Estelle Doineau
email: doineau.estelle@orange.fr

Guillaume Bauer

Léo Ensenlaz

Bruno Novales

Cécile Sillard

Jean-Charles Bénézet

Julien Bras

Bernard Cathala

Nicolas Le Moigne
email: nicolas.le-moigne@mines-ales.fr

Adsorption of xyloglucan and cellulose nanocrystals on natural fibres for the creation of hierarchically structured fibres

Keywords:

Green treatment of natural fibres is a major issue in paper, textile and biocomposites industries to design in-novative and eco-friendly products. In this work, hierarchical structuring of flax woven fabrics by the adsorption of xyloglucan (XG) and cellulose nanocrystals (CNC) is studied. Indeed, CNC have high mechanical properties, high specific surface area and great potential for functionalization. The adsorption of XG and CNC has been investigated in terms of localization by confocal and scanning electron microscopy (SEM) and quantification through adsorption isotherms. Adhesion force measurements have also been performed by Atomic Force Microscopy (AFM). XG and CNC are homogeneously adsorbed on flax fabric and adsorption isotherms reach plateau values around 20 mg /g fibres for both. The pre-adsorption of XG on flax fabric influences the amount of adsorbed CNC in the high concentrations and also creates entanglements and strong interactions between XG and CNC with the formation of an extensible network.

Introduction

Surface treatments of natural fibres (chemical, physical, enzymatic...) are commonly used in a wide range of industrial sectors like textile, paper or biocomposites to bring them specific properties according to the targeted applications [START_REF] Kalia | Surface modification of plant fibers using environment friendly methods for their application in polymer composites, textile industry and antimicrobial activities : A review[END_REF]. For instance, in textile applications, surface treatments primarily concern the dyeing and the finishing while in paper industry, surface treatments target functional properties such as printability, bleaching, hydrophobicity, gas barrier, softness, strong mechanical properties. In the more recently developed field of biocomposites, most of the natural fibres surface treatments are developed to improve wettability and compatibility with polymeric matrices. In all these applications, environmental concerns prompt the search and the development of environmental friendly surface treatments procedures in order to decrease the use of polluting chemical products [START_REF] Araújo | Application of enzymes for textile fibres processing[END_REF][START_REF] Faruk | Biocomposites reinforced with natural fibers : 2000-2010[END_REF][START_REF] Haider | Minireview : Selective production of hydrogen peroxide as a clean oxidant over structurally tailored carbon nitride photocatalysts[END_REF][START_REF] Le Moigne | Surfaces and interfaces in natural fibre reinforced composites[END_REF][START_REF] Rastogi | Bio-based coatings for paper applications[END_REF][START_REF] Sun | Surface modification of natural fibers using plasma treatment[END_REF][START_REF] Wei | 14-Emerging approaches to the surface modification of textiles[END_REF].

In the search of innovative and greener surface modifications of natural fibres, functionalization with nanoparticules or biopolymers knows a growing interest [START_REF] Hajlane | Chemical modification of regenerated cellulose fibres by cellulose nano-crystals : Towards hierarchical structure for structural composites reinforcement[END_REF][START_REF] Dai | Green modification of natural fibres with nanocellulose[END_REF][START_REF] Hajlane | Cellulose nanocrystal deposition onto regenerated cellulose fibres : Effect on moisture absorption and fibre-matrix adhesion[END_REF][START_REF] Idumah | Infuence of chemical modification of kenaf fiber on xGNP-PP nano-biocomposites[END_REF][START_REF] Lee | Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement[END_REF][START_REF] Malik | Organically modified nanoclay and aluminium hydroxide incorporated bionanocomposites towards enhancement of physico-mechanical and thermal properties of lignocellulosic structural reinforcement[END_REF][START_REF] Oksanen | Improving wet web runnability and paper quality by an uncharged polysaccharide[END_REF][START_REF] Zhou | The influence of surface chemical composition on the adsorption of xyloglucan to chemical and mechanical pulps[END_REF][START_REF] Zhuang | Multifunctional multi-walled carbon nanotube-jute fibres and composites[END_REF]. With this long term goal, this work proposes to combine adsorption capacity of xyloglucan (XG) on cellulosic substrates with the outstanding properties of cellulose nanocrystals (CNC) to develop hierarchically structured assemblies at the surface of flax woven fabrics. XG is an hemicellulose found in the primary cell wall of plants, which is involved in the wall extensibility and its integrity, through its strong affinity with cellulose microfibrils [START_REF] Fry | The structure and functions of xyloglucan[END_REF][START_REF] Fry | An unambiguous nomenclature for xyloglucan-derived oligosaccharides[END_REF][START_REF] Hanus | The xyloglucan-cellulose assembly at the atomic scale[END_REF][START_REF] Hayashi | Characterization of the adsorption of xyloglucan to cellulose[END_REF]Yong Bum Park & Cosgrove, 2015;[START_REF] Pauly | Molecular domains of the cellulose/xyloglucan network in the cell walls of higher plants[END_REF]. On the other hand, CNC are used in many applications thanks to their amazing combination of XG and CNC adsorption may yield different to final organization to achieve hierarchical materials combining microscale fibres displaying nanostructured surfaces. Moreover, this study has been achieved on substrates of industrial interest, i.e. flax woven fabrics that present additional levels of complexity when compared to model substrates or unitary natural fibres.

The adsorption of XG and CNC on flax woven fabrics is investigated in terms of localization by confocal and scanning electron microscopy. In order to quantify the adsorbed amount of XG and CNC on flax woven fabrics, adsorption isotherms are established by an UV spectroscopy method. Different adsorption sequences on flax woven fabrics are evaluated: (i) adsorption of XG or CNC (ii) successive adsorption of XG then CNC, (iii) successive adsorption of CNC then XG. Finally, Atomic Force Microscopy (AFM) with XG and CNC modified tips was used to analyze the adhesion between XG, CNC and flax fibres.

Materials and methods

Materials

A twill 2 × 2 flax woven fabric 200 g/m² was provided by SF Composites (France). Cellulose nanocrystals (CNC) were produced by acid hydrolysis of wood pulp and purchased in spray-dried powder from CelluForce (Quebec, Canada). CNC surface charge density : 0.023 mmol/g; Crystalline fraction : 0.88; CNC lateral dimensions : 2-5 nm; CNC length : 50-110 nm. Xyloglucan Glyloid 6 C was obtained from tamarind seed gum and purchased from DSP Gokyo Food & Chemical. M w = 840 000 g/mol; M w /M n = 1.24, R g 72 nm; monosaccharide composition : Glucose 50.7 %; Xylose 31.7 %; Galactose 16.0 %; Arabinose 1.6 %. Fluorescent agents RBITC (Rhodamine B isothiocyanate) and FITC (Fluoresceine isothiocyanate) were purchased from Sigma-Aldrich.

