accordable) sur les ions Le processus d'IRMPD provoque la dissociation de plus basse énergie, dans notre cas: AA-Na +  AA + Na +  L'apparition de l'ion Na + (masse 23 uma) correspond à une bande IR 311+G(2d,2p)//B3LYP/6-31G* Conclusion • Mise en évidence expérimentale de la forme : * zwitterionique de Pro dans le complexe Pro-Na + * neutre de Gly dans le complexe Gly-Na + en accord avec les calculs • Les ions formés par la méthode MALDI sont les isomères les plus bas en énergie Méthodes Bibliographie Considérons l'absorption d'un mode particulier Scaling factor des fréquences 0.98 pour B3LYP/6-31G* 0.96 pour MP2/6-31G*  as CO 2Spectres IR de complexes acides aminés/Na + produits par MALDI dans un spectromètre de masse FT-ICR

cellule ICR de MICRA 1 matrice 2 NaCl 3 AA 4 matrice matrice Pro-Na +

				CS1		SB
	Pro-Na + (Kcal/mol)	5.6		0.0
	Gly-Na + (Kcal/mol)	0.0		2.7
							Pro-Na+ CS1	Pro-Na+ SB	Gly-Na + CS1 Gly-Na + SB
			 L'objectif est donc d'identifier la structure des complexes
	Gly-Na +	et Pro-Na +	en phase gazeuse par spectroscopie infrarouge, grâce à une
	expérience couplant un spectromètre de masse de résonance cyclotronique ionique à
		transforméee de Fourier (FT-ICR) et un laser à électrons libres (LEL) [3]
	solvant	acétone/eau (1/1)	eau	eau	(1/1) acétone/eau	a-HCCA
	[ ] (mol/L) 3.75 10 -2 2.5 10 -2 2.5 10 -2 3.75 10 -2
	V (µL)		15	5	10	15
	équivalent molaire		4.5	1	2	4.5
	Gly-Na +


Joël Lemaire

Luke Macaleese

Catherine Kapota

Gilles Ohanessian

Pierre Boissel

Jean-Michel Ortega

Philippe Maıtre

la diffusion de documents scientifiques

de niveau recherche, publiés ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Vibrational spectroscopy of selectively prepared gas phase ions by coupling a permanent magnet based FT-ICR-MS with a Free Electron Laser

Vibrational spectroscopy of selectively prepared gas phase ions by coupling a permanent magnet based FT-ICR-MS with a Free Electron Laser

Joël Lemaire, a Luke MacAleese a , Catherine Kapota, c

Gilles Ohanessian, c Pierre Boissel, a Jean-Michel Ortega, b Philippe Maître a a Laboratoire de Chimie Physique, UMR 8000 du CNRS, Université de Paris-Sud, 91405 Orsay, France.

InfraRed spectroscopy of metalligand complexes : differentiation of isomers

Fe + (i-butene) is prepared by deshydrogenation of i-butane by mass selected Fe + Fe + (1-butene) is formed by ligand displacement from mass selected FeCO + by 1-butene Fe + (i-butene)

Fe + (1-butene)

The geometry of the ICR cell allows an easy optical access to the cell center : the cell is derived from a cubic cell where the two excitation plates are replaced by two tunnels made of four interconnected electrodes.

IRMPD rate efficiency as a function of laser intensity

The dependance of the multiphoton dissociation rate with laser intensity has the form of an S shape curve exhibiting a threshold behavior at low intensity and a saturation behavior at high intensity.

The threshold intensity under which no dissociation is observed can be very different from one system to another.

The spectra are generally realised in the region where the behavior is close to linearity. They are transferred in the ICR cell, trapped and irradiated with the IR beam. The resulting experimental dissociation spectra are compared with the calculated spectra for the low energy isomeric forms.