Labelling of XG and CNC with RBITC and FITC

The fluorescent labelling of XG and CNC were performed according to the method reported by Belder A & Granath K (de [START_REF] De Belder | Preparation and properties of fluorescein-labelled dextrans[END_REF]. The reaction occurring is the formation of covalent bonds between hydroxyl groups of polysaccharides and isothiocyanate groups of the fluorescent agents [START_REF] Iwakura | The kinetics of the reaction of organic isothiocyanates with 1-octanol in o-Dichlorobenzene[END_REF]. First, 1 g of CNC or XG was solubilized in 40 mL of DMSO under stirring and heated at 65 °C. After 15 min, few drops of pyridine, 50 μL of dibutyltin dilaurate and 7.5 mg of RBITC (Rhodamine B Isothiocyanate) or FITC (Fluoresceine isothiocyanate) were added. The reaction was heated under stirring at 65 °C during 2 h. XG-RBITC and CNC-RBITC (or CNC-FITC) were recovered by precipitation in ethanol/acetone mixture for XG and NaCl 1 M solution for CNC followed by centrifugation (10 min; 10,000 g). Three dispersion/centrifugation cycles were achieved and final precipitates were dissolved / dispersed in deionized water at average concentration of 1 wt% and dialyzed (12-14000 MWCO) during one week against ultrapure water.

Adsorption experiments on flax woven fabrics

The different steps of adsorption experiments are illustrated in Fig. 1 and detailed below.

Simple adsorptions

Small pieces of flax fabrics between 8-12 mg were cut and put in test tubes. To avoid the quenching of RBITC or FITC, all samples need to be in the dark (tubes were wrapped in aluminium film). XG-RBITC or CNC-RBITC solutions of 0.5 g/L were diluted with water in test tubes (final volume 5 mL) to obtain final concentrations ranging between 0.05 g/L and 0.45 g/L. Reference samples were prepared in the same way without flax woven fabrics. Test tubes were gently shaken during 16 h. Other samples were prepared with CNC-FITC in the same properties [START_REF] Habibi | Cellulose nanocrystals : Chemistry, selfassembly, and applications[END_REF]. They have outstanding mechanical properties, i.e high Young's Modulus estimated between 100 and 150 GPa, a high specific surface area [START_REF] Dufresne | Nanocellulose : From nature to high performance tailored materials[END_REF][START_REF] Habibi | TEMPO-mediated surface oxidation of cellulose whiskers[END_REF][START_REF] Nagalakshmaiah | Ionic compatibilization of cellulose nanocrystals with quaternary ammonium salt and their melt extrusion with polypropylene[END_REF][START_REF] Rusli | Determination of the stiffness of cellulose nanowhiskers and the fiber-matrix interface in a nanocomposite using Raman spectroscopy[END_REF][START_REF] Yue | Surface-modified cellulose nanocrystals for biobased epoxy nanocomposites[END_REF] and can be chemically functionalized through substitution of their hydroxyl groups. Xyloglucan interactions with cellulosic surfaces have been intensively studied in vitro and in vivo [START_REF] Hayashi | Characterization of the adsorption of xyloglucan to cellulose[END_REF][START_REF] Dick-Pérez | Structure and interactions of plant cell-wall polysaccharides by two-and three-dimensional magic-angle-Spinning solid-state NMR[END_REF][START_REF] Hayashi | Pea xyloglucan and cellulose V: Xyloglucan-cellulose interactions in vitro and in vivo[END_REF][START_REF] Morris | The self-assembly of plant cell wall components by single-molecule force spectroscopy and Monte Carlo modelling[END_REF]Young B. Park & Cosgrove, 2012;[START_REF] Vincken | Fractionation of xyloglucan fragments and their interaction with cellulose[END_REF], and are known to be an entropy driven process [START_REF] Benselfelt | Adsorption of xyloglucan onto cellulose surfaces of different morphologies: An entropy-driven process[END_REF]. From the physico-chemical point of view, glucan chains will interact each other through Hydrogen bonds, van der Waals and polar bonds with some reorganizations of the conformation of XG depending on the polymer concentration [START_REF] Villares | Kinetic aspects of the adsorption of xyloglucan onto cellulose nanocrystals[END_REF]. The final architecture of the cellulose/xyloglucan assemblies is related to concentrations of the building blocks, the molar mass and other adsorption processing parameters [START_REF] Benselfelt | Adsorption of xyloglucan onto cellulose surfaces of different morphologies: An entropy-driven process[END_REF][START_REF] Dammak | Exploring architecture of xyloglucan cellulose nanocrystal complexes through enzyme susceptibility at different adsorption regimes[END_REF][START_REF] Hanus | The xyloglucan-cellulose assembly at the atomic scale[END_REF][START_REF] Hayashi | Pea xyloglucan and cellulose : I. Macromolecular organization[END_REF][START_REF] Hayashi | Pea xyloglucan and cellulose V: Xyloglucan-cellulose interactions in vitro and in vivo[END_REF][START_REF] Lima | Xyloglucan-cellulose interaction depends on the sidechains and molecular weight of xyloglucan[END_REF][START_REF] Lopez | Enthalpic studies of xyloglucan-cellulose interactions[END_REF][START_REF] Stiernstedt | Friction between cellulose surfaces and effect of xyloglucan adsorption[END_REF][START_REF] Stiernstedt | Friction and forces between cellulose model surfaces : A comparison[END_REF][START_REF] Vincken | Fractionation of xyloglucan fragments and their interaction with cellulose[END_REF]. In particular, the strong affinity of XG for cellulose surfaces has attracted researchers for the elaboration of cellulose-based materials as natural cross-linker. For instance, XG and CNC have been already used for the implementation of thin films displaying structural colors or for the reinforcement of nanocellulose aerogels (C. [START_REF] Cerclier | Elaboration of spincoated cellulose-xyloglucan multilayered thin films[END_REF], 2011;[START_REF] Cerclier | Xyloglucan-cellulose nanocrystal multilayered films : Effect of film architecture on enzymatic hydrolysis[END_REF][START_REF] Dammak | Influence of cellulose nanocrystals concentration and ionic strength on the elaboration of cellulose nanocrystals-xyloglucan multilayered thin films[END_REF][START_REF] Jean | Non-electrostatic building of biomimetic cellulose-Xyloglucan multilayers[END_REF]Sehaqui, Zhou, & Berglund, 2011;[START_REF] Sehaqui | Strong and tough cellulose nanopaper with high specific surface area and porosity[END_REF].

Ligno-cellulosic fibres properties have also been improved by the adsorption of either XG or nanocellulose. For example, addition of XG by spraying on a bleached birch kraft pulp increases its wet web strength, tension holding, drying tension and also its smoothness [START_REF] Oksanen | Improving wet web runnability and paper quality by an uncharged polysaccharide[END_REF]. Similarly, when XG is used as a wet end additives in paper making process, paper formation is facilitated and final paper strength is increased [START_REF] Christiernin | The effects of xyloglucan on the properties of paper made from bleached kraft pulp[END_REF][START_REF] Yan | Some ways to decrease fibre suspension flocculation and improve sheet formation[END_REF]. In fact, fibre bonds in pulps treated with XG seem to give high adhesion levels. To go further in this way, friction measurements have been performed by AFM between cellulose and XG (Stiernstedt, Brumer et al., 2006;[START_REF] Stiernstedt | Friction and forces between cellulose model surfaces : A comparison[END_REF]. These studies show the irreversibility side of the adsorption of XG to cellulose. Moreover, the adhesion between the cellulose surfaces in water, which is often very small, is increased with the presence of XG on the surfaces. It appears that specific bonds are formed between XG and cellulose surfaces, creating a strong bridging between cellulose fibres. Nanocellulose, and more especially bacterial cellulose, has also demonstrated its capacity to increase the strength of the interfacial adhesion between natural fibres and polymeric matrices as reported by Bismarck's group (Fortea-Verdejo, [START_REF] Fortea-Verdejo | Upgrading flax nonwovens : Nanocellulose as binder to produce rigid and robust flax fibre preforms[END_REF][START_REF] Juntaro | Nanocellulose enhanced interfaces in truly green unidirectional fibre reinforced composites[END_REF][START_REF] Lee | Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement[END_REF][START_REF] Lee | Hierarchical composites reinforced with robust short sisal fibre preforms utilising bacterial cellulose as binder[END_REF][START_REF] Pommet | Surface modification of natural fibers using Bacteria : Depositing bacterial cellulose onto natural fibers to create hierarchical Fiber reinforced nanocomposites[END_REF]. For example, the creation of "dense" or "hairy" coated sisal fibres via the dipping of sisal fibres in a slurry of bacterial cellulose (BC) has been proposed to reinforce PLA based biocomposites [START_REF] Lee | Short sisal fibre reinforced bacterial cellulose polylactide nanocomposites using hairy sisal fibres as reinforcement[END_REF]. The results of this study were promising with an increase of the specific surface area up to 800 % compared to neat sisal fibres. In terms of mechanical properties, the storage modulus of the PLA-BC coated sisal fibresbiocomposite was increased by around 20 % in comparison with PLA-neat sisal fibres. However, up to our knowledge there is no report of combined adsorption of XG and nanocellulose on (ligno-)cellulosic substrates. In this work, the challenge addressed is to characterize the adsorption of XG and CNC on flax fibres and the potential synergistic effect of the combination of the two biobased building blocks. Indeed, similarly to the plant wall architecture, conditions for confocal microscopic observations. At least triplicate was performed.

Successive adsorptions

For successive adsorptions of XG / CNC-RBITC and CNC / XG-RBITC on flax woven fabrics, similar protocol as for labeled-CNC or -XG adsorption was used preceded by adsorption of XG or CNC. A solution of non-labeled XG or CNC at 0.4 g/L was added in test tubes. After 5 h, the flax woven fabrics were washed with deionized water. Labeled CNC or XG were added at concentrations ranging between 0.05 g/L and 0.45 g/ L, during 16 h. Other samples were prepared with both XG-RBITC and CNC-FITC in the same conditions for confocal microscopic observations. At least triplicate was performed.

After adsorption experiments, the supernatant was collected and the absorbance of Rhodamine at 558 nm for XG-RBITC or CNC-RBITC was measured by UV spectroscopy. The amount of adsorbed XG-RBITC and CNC-RBITC on the fibres was calculated by the difference between measured absorbance in test tubes with flax woven and the absorbance of reference tubes (without flax woven).

Scanning electron microscopy

Elementary flax fibres extracted from the raw and treated flax woven fabrics were observed with an Environmental Scanning Electron Microscope (ESEM,FEI Quanta 200 FEG,Netherlands). The samples were pasted on an adhesive wafer and metallized in high vacuum by depositing a thin layer of carbon on their surface using a BALZERS CED 030. SEM micrographs were obtained under high vacuum at an acceleration voltage of 3 keV, a working distance of 9 mm and magnification up to 50 000 × . At least 10 micrographs per sample were performed and the most representative pictures were conserved for the discussion.

Confocal microscopy

The confocal microscopy images were obtained using an inverted Nikon A1 laser scanning confocal microscope. Several pieces of yarns (raw or treated) were deposited between slide and slip cover with one spacer and deionized water, avoiding the prolonged light contact. The samples were viewed with a Plan Fluor 4× or with a Plan Apo 20× Nikon objective by scanning using a laser beam at a wavelength of 488 nm and 561 nm, for imaging FITC and RBITC fluorescences respectively. The emissions of fluorescence were recorded via a photomultiplier through 500-530 nm and 570-620 nm band-pass filters, respectively. Images were processed using the NIS-Element software (Nikon). At least triplicate was performed.

Atomic Force Microscopy (AFM)

AFM force curves (Bruker Icon dimension) between the tip and the substrate have been performed in the air to study the adhesion between xyloglucan (XG), cellulose nanocrystals (CNC) and flax fibres. Shortly, during the adhesive force measurements, the cantilever tip approached the surface of the substrate at a speed of 1.4 μm/s until contact, and then separated. Before all these measurements, calibrations have to be performed on the cantilever Sharp Nitride Lever (SNL-10, Bruker) with a tip radius below 10 nm. First, the stiffness of the cantilever was determined by the thermal tune method and gave a spring constant of 0.098 N/m. Secondly, the sensitivity of the cantilever has to be measured. To achieve this calibration, force curves have been performed on a sapphire in contact mode. By measuring the yield of the curve, made possible by a functionality of the AFM device, the sensitivity of the cantilever can be determined. Then, the different force curves between cantilever tips and substrates were measured [START_REF] Cappella | Force-distance curves by atomic force microscopy[END_REF]; [START_REF] Ralston | Atomic force microscopy and direct surface force measurements (IUPAC Technical Report)[END_REF]. The raw data were converted by a calculation algorithm detailed in Fig. 2. The method is based on the conversion of the raw data graph (deflection of the cantilever vs piezo position) into a force-distance curve representing the force vs the tip-substrate distance (D). The baseline was done and the zero separation was obtained from the gradient of the constant compliance region. Different parameters are required like the cantilever rest position (Z), the deflection of the cantilever (δ c ), the deformation of the surface of the substrate (δ s ) and the constant spring of the cantilever (k c ).

In order to perform the experiments, two different tips functionalization have been done. First, one consisted on the dipping of the SNL-10 cantilever tip in a drop of XG solution at 20 g/L during 1 min and then a rinsing in water during 1 min before a drying at 60 °C during 30 min. This functionalized tip is named XG-tip within this article. The second one consisted on a tip's functionalization with CNC. The precoating of the tip by polycations poly(ethylene imine) (PEI, Mw = 750 000, Sigma Aldrich) was made to decrease the time of dipping and obtain a uniform coating of CNC [START_REF] Kolasinska | The effect of nature of polyions and treatment after deposition on wetting characteristics of polyelectrolyte multilayers[END_REF][START_REF] Podsiadlo | Layerby-Layer assembled films of cellulose nanowires with antireflective properties[END_REF]. The different steps of the dipping are described in the study of [START_REF] Jean | Structural details of cellulose nanocrystals/polyelectrolytes multilayers probed by neutron reflectivity and AFM[END_REF]). This functionalized tip is named CNC-tip within this article. Finally, a mica substrate was also spin coated (POLO SPIN150i, SPS-Europe) by PEI and CNC following the protocol of [START_REF] Aulin | Nanoscale cellulose films with different crystallinities and mesostructures-Their surface properties and interaction with water[END_REF], and is named CNC-surface throughout the article. Adhesion measurements were taken in contact mode at three localizations, 35 on each, i.e. 105 measurements in total for each configuration. Outliers were identified and rejected considering the interquartile range, i.e. values inferior to Q1 -1.5 x (Q3-Q1) or superior to Q3 + 1.5 x (Q3-Q1). No obvious irreversible deformation or damaging of the substrates or the tip was detected during the measurements.