In the case of Pro-Na + the complex is found to have a salt bridge structure while in the case of Gly-Na + the observed spectra corresponds to charge solvation by chelation of Na + between nitrogen and the carbonyl oxygen. Gly-Na + Wavenumber (cm -1 )

Sodiated amino acids

b

  Laboratoire pour l'Utilisation du Rayonnement Electromagnétique, UMR 130 du CNRS, Université de Paris-Sud, CLIO-BP34, 91405 Orsay, France. c Laboratoire des Mécanismes Réactionnels, UMR 7651 du CNRS, Ecole Polytechnique, 91128 Palaiseau CEDEX, France. The InfraRed Multiphoton Dissociation experiments are realised with a Mobile Ion Cyclotron Resonance Analyzer (MICRA), associated to the Orsay Free Electron Laser CLIO. The mass spectrometer is based on the use of a permanent magnet assembly (Halbach cylinder) producing a nominal magnetic field of 1.25T. Available ionisation modes : -Electron impact -Formation of metal cations by laser ablation ionisation on a metallic target -MALDI Several ion reaction and mass selection steps can be performed before irradiation of the ions with the focussed IR beam. Characteristics of the Free Electron Laser CLIO Tuning range : 3 -120 μm Average power : 1W peak power 40 MW Continuously tunable over Δλ/λ =2.5 Bandwidth 0.5% -1% of the central wavelength 8 μs long macropulses, 25 Hz repetition rate A macropulse contains 500 micropulsesThe laser is focussed by a 1m focal length spherical mirror (waist diameter ~200 μm for λ=5 μm)

  of glycine and proline are formed by MALDI on a target located in front of the trapping plate.


Yan Hao
email: haoyannk@163.com

Zhenpeng Cui

Hui Yang

Guibao Guo

Jinyan Liu

Zhixiang Wang

Ariane Deniset-Besseau

Samy Remita
email: samy.remita@u-psud.fr

Adsorption of Cr (VI) by cellulose adsorbent prepared using ionic liquid as green homogeneous reaction medium

Keywords: microcrystalline cellulose (MCC), ionic liquids (ILs), homogeneous phase graft copolymer, metal ion adsorbent

Cellulose-based adsorbent was prepared for the removal of Cr (VI) by chemical induced grafting of styrene onto microcrystalline cellulose (MCC) followed by chloroacetylation, amination and protonation processes in 1-butyl-3-methylimidazolium chloride ([Bmim]Cl) ionic liquid. The shape and size of adsorbents can be easily adjusted according to the requirements of wastewater treatment. FTIR and AFM-IR characterizations demonstrate grafting and copolymerization of styrene onto MCC substrates. The grafting yield can be easily controlled by changing the experimental conditions, such as monomer concentration, reaction time and temperature. The adsorption of Cr (VI) ions has been investigated as a function of contact time and pH. The adsorption kinetics revealed that the equilibrium adsorption could be achieved within 30 min. The adsorption behavior of the resultant absorbent could be described with the Langmuir mode.

INTRODUCTION

Heavy metal pollution, which is characteristic of covert, long-term and irreversible, mainly results from the heavy metal ions released by industrial wastewater. Heavy metal ions are difficult to be eliminated due to their transformation among different states rather than degradation by microbes. 1 The removal of heavy metal ions has become an urgent issue as they can not only lead to direct and indirect health risk to human being but also disserve the whole ecosystem. For instance, hexavalent chromium, Cr (VI), can invade human body through digestive tract, skin and mucosa bringing about ulceration, dermatitis, nervous poison and even cancer. [2][3][4] Several techniques have been reported to address this problem, such as precipitation, ion-exchange, reverse osmosis, filtration, extraction, redox and adsorption. 5,6 Among various methods, adsorption is a green, economic and effective approach, achieving efficient separation without importing other metal ions and extra energy, which is in accordance with presently advocated green chemistry. In order to effectively adsorb heavy metal ions, selecting appropriate adsorbents with high adsorption capacity is critical and the development of low-cost and high-efficiency new adsorbents has attracted great interest.