Results and discussions

Adsorption behavior of XG and CNC on flax woven fabrics

In order to assess XG and CNC adsorption behaviors qualitatively and quantitatively, CNC and XG were labeled with fluorescent dies. In a first approach, confocal microscopy was used to study the localization and homogeneity of XG and CNC adsorption on flax fibres. Using the UV absorbance of the dies, the amount of XG and CNC adsorbed on flax fabrics were then used to determine adsorption isotherms.

Microscopic observations

Confocal microscopy images on Fig. 3 present two different views of XG-RBITC (Fig. 3a) and CNC-FITC (Fig. 3b) adsorbed on flax woven fabrics, at the scale of a yarn and an elementary fibre. Both adsorption of XG-RBITC and CNC-FITC is homogeneous with no aggregates on the surface of yarns and elementary fibres. Some microfibrils functionalized by XG-RBITC or CNC-FITC were also observed at the surface of elementary fibres. These microfibrils probably originate from the peeling of fibre cell walls and/or the presence of impurities and residues at the surface of flax fibres generated during the manufacturing process of flax woven fabrics.

Some elementary flax fibres were extracted from the yarns of treated flax woven fabrics to compare qualitatively the homogeneity of XG-RBITC (Fig. 3c) and CNC-FTIC (Fig. 3d) adsorption across flax yarns diameter. The elementary fibres collected at the outer part of flax yarns show higher intensities for both CNC-FTIC and XG-RBITC, compared to fibres extracted within the yarns. These results highlight differences in penetration of CNC-FTIC and XG-RBITC within the complex and multiscale architecture of flax woven fabrics. Indeed the elementary fibres are twisted together to form flax yarns that are then woven to form the fabric. This puts forwards the issues related to the homogeneous treatment of industrial ligno-cellulosic substrates as natural woven fabrics having complex architectures and possibly impurities.

The surface of flax fibres treated with CNC was imaged by SEM. Cellulose nanocrystals at the surface of treated flax fibres are clearly visible in Fig. 4b (see also insert), compared to raw flax fibres (Fig. 4a) with no visible rod-like shaped nanoparticles at their surface. Adsorbed CNC appear homogeneously distributed and randomly oriented at the surface of flax fibres. This clearly demonstrated the creation of hierarchical structuration at the surface of the fibres by the formation of a nanostructured layer. This microscale structuration echoes the organization of the cellulosic elements of the flax fibres. It should be noticed that CNC were not present on all elementary fibre surfaces. This has to be related to the accessibility of fibre surfaces during adsorption treatments. Indeed, the twisted structure of flax yarns and physical contacts between elementary fibres likely limit the exposure of flax surface upon adsorption. As observed by confocal microscopy (Fig. 3), flax fibres from the inside of the yarns are less accessible and hence likely treated to a lower extent.

Adsorption isotherms

Xyloglucan (XG) and cellulose nanocrystals (CNC) adsorption processes are complex phenomena, and the final adsorbed amount and conformation over substrate's surface are related to several parameters such as the molar mass, concentrations, solubility, etc. [START_REF] Ahola | Effect of polymer adsorption on cellulose nanofibril water binding capacity and aggregation[END_REF][START_REF] Villares | Kinetic aspects of the adsorption of xyloglucan onto cellulose nanocrystals[END_REF]. Additionally, flax woven fabrics are industrial materials and as such are far from being model substrates. For instance, flax fibres swelling and destructuration of the fabrics occurring during treatment might influence the adsorption kinetics. Indeed it changes the available surfaces for sorption and their accessibility, as well as the particular microstructure of natural fibres with its porosities and the lumen. Based on preliminary adsorption trials, an adsorption time of 16 h was chosen. These trials also highlighted that further investigations on the adsorption kinetics of XG and CNC on flax fibres would be of interest.

The obtained adsorption isotherms are displayed in Fig. 5, for XG and CNC on flax woven fabrics. Adsorption was determined by comparing the UV absorbance of reference labeled CNC-RBITC or XG-RBITC solution to those incubated with flax woven fabrics. First, the Fig. 5a represents the adsorption isotherm of XG on flax woven fabrics. The insert shows the related confocal images of the yarns at low and high adsorption ratios. The difference in intensity of XG-RBITC on confocal images is nicely correlated with the increase of adsorbed amount of XG on flax fibres depending on initial concentrations of XG in solutions. A plateau is observed at average 15-20 mg/g. In literature, the adsorption of xyloglucan has already been studied on different ligno-cellulosic substrates. For example, paper fibres like bleached softwood kraft pulp (BSKP) [START_REF] Moser | Xyloglucan for estimating the surface area of cellulose fibers[END_REF] or chemical and mechanical pulp [START_REF] Zhou | The influence of surface chemical composition on the adsorption of xyloglucan to chemical and mechanical pulps[END_REF]. For BSKP fibres, the amount of adsorbed XG was between 4 and 12 mg/g pulp depending on the refining time. In the case of thermomechanical pulp (TMP) and chemithermomechanical pulp (CTMP), refined or unrefined pine/spruce and birch pulp, an adsorption of XG from 15 to 30 mg/g pulp was reached. In our study, the adsorbed amount of XG on flax woven fabrics seems to be in the same range as TMP and CTMP pulps. Previous studies also reported the adsorption of xyloglucan on cellulose nanocrystals [START_REF] Dammak | Exploring architecture of xyloglucan cellulose nanocrystal complexes through enzyme susceptibility at different adsorption regimes[END_REF][START_REF] Gu | The impact of cellulose structure on binding interactions with hemicellulose and pectin[END_REF][START_REF] Lopez | Enthalpic studies of xyloglucan-cellulose interactions[END_REF] with a maximal adsorption between 98-333 mg XG /g cellulose . The higher amount of adsorbed XG on cellulose nanocrystals as substrates is due to the much higher specific surface area of CNC that range from to 800 m²/g [START_REF] Brinkmann | Correlating cellulose nanocrystal particle size and surface area[END_REF][START_REF] Habibi | Key advances in the chemical modification of nanocelluloses[END_REF], as compared to that of flax fibres, roughly 1 m²/g [START_REF] Bismarck | Surface characterization of flax, hemp and cellulose fibers; Surface properties and the water uptake behavior[END_REF][START_REF] Le Duigou | Improving the interfacial properties between flax fibres and PLLA by a water fibre treatment and drying cycle[END_REF][START_REF] Legras | Inverse gas chromatography for natural fibre characterisation : Identification of the critical parameters to determine the Brunauer-Emmett-Teller specific surface area[END_REF][START_REF] Müssig | Testing methods for measuring physical and mechanical fibre properties (plant and animal fibres). Industrial applications of natural fibres[END_REF], even if fibre swelling could contribute to improve specific surface area.

Fig. 5b represents the adsorption isotherm of CNC on flax woven fabrics. As for XG adsorption, the difference in intensity of CNC-FITC on confocal images is well correlated with the increase amount of adsorbed CNC on flax fibres depending on initial concentrations of CNC in suspensions. A plateau is reached at average 15-20 mg/g. Plateau values of the amount of adsorbed XG and CNC are thus similar for the studied flax woven fabric. This points out that both XG and CNC can be adsorbed in similar quantities on flax woven fabrics, and suggests that the surface availability of the substrate has more influence on the amount of adsorbed polymers/particles than its nature and physico-chemical characteristics. It should be noticed that the adsorbed amounts of XG and CNC are rather high when considering the low specific surface area of flax woven fabric. It might be suggested that adsorption occurs in multilayered structure and not only in monolayer as usually proposed in adsorption studies based on model substrates. Other parameters than adsorption mechanisms cannot be ruled out to explain our findings. As discussed above, other processes such as the swelling of flax yarns and fibres with an increase of the available surface for adsorption, the complex and heterogeneous surface chemistry of flax fibres as well as a significant fibrillation rate due to flax processing (retting, scutching, weaving etc.) should also be considered.

Successive XG and CNC adsorptions on flax woven fabrics

Two different successive adsorption procedures of XG and CNC have been tested on flax woven fabrics, i.e. adsorption of XG then CNC (XG-CNC) and vice versa (CNC-XG). Both had the aim to observe a possible synergy between XG and CNC during their adsorption on flax woven fabrics.

Microscopic observations

Fig. 6 shows confocal microscopy images of XG-RBITC (Fig. 6a) and CNC-FITC (Fig. 6b) on the same elementary flax fibre.

RBITC and FTIC were successively imaged to investigate their potential co-localization. Globally XG and CNC are homogeneously adsorbed on flax fibres, recovering the entire surface of the fibre on both Fig. 6a andb, even if some difference can be seen between the two images. As for the simple adsorptions of XG or CNC on flax woven fabrics, some microfibrils around the flax fibre are visible and are likely originated from the peeling of fibre cell walls and/or residues produced during the manufacturing process of the industrial flax woven fabric. These residues display RBITC labeling and seem thus to have a preferential adsorption of xyloglucan (Fig. 6a) than cellulose nanocrystals (Fig. 6b). The confocal plan used allows also the imaging of the inner part of the fibre. It has to be noticed that any fluorescence can be detected in the bulk of the fibres indicating that the adsorption of both XG and CNC remain located at the surface of the fibres.

Surface of XG-CNC treated flax fibres were imaged by SEM. As for simple CNC adsorption (Fig. 4), cellulose nanocrystals are clearly visible at the surface of treated flax fibres in Fig. 7b (see also insert). Adsorbed CNC appear homogeneously distributed and randomly oriented at the surface of flax fibres. As mentioned above, CNC were not present on all elementary fibre surfaces which could be explained by differences in fibre surfaces accessibility and chemistry during adsorption treatments.

Adsorption isotherms of successive XG and CNC

Adsorption isotherms of the successive adsorptions CNC-XG and XG-CNC have been plotted in Fig. 8. The goal was to investigate if the pretreatment of flax woven fabric by CNC or XG had an influence on the adsorbed amount of the second compound (Fig. 8a;b).

It seems that the pre-adsorption of CNC on flax fibres, Fig. 8a, has no influence on the adsorption behavior of XG. Indeed, the two adsorption isotherms are very similar with the same plateau around 15-20 mg/g. However, it should be noticed that the plateau seems to be reached at lower initial concentrations of XG in solution with the presence of CNC on flax woven fabric. The pre-adsorption of XG on flax fibres, Fig. 8b, appears to have a slight influence on the amount of adsorbed CNC. In fact, the adsorption isotherm for XG-CNC reaches a plateau at around 25-30 mg/g for against 15-20 mg/g for simple CNC adsorption. These results demonstrate that CNC have a good affinity for flax fibres surface, the pre-adsorption of XG increasing the amount of adsorbed CNC at high initial concentration of CNC in the suspension.

Adhesive force measurements by Atomic Force Microscopy (AFM)

In order to get a deeper understanding of the affinity between xyloglucan (XG), cellulose nanocrystals (CNC) and flax woven fabric, we performed different adhesive force measurements by the atomic force microscopy (AFM). All of these results are detailed in Fig. 8. Different configurations tip / substrate were analyzed:

(a) neat-tip or XG-tip / CNC-surface obtained by deposition of CNC on PEI pre-coated mica substrate by spin-coating. The adhesion between the tip and the substrate is due to the chemical interactions between the two surfaces into contact. The force curves measurements allow the quantification of these interactions. The rupture distance corresponds to the distance tip-substrate needed to completely separate the tip and the substrate. Finally, the work of adhesion is the energy required to separate the tip from the substrate.

All the results in Fig. 9 show high standard deviations, probably due to differences in surface roughness and chemistry between the different surfaces.

First, the configuration (a) with the neat-tip or the XG-tip approaching the CNC-surface was used as a supplementary control of the dipping of the XG-tip. The adhesion force and the work of adhesion between the XG-tip and the CNC-substrate are respectively 7 times and 25 times higher than the measurements with the untreated nitride tip. In this configuration, the rupture distance is unchanged. These results show that the XG-tip has a high adhesion force with the CNC-substrate, which is correlated with the high affinity between XG and CNC with strong hydrogen and van der Waals bonds.

Then force curves measurements were performed with the configuration (b), XG-tip in contact with the raw flax or the flax-CNC and results are detailed in Fig. 9b. We can observe that the adhesive force is the same, around 6 nN for both, with more heterogeneous measurements for the raw flax substrate (between 3 and 15 nN). The work of adhesion is also in the same range with 60-90 nN.nm. However the rupture distance seems to be slightly increased with the presence of CNC on the flax woven fabric with a maximum of 12 nm against 5 nm for the raw flax. This result shows a possible increase of the interactions zone between biopolymers XG and adsorbed CNC with the creation of an extended network. The configuration (c) is the opposite of the configuration (b), with the raw flax or flax-XG as substrates and CNC coated on the AFM tip. The adhesion force and the work of adhesion are higher between the CNC-tip and the raw flax than the flax-XG substrate with respectively 35 nN against 20 nN and 750 nN.nm against 3100 nN.nm. It means that CNC seem to have very good interactions with the flax woven fabrics, without the presence of adsorbed XG on the surface. On the other side, the rupture distance is strongly improved between the CNC-tip and the flax-XG with a median around 23 nm against 5 nm for the raw flax. That means, as observed with the configuration (b), that the interactions between CNC and adsorbed XG create specific bonds and entanglements with an extensible network, illustrated in Fig. 10. Note that adhesion force, rupture distance and work of adhesion obtained with the CNC-tip are much higher than those obtained with the XG-tip, probably because the tip coated by CNC offers more available surface with more available OH groups due to high specific surface area of CNC (more than 100 g/m²) as compared to the XG biopolymer. Despite the similar adsorbed amounts are similar for CNC and XG whatever the adsorption process, the AFM results suggest that a change in the sequence of adsorption and the combination of the two biobased building blocks can be a mean to vary the structuration of the surface organization. Accordingly, future studies will focus on the impact of hierarchically structured surfaces obtained by varying the adsorption sequence on the macroscopic properties of flax fibres based materials.