Generally, adsorbents can be prepared by modifying various substrates such as polyethylene (PE), polypropylene (PPE) through monomer grafting. However, these substrates may cause serous pollution to the environment due to hard decomposition. 7,8 In contrast, cellulose is the most abundant, widely distributed, renewable, biodegradable and biocompatible natural polymer in nature enabling both landfill reduction and environment preservation. 9,10 As the function of natural cellulose is limited, modification of cellulose by graft polymerization with natural or synthetic macromolecules provides a meaningful approach to extend its range of application. 11 When cellulose is treated with initiator or high energy radiation, active sites emerge and functional monomers are subsequently grafted onto its backbone, which can not only maintain its original structure but also endow cellulose new properities by functionalization with selected monomers. John et al. grafted cardanol onto filter paper cellulose using cationic initiator (BF3-Et2O) and improved its waterproofness. 12 Kumar et al. applied co-radiation graft copolymerization method to prepare cotton cellulose grafted with vinylbenzyl chloride for adsorption of protein. 13 Hajeeth et al. reported the grafting modification of extracted cellulose with acrylic acid in the presence of ceric ammonium nitrate redox initiator. The prepared graft copolymer was used as adsorbent for the removal of Cr (VI) from the aqueous solution, which was fitted well to the Freundlich adsorption isotherm. 14 Navarro et al. fabricated cellulose grafted with alkyl ether to adsorb Hg (II) by treating cellulose with epichlorohydrin for etherification as well as further reactions between amino groups of polyethyleneimine and epoxy groups of cellulose ether. 15 The insolubility of cellulose has become a bottleneck for its application. Since cellulose is nearly insoluble in water and most organic solvents, modification of cellulose is mainly carried out in heterogeneous, non-green solvent systems. However, inhomogeneous substitution is often formed in the heterogeneous system resulting from unequal accessibility of OH groups between the amorphous regions and crystalline regions. 16 In addition, heterogeneous reaction and non-green solvent have many limitations in terms of volatility, toxicity and instability. 17 Sun synthesized the amino-functionalized magnetic composite of cellulose that was able to remove Cr (VI). 18 The above preparation was carried out in NaOH/urea aqueous solvents while big amounts of acids and bases were used making the recycling of cellulose solvents difficult. It is worth noting that Swatloski et al. found that room temperature ionic liquids (RTILs) could efficiently dissolve cellulose, thus initiating great interest in solubilization and modification of cellulose in this new solvent system. 19 RTILs, which are regarded as intrinsically ''green" due to their negligible vapor pressure, have attracted great attention for their good thermal stability, high conductivity, low melting point and wide electrochemical window. 20,21 In this work, 1-butyl-3-methylimidazolium ([Bmim]Cl) ionic liquid was used as the solvent for the homogeneous modification of cellulose to produce a cellulose-based absorbent to adsorb Cr (VI). First, styrene monomers were grafted onto cellulose substrates and further modified by chloroacetylation, amination and protonation to obtain target cellulose-based adsorbent. The shape and size of adsorbents can be easily adjusted according to the requirements of wastewater treatment. Then, influencing factors on the grafting yield (GY) and the adsorption kinetics of Cr (VI) were systematically investigated.

EXPERIMENTAL

Materials

MCC with a degree of polymerization of 215~240 and styrene (St) monomer were obtained from Shanghai Kefeng Chemical Reagents CO. Ltd. [Bmim]Cl (>99%) was purchased from Lanzhou Institute of Chemical Physics (Lanzhou, China) and the water content was tested to be less than 0.9% by Karl-Fischer titration. Dimethyl sulfoxide (DMSO) and methylene-bis-acrylamide (MBA) were provided by Aladdin Industrial Corporation. Ammonium sulfate ((NH4)2S2O8) was obtained from Tianjin Chemical Reagent Co., Ltd. Other chemicals were analytical reagents and used as received.

Synthesis of Cellulose-g-PS

First, MCC was dispersed into [Bmim]Cl (ca. 5 g MCC/45 g [Bmim]Cl) and stirred for 24 h at 80 ℃, forming a clear solution with 10% (wt%) of MCC. The obtained solution was diluted by adding DMSO (50 g). Then, chemical initiator ((NH4)2S2O8) was added into the diluted solution at 60 ℃ and stirred for 20 min under nitrogen atmosphere followed by the addition of St monomer and MBA cross-linking agent. All the conditions were kept the same until the grafting reaction terminated. The solution after grafting was divided into two parts: one part was modified by chloroacetylation; the other part was used to calculate the grafting yield (GY). The procedures were as follows: the obtained solution after grafting was transferred to culture dish to form membrane and washed with ethanol to regenerate cellulose; the regenerated cellulose membrane was covered by filter paper and washed in soxhlet extractor for 24 h to wash away homopolymer, unreacted monomer and [Bmim]Cl. The obtained copolymer of cellulose and styrene was dried in vacuum drier at 70 ℃ until constant weight and named as Cellulose-g-PS.

The GY was determined as follows: (1) where W0 is the weight of MCC, W1 is the weight of Cellulose-g-PS.