Fig. 1 .

 1 Fig. 1. Illustration of the different steps of adsorption processes on flax woven fabric with labeled cellulose nanocrystals (CNC) and xyloglucan (XG). For clarity reasons, XG, CNC and flax fibres are not scaled.

Fig. 2 .

 2 Fig. 2. Algorithm employed to convert the raw data graph of the force measurements obtained by AFM to the force-distance curve.

Fig. 3 .

 3 Fig. 3. Confocal microscopy images of (a) XG-RBITC and (b) CNC-FITC adsorption on flax yarns and elementary fibres; and of (c) XG-RBITC and (d) CNC-FITC adsorbed on elementary flax fibres extracted from different positions across the diameter of flax yarns.

Fig. 5 .

 5 Fig. 5. Adsorption isotherms of (a) XG and (b) CNC on flax woven fabric and associated confocal images of XG-RBITC and CNC-FITC at low and high adsorption ratios.

Fig. 6 .

 6 Fig. 6. Confocal microscopy images of successive adsorption of XG-CNC on an elementary flax fibre with (a) XG-RBITC (b) CNC-FITC.

Fig. 8 .

 8 Fig. 8. Adsorption isotherms of (a) XG and (b) CNC for simple and successive adsorptions (XG-CNC and CNC-XG) on flax woven fabrics and associated confocal images of XG-RBITC and CNC-FITC at low and high adsorption ratios.

Fig. 9 .

 9 Fig. 9. Adhesion force, distance and work of adhesion obtained with force measurements by AFM in different tip / substrate configurations (a) neat-tip or XGtip / CNC-surface, (b) XG-tip / raw flax or flax-CNC, (c) CNC-tip / raw flax or flax-XG.

  confocal microscopy and force measurements by Atomic Force Microscopy (AFM) have been used to characterize different configurations of xyloglucan (XG) and cellulose nanocrystals (CNC) adsorptions on an industrial flax woven fabric. This industrial fabric has a complex architecture with twisted yarns composed of several hundred elementary flax fibres. Moreover, some residues likely to be generated by the manufacturing process of the fabric are present at the surface and seem to have affinities mainly with xyloglucan and also cellulose nanocrystals. The confocal images show the homogeneous distribution of XG and CNC on the flax fabric for the simple and successive adsorptions with no aggregates. However, SEM images show non-treated surfaces, which can be explained by the complex architecture of the fabric with porosities, entanglements etc. Moreover, phenomena such as swelling and destructuration occur during the dipping in water suspension and can influence the adsorption. The adsorption isotherms of XG and CNC on the flax woven fabric are very similar for the simple and successive adsorptions with a plateau around 20 mg/g fibres , highlighting the strong influence of available surfaces on CNC or XG adsorption onto flax fibres. The pre-adsorption with XG on flax woven fabrics increases the adsorbed amount of CNC in the high concentrations and seems to change the architecture of the biopolymers network. In fact the AFM adhesive force measurements show first a good affinity between CNC and XG and also higher adhesive force and work of adhesion between CNC and the neat flax woven fabric. However, it seems that the pre-adsorption of XG on the fabric increases the rupture distance with the CNC-tip. The combined XG and CNC, adsorbed with strong interactions on the flax woven fabric, create an extensible network. These results open perspectives in the treatment of natural fibre fabrics and the possibility to change their surface and architecture with the adsorption of biopolymers like xyloglucan or nanoparticles like cellulose nanocrystals. However, we have to keep in mind the complexity of the hierarchical architecture of the industrial flax woven fabric for such treatments in terms of homogeneity, residual components and swelling effect. CRediT authorship contribution statement Estelle Doineau: Methodology, Investigation, Funding acquisition, Writing -original draft, Writing -review & editing. Guillaume Bauer: Formal analysis, Methodology. Léo Ensenlaz: Formal analysis, Methodology. Bruno Novales: Investigation, Methodology. Cécile Sillard: Investigation, Methodology, Supervision, Validation. Jean-Charles Bénézet: Conceptualization, Methodology, Validation, Resources, Writing -review & editing, Supervision, Project administration. Julien Bras: Conceptualization, Methodology, Validation, Resources, Writing -review & editing, Supervision, Project administration. administration, Resources, Supervision, Validation, Visualization, Writing -review & editing. Nicolas Le Moigne: Conceptualization, Methodology, Validation, Resources, Writing -review & editing, Supervision, Project administration.

Fig. 10 .

 10 Fig. 10. Illustration of the adhesive force measurements by AFM with the CNC-tip when comparing substrates: (A) a non-treated flax fibre and (B) a flax fibre treated with XG.

Acknowledgements

Estelle Doineau thanks IMT Mines Alès and Doctoral School GAIA for funding his PhD work.

A part of this work was supported by Glyco@alps (ANR-15-IDEX-02), Idex UGA. LGP2 is part of the LabEx Tec 21 (Investissements d'Avenirgrant agreement n°ANR-11-LABX-0030) and of PolyNat Carnot Institute (Investissements d'Avenirgrant agreement n°ANR-16-CARN-0025-01).

Nadège Leray is acknowledged for her efficient help for CNC and XG labeling and adsorption experiments.


Rahaf Rahal
email: rahaf.rahal@irisa.fr

Firas Abi-Farraj
email: firas.abifarraj@gmail.com

Paolo Robuffo Giordano

Claudio Pacchierotti
email: claudio.pacchierotti@irisa.fr

Haptic Shared-Control Methods for Robotic Cutting

I. INTRODUCTION

Shared control allows to share the available degrees of freedom of a robotic system between the operator and an autonomous controller, to facilitate the task and improve the efficiency of the system [START_REF] Abbink | Haptic shared control: smoothly shifting control authority?[END_REF], [START_REF] Sheridan | Human-robot interaction: Status and challenges[END_REF], e.g., in robotic teleoperation [START_REF] Selvaggio | Haptic-based shared-control methods for a dual-arm system[END_REF]- [START_REF] Abi-Farraj | User evaluation of a haptic-enabled shared-control approach for robotic telemanipulation[END_REF]. Robotic cutting is particularly interesting for shared control, as it is employed in various sensitive applications which range from surgical cutting [START_REF] Patnaik | Surgical incisionstheir anatomical basis part iv-abdomen[END_REF] to nuclear decommissioning [START_REF] Park | Enhanced teleoperation for d and d[END_REF] and disaster response [START_REF] Abi-Farraj | Humanoid teleoperation using task-relevant haptic feedback[END_REF]. Moreover, cutting applications feature a variety of constraints which can have a high impact on the task. For example, to avoid damaging the environment, the cutting tool should neither perform pure lateral motion nor rotate in place. Indeed, unicycle and car-like kinematic models have been used for modeling the cutting task to reflect its nonholonomic nature [START_REF] Bloch | Control and stabilization of nonholonomic dynamic systems[END_REF], [START_REF] Mcclamroch | Knife-edge motion on a surface as a nonholonomic control problem[END_REF].

Several shared-control architectures have been proposed in the literature to tackle different cutting applications [START_REF] Prada | A study on design and analysis of virtual fixtures for cutting in training environments[END_REF]. One example is enforcing nonholonomic constraints on multipurpose robots through control [START_REF] Li | Task frame estimation during model-based teleoperation for satellite servicing[END_REF], [START_REF] Vozar | Preliminary study of virtual nonholonomic constraints for time-delayed teleoperation[END_REF]. In [START_REF] Vozar | Preliminary study of virtual nonholonomic constraints for time-delayed teleoperation[END_REF], Vozar et al. design four shared-control approaches for cutting straight lines into MLI blankets under a time delay. In all modes, the master interface was free to move in any direction, and the constraints were implemented only at the remote side.

This work targets the limitations of the above-described architectures. It presents the design and evaluation of two shared-control approaches for commanding a manipulator in a cutting scenario while enforcing constraints associated with the task itself, e.g., limiting lateral motions, rotations in place, and sharp turns of the tool. The user is provided with information about the enforced nonholonomic constraints (alongside contact forces) via haptic feedback on the master device.

II. METHODS

The robotic system is composed of a master 6-DoF haptic interface and a remote 7-DoF torque-controlled manipulator, equipped with a scalpel. A planar object to cut is placed on a table in front of the robot (see Fig. 1). The scalpel and the end-effector of the master device can move along the three translational directions but their orientation is constrained via control to only rotate around the vertical axis z b .

We present three different control approaches which are shown in Fig. 2. More details on the methods are given in [START_REF] Rahal | Haptic shared-control methods for robotic cutting under nonholonomic constraints[END_REF] and in the video available as supplemental material and at https://youtu.be/DkW4OcjgX9M.

A. Standard haptic teleoperation (condition T)

This mode is a simple teleoperation, with no added constraints related to the cutting task. The manipulator receives torque commands based on impedance control, relating the pose of the remote robot to the pose of the master.

The external forces applied by the environment on the remote robot are fed back to the user through the master interface, such that τ m = τ c + τ nc , where τ c represents the forces applied along the constrained directions, and τ nc the ones applied along the non-constrained directions. In this mode, nonholonomic constraints are not applied, so external forces are reflected on all directions controlled by the operator.

B. Unicycle approach (condition U)

While T guarantees high flexibility, the cutting scenario enables us to introduce additional constraints which can make the teleoperation easier and safer. In particular, any lateral motion of the scalpel during cutting can induce significant damage on the material (and even on the scalpel itself). We thus impose nonholonomic constraints on the robot motion, such that the scalpel can only move along two translational directions: its cutting direction u s , and its vertical direction z b ; however, it cannot translate laterally. The scalpel can always rotate around its axis z s .

To achieve this desired behavior, we also constrain the master device such that the user is allowed to move along x m and z m in translation, as well as to rotate around z m . The motion around y m is, however, blocked. In addition to the forces imposing the constraints on the master interface, the user also receives haptic feedback τ nc from the environment along the directions not constrained by the control.

C. Car-like approach (condition C)

The previous approach prevents the user from moving the scalpel laterally. However, it does not prevent rotation in place or sharp turns. To limit these undesired behaviors, we impose additional constraints executing rotations only if the scalpel moves along u s . Moreover, the user is given control over the of curvature of the trajectory, R d . Similarly to driving a car, the user does not directly control the angular velocity of the remote robot but rather the steering angle.

As in Sec. II-B, we constrain the master interface with a hard spring blocking any lateral motion. A soft spring is applied around z m to fix the orientation of the master device at a particular pivot angle, which ensures that the master and the robot are aligned at all times. As before, the user also receives τ nc along the directions not constrained by the control.

III. EXPERIMENTS AND RESULTS

To evaluate the effectiveness of our shared-control approaches, we carried out a human subject experiment with twelve participants. Participants used the master device to control the robot and cut a target shape in the modeling clay. The three modes T, U and C were considered. For each mode, participants were asked to carve three different shapes: a straight line (L), a bent line (B), and a sinusoidal shape (S).

The different metrics and results are summarized in Fig. 3. A questionnaire also showed that eight subjects found condition U to be the most effective for the cutting task. Three subjects preferred condition T while one preferred C.

Results show that the proposed shared-control approaches C and U outperform standard teleoperation in all metrics but completion time. These results are sustained across the three considered shapes. This proves our hypothesis that shared control is an effective approach to improve currently-available teleoperation systems for cutting tasks, which is in agreement with previous results in the literature. Comparing performance between U and C, we can see that limiting the maximum radius of curvature and preventing rotations in place (C) significantly lowers the lateral forces w.r.t. U, where these constraints were not enforced. Moreover, the error metric shows significant differences among all pairs, ranking C first (lowest error), followed by U and T (highest error). Finally, comparing performance among the target shapes, we can see that, as the shapes become more complex, their performance degrades.

For most metrics, as the shapes become more complex, the difference of T vs. U and C increases. This is quite expected, as users need more help when cutting more complex shapes. Surprisingly, the subjective metrics did not always agree with the above results. In fact, users preferred T and U over C, as many constraints imposed in the C modality created the impression of conditions difficult to use. These results might change in the presence of experienced users.

Fig. 1 .

 1 Fig. 1. Experimental setup and frames used for the shared-control methods that enforced various nonholonomic-inspired constraints.

Fig. 2 .

 2 Fig. 2. Summary of the shared control modes. Black arrows are directions the user is allowed to control, red arrows directions which are blocked, dashed red lines sample trajectories. (T) Teleoperation. The user has control over all planar motions and the vertical movement. (U) Unicycle. Pure lateral motions are blocked. (C) Car-like. In addition to blocking the lateral motion, rotations in place and sharp turns are also avoided. The user controls the radius of curvature of the steering. A spring informs the user about the master position corresponding to a zero radius of curvature.

Fig. 3 .

 3 Fig. 3. Human subjects experiment. Objective metrics. Mean and standard error of the mean of (a) completion time, (b) error in carving the target shape, (c) mean lateral force, and (d) roughness of the cut profile for the three control conditions (T, U, C) and the three target shapes (L, B, S).


Dr Dae Seok Kim

Dr Wonsuk Lee

Dr Teresa Lopez-Leon
email: teresa.lopez-leon@espci.fr

Dong Ki Yoon

Prof. D K Yoon

Self-regulated smectic emulsion with switchable lasing application

Keywords: emulsion, liquid crystal, self-regulation, switchable lasing

We present a structurally reversible smectic liquid crystal (LC) emulsion made of semifluorinated rod-type molecules in silicon oil, which is controlled by simple heating and cooling. Without adding any kind of additives, such as surfactants, polymers or emulsifiers, and without using any special tools, such as microfluidics or gas bubbling, the LC molecules spontaneously form monodisperse spherical and myelin-like structures upon cooling from the isotropic temperature. The LC emulsion can easily trap guest materials, providing a platform for repeatable and reliable switchable emulsification. For example, this interesting system enables to realize an on-off lasing system by confining fluorescent dyes in the LC droplets.