Preparation of absorbent

Cellulose-g-PS (5.0798 g) was added with CH2Cl2 (50 ml), CH2ClCOCl (5.5140 g) and AlCl3 (12.6744 g) in sequence. After reacting for 7 h at room temperature, the resulting product was first washed with THF, HCl (3%), deionized water, methanol and then extracted in soxhlet extractor for 24 h with ethanol. The as-prepared product was dried in vacuum drier at 70 ℃ for 24 h and noted as chloroacetylation product (CPC). Subsequently, CPC (2.3160 g) was swelled in THF and reacted with NH(C2H5)2 (10.5541 g) as well as NaHCO3 (0.9353 g). As the reaction continued, the color of product changed from white to yellow and amination product (CPCN) was prepared by washing with deionized water. After above reactions, the solution was cast onto a glass plate and then immediately coagulated in ethanol to obtain the regenerated cellulose film with a thickness of about 0.50 mm. 22 In addition, the spherical cellulose adsorbent also could

1 0 0 (%) W W GY W - =
be obtained through a water-in-oil suspension technique. Thus, the shape and size of adsorbents can be easily adjusted according to the requirements of wastewater treatment. Finally, CPCN was stirred in HCl (1 mol L -1 ) at room temperature, washed with deionized water and dried in vacuum drier at 70 ℃ for 24 h to get target CPN adsorbent. The synthetic routes of CPN are illustrated in Scheme 1.

Scheme 1

Characterization of CPN

Micro-FTIR spectra of original MCC, regenerated cellulose, Cellulose-g-PS, CPC and CPN were recorded on a Nicolet (Magna-IR 750) spectrometer within the wavenumber range of 4000~600 cm -1 . AFM-IR measurement was carried out by depositing adsorbent onto a ZnSe prism (transparent in the mid-IR). The deposit was observed by nanoIRTM (@Anasys Instruments), an AFM-IR system that combines AFM with a pulsed infrared OPO laser to perform spectromicroscopy. The morphologies of samples before and after grafting were observed by SEM (HITACHI S-4300) and all the samples were treated by spray-gold. The thermal stability and composition analysis were performed on a thermogravimetric analysis instrument TGA Q600 under a nitrogen flow of 40 mL min -1 . The temperature ranged from 30 to 700 ℃ at a heating rate of 15 ℃ min -1 .

Adsorption capacity of Cr (VI)

The stock Cr (VI) solutions were prepared by using the same procedures as reported. 3 Certain amount of HCl (0.02 mol L -1 ) was added to stabilize Cr (VI) ions in the stock solution. The above solution was subsequently diluted with deionized water to obtain desired concentrations. The concentration of Cr (VI) was determined by UV-vis spectrophotometry at 258 nm. 23,24 The pH value of solution was adjusted with HCl (1 mol L -1 ) or NaOH (1 mol L -1 ) and measured by pH-meter (PHSJ-3F).

The kinetics study was conducted in a 50 mL 35 mg L -1 Cr (VI) solution with 20 mg CPN adsorbent. All the solutions were covered and kept in a constant temperature incubator at 25 ℃ for different time intervals. The Cr (VI) uptake (q) was calculated as follows:

(2) where C0 and Ct (mg L -1 ) are the concentrations of Cr (VI) before and after adsorption, respectively; V (mL) is the volume of Cr (VI) solution and m (mg) is the weight of CPN adsorbent.

RESULTS AND DISCUSSION

Preparation of CPN adsorbent

The influences of concentrations of monomer and initiator, reaction time, reaction temperature on the grafting yield (GY) were studied in detail and shown in Fig. 1.

From Fig. 1a, it is clear that GY goes up and reaches a plateau as the concentration of monomer increases. When the amount of styrene (St) rises, reactive radicals of MCC have more access to St monomer and graft more resulting in growing GY. However, St monomer conducts copolymerization rather than grafting as its content is in excess (wt% > 11.1%) leading to maximum GY. Therefore, the appropriate mass percent of St monomer is fixed at 11.1%. Fig. 1b shows that GY increases as the concentration of initiator rises and decreases when the amount of (NH4)2S2O8 is over 0.3% (wt%). It is easy to understand that more reactive radicals are produced as the content of (NH4)2S2O8 goes up accelerating reaction between MCC and St monomer, which accounts for increasing GY. However, excess (NH4)2S2O8 also enhances the homopolymerization of St monomer leading to decreased GY. As a result, the mass percent of (NH4)2S2O8 initiator is chosen as 0.3%. The effects of reaction time and reaction temperature on GY are presented in Fig. 1c and1d, respectively. It is found that GY increases first and maintains nearly the same after 2 h indicating complete grafting of St monomer. As a result, the optimal reaction time is 2h. Also, the suitable reaction temperature is found to be 60 ℃. This observation is explained by that with an increase in reaction temperature, more reactive radicals are generated, which promotes the increasing of GY. However, when the temperature is too high, it also increases the rate of chain termination, chain transfer and homopolymerization reaction, which results in the decrease of GY. 25 According to the above results, the preparation of CPN adsorbent was carried out with St monomer of 11.1% (wt%) and (NH4)2S2O8 initiator of 0.3% (wt%) for 2 h at 60 ℃. For comparison and further characterizations, products with a calculated GY of 68.1% were selected as a representative.

Characterization of CPN

In order to confirm successful grafting of St and study its effect on thermal stability, original MCC, regenerated cellulose, Cellulose-g-PS (GY=68.1%), CPC and CPN absorbent were analyzed by micro-FTIR and TGA. Fig. 2 presents the micro-FTIR spectra of all the samples. The spectra of original MCC (Fig. 2a) and regenerated cellulose (Fig. 2b) are similar and the following characteristic bands are observed: a large band between 3500~3300 cm -1 corresponding to stretching vibration of OH groups; a small band between 2900~2700 cm -1 owing to -CH2 groups; characteristic bands at 1160 arising from C-O-C of glucosidic units. 26 Grafting of styrene is verified by the characteristic bands of aromatic ring at 1490 and 1600 cm -1 which are attributed to C=C in-plane stretching vibration. The sharp peaks at 698 and 760 cm -1 of cellulose-g-PS (Fig. 2c) are assigned to aromatic C-H deformation of substituted benzene ring. The new band at 3025 cm -1 is ascribed to =C-H stretching vibration of aromatic ring. 27 The stretching vibration of carbonyl (C=O) of CPC is located at 1750 cm -1 (Fig. 2d) indicating that the chloroacetyl groups have been attached to original MCC. 28 In Fig. 2e, the C=O band of CPN at 1750 cm -1 becomes is due to the substitution of chloroacetyl groups by amination. Therefore, the results of micro-FTIR spectra proved that CPN adsorbent was successfully synthesized. To observe the surface morphology of CPN adsorbent and identify the distribution of functional groups in it, the as-prepared adsorbent was characterized by AFM-IR. AFM was used for the surface morphology observation in contact mode. For the distribution of functional groups study, the combination of AFM and IR laser pulse was applied to draw the chemical maps of the characteristic bands of functional groups. 29 The results are shown in Fig. 3.

Figure 3

According to the topographic image taken by AFM in Fig. 3a, CPN adsorbent appears as big aggregates (~2.5 µm) composed by small particles (marked by arrows). Then, the chemical maps of CPN adsorbent are recorded by tuning IR source at 1160, 1490 and 1735 cm -1 which correspond to original MCC, in-plane stretching vibration of aromatic C=C and C=O group, respectively. As is shown in Fig. 3b, the shape of chemical map obtained with the characteristic band of original MCC is the same as topography of CPN adsorbent. Therefore, it is clear that CPN adsorbent contains MCC substrate and the red areas indicate that more MCC exists in these regions. Similarly, the chemical maps recorded with the characteristic peaks of C=C and C=O groups show quite similar features as that of original MCC. These results further demonstrate successful grafting of styrene and chloroacetylation onto original MCC and prove relatively homogeneous distribution of C=C and C=O groups in CPN adsorbent. The AFM-IR results are in accordance with that of micro-FTIR and further characterize the morphology of CPN adsorbent and its composition.

In order to study the thermal stability and further determine the amount of grafting groups onto cellulose substrate, TGA curves of all the samples were measured and shown in Fig. 4. All the samples have a small weight loss below 120 ℃ caused by the loss of moisture. Compared with original MCC (Fig. 4a), the regenerated cellulose (Fig. 4b) exhibits a lower onset temperature of decomposition but gives a higher char yield after pyrolysis. In addition, the residual of original MCC is 1.6% at 600 ℃, whereas 17.8 % is remained of regenerated cellulose at the same temperature due to the formation of nonvolatile carbonaceous material. 19 Cellulose-g-PS (Fig. 4c) undergoes two-step degradation in which degradation of MCC first occurs and then decomposition of grafted styrene proceeds resulting in a weight loss of 66.6% which is close the calculated GY (68.1%). Moreover, the residual of Cellulose-g-PS decreases compared with that of regenerated cellulose implying the covalent attachment of the polystyrene chains to original MCC. 30 TGA curves of CPC (Fig. 4d) and CPN (Fig. 4e) show that chloroacetylation and amination of cellulose have no effect on the thermal stability of cellulose which is the same as reported results. 31 However, it is found that that the decomposition temperature (Td) of all the cellulose derivatives is lower than that of original MCC. This decrease of Td may arise from the decreasing crystalline degree of MCC in all the cellulose derivatives. 32 Therefore, TGA analysis agrees well with calculated GY of St and proves that functionalization has no effect on the thermal stability original MCC but lowers its crystalline degree.

The morphology of regenerated cellulose and cellulose-g-PS with different GY (68.1% and 97.3%) were observed by SEM and the typical images are shown in Fig. 5. It is clear that the regenerated cellulose (Fig. 5a) shows smooth surface while cellulose-g-PS (Fig. 5b and5c) appears as rough nano-scaled particles. This morphological difference indicates the existence of homopolymer of St in regenerated cellulose and complete elimination in cellulose-g-PS after washing. As the GY increases, the roughness increases arising from the enlarged particle size. Thus, the observed morphology of cellulose-g-PS by SEM is similar to that of the topography recorded by AFM and the increased particle size roughens the surface of cellulose-g-PS.

Adsorption of Cr (VI)

Adsorption kinetics

The adsorption kinetics of Cr (VI) by CPN adsorbent was studied and plotted in Fig. 6. Obviously, CPN adsorbent rapidly adsorbs Cr (VI) within a contact time of 20 min and reaches adsorption equilibrium after 30 min. The adsorption kinetics can be analyzed by pseudo-first-order and pseudo-second-order kinetic models and the corresponding equations are as follows: 33 Pseudo-first-order:

(

Pseudo-second-order:

Where qe and qt are the concentrations of Cr (VI) reaching adsorption equilibrium and at contact time t (min); k1 and k2 are the corresponding kinetic rate constants. The fitting results according to the two different models are summarized in Table 1. According to the correlation coefficient (R 2 ), adsorption of Cr (VI) by CPN adsorbent obeys the pseudo-second-order kinetics (R 2 =0.999) which suggests that chemical interaction is involved in the adsorption of Cr (VI) by CPN adsorbent. 34 Table I

Effect of pH

As pH influences both surface charge of adsorbent and existing form of Cr (VI), it is necessary to study the Cr (VI) uptake at different pH values and the result is shown in Fig. 7.

It is clear that CPN absorbent can work under a wide pH range from 2 to 10. As the pH increases from 2.0 to 4.0, the Cr (VI) uptake increases from 22.5 to 31.2. It is well known that Cr (VI) can be stabilized as Cr2O7 2-, HCrO4 -, H2Cr2O7 and CrO4 2-and their relative abundance depends on the pH and Cr (VI) concentration. When pH is lower than 1.0, H2CrO4 is dominant while HCrO4 -predominates in the pH range of 1.0 to 6.0. 3 When the pH is below 7.0, the functional amino groups of CPN adsorbent will accept H + forming -NH3 + adsorption centers which adsorb Cr (VI) through electrostatic interaction. However, the Cr (VI) uptake decreases obviously when the pH is over 7.0. Under alkaline condition, Cr (VI) mainly exists as CrO4 2-and positively charged adsorption centers reduce resulting from competition of OH -and deprotonation of the -NH + (C2H5)2 groups leading to low adsorption capacity of CPN adsorbent. Thus, the optimal pH is in the range of 2.0 to 7.0.

ln(

) ln e t e q q q k t -= -2 2 1 t e e t t q k q q = + Figure 7

Isotherm of Cr (VI) adsorption

The Langmuir isotherm model is applied to establish the relationship between Cr (VI) uptake and equilibrium concentration in aqueous solution. The data is collected and analyzed by the Langmuir isotherm model described in equation ( 5): (5) where Ce is the equilibrium concentration of Cr (VI) (mg L -1 ) and qe is the amount of Cr (VI) adsorbed by per gram of adsorbent (mg g -1 ). qm and KL are Langmuir constants related to the adsorption capacity (mg g -1 ) and the adsorption energy (L g -1 ), respectively. qm and KL are calculated according to the slope and the intercept of Ce/qe versus Ce, respectively. The Langmuir isotherm plot for Cr (VI) adsorption using CPN absorbent is plotted in Fig. 8. The linear relation coefficient R is calculated to be 0.99 revealing that the adsorption of Cr (VI) by CPN adsorbent fits well the Langmuir model. From the linear relationship, the isotherm constants qm and KL are determined as 32.5 mg g -1 and 0.285 L g -1 , respectively. It has been reported that maximal uptake of Cr (VI) by different adsorbents ranges from 0.15 to 256 mg g -1 by a variety of modified and unmodified biosorbents. 35,36 Compared with the reported adsorption capacity of Cr (VI) uptake, the CPN absorbent has a moderate adsorption capacity of Cr (VI). In addition, the CPN absorbent also has the adsorption capacity for other heavy metal ions, such as Cu (II) ions. Therefore the CPN absorbent could be expected to have a potential application in adsorption materials, which will be investigated in our further research work.

CONCLUSIONS

A novel amino-functionalized cellulose adsorbent was prepared by grafting of styrene monomer onto MCC substrate in ionic liquid homogeneous system, followed by acetylation, amination and protonation processes. Micro-FTIR and AFM-IR characterizations confirmed successful grafting and copolymerization of styrene onto MCC substrate along with homogenous distribution of functional groups. The GY could be controlled by changing the concentrations of monomer and initiator, reaction time and reaction temperature. Adsorption of Cr (VI) from an aqueous solution by the resulting CPN adsorbent was tested and adsorption equilibrium was reached within 30 min. The adsorption capacity strongly depends on the pH value and the optimal pH is in the range of 2.0 to 7.0. The Cr VI) adsorption follows pseudo-second order mode suggesting that the adsorption is a process of ion exchange. The adsorption isotherm indicated the adsorption behavior of CPN was fitted with the Langmuir model with a maximum Cr (VI) uptake of 32.5 mg g -1 . Therefore, this work provides a new cellulose-based adsorbent for Cr (VI) removal from wastewater.

Figure 1

 1 Figure 1

Figure 2

 2 Figure 2

Figure 4

 4 Figure 4

Figure 5

 5 Figure 5

Figure 6

 6 Figure 6

Figure 8

 8 Figure 8

Scheme 1

 1 Figure Captions:Scheme 1 Synthesis routes of CPN absorbent in [Bmim]Cl ionic liquid Figure 1 Influences of concentration of St monomer (a), chemical initiator (NH4)2S2O8 (b), reaction time (c) and reaction temperature (d) on the GY.

Figure 2 FT

 2 Figure2FT-IR spectra of the original MCC (a), regenerated cellulose (b), Cellulose-g-PS(GY=68.1%), (c), CPC (d), CPN (e). Figure3Topographic image of CPN adsorbent (a) and chemical maps of CPN adsorbent obtained with wavenumbers of 1160 cm -1 (b), 1490 cm -1 (c) and 1735 cm -1 (d). Figure4TGA curves of the original MCC (a), regenerated cellulose (b), cellulose-g-PS (GY= 68.1%), (c), CPC (d), CPN (e). Figure5SEM images of regenerated cellulose (a); cellulose-g-PS with GY of 68.1% (b) and 97.3% (c). Figure6Adsorption kinetics of Cr (VI) by CPN adsorbent in 50mL solution (Cr (VI): 35mg L -1 ). Figure7Effect of pH on Cr (VI) uptake. (Cr (VI) concentration: 35mg L -1 ; absorbent feed: 0.7g L -1 ; stirring time: 40min; V: 50mL) Figure8Langmuir isotherm plot of Cr (VI) adsorption by CPN absorbent. Table I Kinetic parameters of model fitting for adsorption of Cr (VI) by CPN absorbent.

Figure 3

 3 Figure2FT-IR spectra of the original MCC (a), regenerated cellulose (b), Cellulose-g-PS(GY=68.1%), (c), CPC (d), CPN (e). Figure3Topographic image of CPN adsorbent (a) and chemical maps of CPN adsorbent obtained with wavenumbers of 1160 cm -1 (b), 1490 cm -1 (c) and 1735 cm -1 (d). Figure4TGA curves of the original MCC (a), regenerated cellulose (b), cellulose-g-PS (GY= 68.1%), (c), CPC (d), CPN (e). Figure5SEM images of regenerated cellulose (a); cellulose-g-PS with GY of 68.1% (b) and 97.3% (c). Figure6Adsorption kinetics of Cr (VI) by CPN adsorbent in 50mL solution (Cr (VI): 35mg L -1 ). Figure7Effect of pH on Cr (VI) uptake. (Cr (VI) concentration: 35mg L -1 ; absorbent feed: 0.7g L -1 ; stirring time: 40min; V: 50mL) Figure8Langmuir isotherm plot of Cr (VI) adsorption by CPN absorbent. Table I Kinetic parameters of model fitting for adsorption of Cr (VI) by CPN absorbent.

Figure 5

 5 Figure2FT-IR spectra of the original MCC (a), regenerated cellulose (b), Cellulose-g-PS(GY=68.1%), (c), CPC (d), CPN (e). Figure3Topographic image of CPN adsorbent (a) and chemical maps of CPN adsorbent obtained with wavenumbers of 1160 cm -1 (b), 1490 cm -1 (c) and 1735 cm -1 (d). Figure4TGA curves of the original MCC (a), regenerated cellulose (b), cellulose-g-PS (GY= 68.1%), (c), CPC (d), CPN (e). Figure5SEM images of regenerated cellulose (a); cellulose-g-PS with GY of 68.1% (b) and 97.3% (c). Figure6Adsorption kinetics of Cr (VI) by CPN adsorbent in 50mL solution (Cr (VI): 35mg L -1 ). Figure7Effect of pH on Cr (VI) uptake. (Cr (VI) concentration: 35mg L -1 ; absorbent feed: 0.7g L -1 ; stirring time: 40min; V: 50mL) Figure8Langmuir isotherm plot of Cr (VI) adsorption by CPN absorbent. Table I Kinetic parameters of model fitting for adsorption of Cr (VI) by CPN absorbent.

Figure 6

 6 Figure2FT-IR spectra of the original MCC (a), regenerated cellulose (b), Cellulose-g-PS(GY=68.1%), (c), CPC (d), CPN (e). Figure3Topographic image of CPN adsorbent (a) and chemical maps of CPN adsorbent obtained with wavenumbers of 1160 cm -1 (b), 1490 cm -1 (c) and 1735 cm -1 (d). Figure4TGA curves of the original MCC (a), regenerated cellulose (b), cellulose-g-PS (GY= 68.1%), (c), CPC (d), CPN (e). Figure5SEM images of regenerated cellulose (a); cellulose-g-PS with GY of 68.1% (b) and 97.3% (c). Figure6Adsorption kinetics of Cr (VI) by CPN adsorbent in 50mL solution (Cr (VI): 35mg L -1 ). Figure7Effect of pH on Cr (VI) uptake. (Cr (VI) concentration: 35mg L -1 ; absorbent feed: 0.7g L -1 ; stirring time: 40min; V: 50mL) Figure8Langmuir isotherm plot of Cr (VI) adsorption by CPN absorbent. Table I Kinetic parameters of model fitting for adsorption of Cr (VI) by CPN absorbent.

Figure 7 Figure 8

 78 Figure2FT-IR spectra of the original MCC (a), regenerated cellulose (b), Cellulose-g-PS(GY=68.1%), (c), CPC (d), CPN (e). Figure3Topographic image of CPN adsorbent (a) and chemical maps of CPN adsorbent obtained with wavenumbers of 1160 cm -1 (b), 1490 cm -1 (c) and 1735 cm -1 (d). Figure4TGA curves of the original MCC (a), regenerated cellulose (b), cellulose-g-PS (GY= 68.1%), (c), CPC (d), CPN (e). Figure5SEM images of regenerated cellulose (a); cellulose-g-PS with GY of 68.1% (b) and 97.3% (c). Figure6Adsorption kinetics of Cr (VI) by CPN adsorbent in 50mL solution (Cr (VI): 35mg L -1 ). Figure7Effect of pH on Cr (VI) uptake. (Cr (VI) concentration: 35mg L -1 ; absorbent feed: 0.7g L -1 ; stirring time: 40min; V: 50mL) Figure8Langmuir isotherm plot of Cr (VI) adsorption by CPN absorbent. Table I Kinetic parameters of model fitting for adsorption of Cr (VI) by CPN absorbent.

  Table I Kinetic parameters of model fitting for adsorption of Cr (VI) by CPN absorbent.

ACKNOWLEDGEMENTS

This work was supported by the National Natural Science Foundation of China (NNSFC, Project No. 21463016).