Emulsions, in which droplets of a liquid are dispersed in an immiscible continuous phase, are widely used in commercial products including milk, paint, cosmetic products and medical treatment. A great amount of attention has recently been paid to switchable emulsions, which enable to realize a rapid transition between the emulsion state and the completely separated state. Such switchable emulsions have potential applications in colloidal science and engineering, for clearance of chemicals, drug delivery, [1,2] switchable optical sensing, [3,4] designing on-off opto-electronic devices, [5] biocatalyst, [6] or colloidal shape control. [7] So far, switchable emulsions have been designed using oil-in-water or water-in-oil emulsions which are stabilized by switchable emulsifiers, [8,9] which activate or deactivate upon the action of specific external stimuli such as pH, [10,11] gas bubbling, [12] light, [6] or magnetic field. [13] However, the use of water-in-oil emulsions enforces the switching mechanism to be a twostep process, since the separated liquids need specific mechanical treatments such as sonication or gas bubbling to return to the initial emulsion form, [8,9] restricting the practical applications. Moreover, the switchable emulsifiers need to be specially designed according to the physical and chemical properties of the host/guest liquids and switching triggers, which makes this method complicated, time consuming, and expensive. Finally, although it has been demonstrated that self-emulsification methods can produce emulsions with small droplet size, i.e. submicron, without any mechanical energy input, these processes typically require high concentration of surfactants and are not reversible. [14][15][16] An ideal switchable emulsion should fulfill several requirements. First, the guest liquid should be able to be emulsified in the host medium without the need of a specific emulsifier, since the presence of surfactants would compromise the purity of the involved liquids and potentially decrease the performance of the system in its active form. Second, for high reproducibility and reliability, the system should be capable of reversibly producing droplets of uniform size so that every single droplet in the emulsion performs its particular function in the same way. Third, the system should be able to undergo rapid and infinitely reversible on-off switching without any mechanical treatment. In order to realize this, the host and guest liquids would need to be semi-miscible with a degree of miscibility that could be modulated through physical parameters, such as temperature of the guest material. Since semi-miscibility requires a mild hydrophobic interface between the two constituents, non-aqueous systems such as oil-in-oil emulsions seem to be appropriate candidates. Yet, oil-in-oil emulsions are usually unstable due to the low surface tension at the oil-oil interface, for instance, ~3 mN/m for hydrocarbon and silicon oils. [17] Stabilizing oil-in-oil emulsions without the use a specifically designed stabilizer is still a challenge. [18] In this paper, we propose a method to fabricate highly stable switchable emulsions that fulfill the requirements mentioned above. The method consists in mixing silicon oil with a semifluorinated liquid crystal (LC) that display two different smectic phases (SmA and SmE) when decreasing temperature from the isotropic phase. We show that emulsification and demulsification processes occur at the smectic A and isotropic phases respectively, enabling fast switching between the two states via temperature. The elasticity and chemistry of the LC guarantees the stability of the emulsified state without the use of any surfactant. Interestingly, we observe two different kinds of morphologies in the emulsified state, spherical droplets and myelin-like filaments, resulting from an interesting interplay between kinetics of nucleation and growth, elasticity of the liquid crystal, and surface tension. Although these liquid crystalline organizations have been reported before, [19][20][21] previous studies only focused on the formation and growth mechanisms of the structures, usually under confined geometries, while little effort has been paid to the realization of a switchable system where these structures could form and disappear in a completely reversible way, providing a solid platform for reconfigurable emulsification. In this paper, we show that the growth of filaments from a state where the oil and LC are fully mixed yields the formation of rather monodisperse emulsions, capable to switch between the emulsion and homogenous mixed state by thermal phase transition. Finally, we show the potential of this system in terms of applications, for instance, as a switchable laser.

In order to make the oil-in-oil emulsion, we synthesized a semi-fluorinated LC molecule, Y002, whose structure is shown in Figure 1a. [22,23] The Y002 was used as guest oil, while silicone oil was used as host, continuous phase. To prepare the emulsion sample, 0.1wt% of the powdered Y002 is dispersed in 1ml of silicon oil and vigorously mixed for 2 min. Then, the mixture is placed on a heating stage and heated up to the isotropic temperature, 200℃, and cooled down at a rate of 10 o C/min to room temperature. A drop of this mixture is confined between a glass slide and a cover slip using ~300μm spacers, and observed utilizing polarized optical microscopy (POM) with varying temperature. On cooling from the isotropic to room temperature, the Y002 adopts three different morphologies as shown in Figure 2 and Video S1: (i) a completely mixed state, when Y002 is in the isotropic phase (above 200 o C), see Figure 2a,d,(ii) a state where myelin-like smectic filaments are dispersed in silicon oil, which forms at the onset of the isotropic-smectic phase transition (at 185 o C when cooling from the isotropic phase), see Figure 2b,e and eventually (iii) a state where onion-like smectic droplets are dispersed in silicon oil, which appears all over the temperature range of the smectic phase, see Figure 2c,f. The emulsion state turned out to be extremely stable. This stability comes from two main factors. First, the semi-fluorinated functional group of the Y002 molecule acts as pseudoemulsifier: it places itself at the interface between silicon oil and Y002, lowering its surface tension. [24] Thus, the semi-fluorinated group in Y002 plays the role of both guest oil and surfactant. The second and probably stronger stabilizing factor comes from the change in elastic properties of Y002 as the system is cooled down from the isotropic phase to the smectic A, smectic E and soft crystal phases. Cooling down to room temperature very quickly makes each droplet enter the soft crystal phase, in which molecular translation and rotation are thermodynamically not allowed, and where the droplets exist as they were solid particles dispersed in the oil.

Once the emulsion is formed, it is possible to make the smectic droplets appear and disappear through thermal phase transition. We tested the switchable behavior of our emulsion platform in terms of switching speed, size uniformity of the droplets and reproducibility. The switching between the mixed and emulsified states occurs in a time interval of less than a second. The process is fully reversible and could be, in principle, indefinitely repeated (Figure 3a).

Interestingly, the LC spherical droplets formed in the emulsification process are rather monodisperse, with a diameter of ~13μm (Figure 3a). Without using any specific tool such as microfluidic set-up, homogenizer, or ultrasonicator, our system produces uniformly sized spheres by temperature-induced self-assembly of the LC molecules. This is possible due to a specific mechanism of nucleation and growth, combined with the elastic properties of the smectic phase, as discussed later.

To get some quantitative information about the self-assembly and dispersion of the Y002 molecules during the emulsification and demulsification processes, we incorporated a red dye that is fully miscible with Y002, but poorly miscible with silicon oil. During repeated heating and cooling cycles, the color of the host medium, here silicon oil, becomes reddish at the dispersed state (200 o C) and whitens at the emulsified state (120 o C), indicating that the Y002 molecules and the dye get mixed with the silicon oil when Y002 is the isotropic phase (200 o C). The dispersed phase, i. e. the spheres and filaments, always exhibits an intense red color, meaning that the emulsion can trap red dyes when it is generated from a fully mixed state (Figure 3c and Video S2). To get some quantitative information on the mixing process, the red-color intensity variation in the silicon oil is measured as a function of temperature (Figure 3b). The intensity gradually increases upon heating to the isotropic phase (200 o C), meaning that the LC molecules are progressively dispersed into the silicon oil, eventually reaching a homogeneous state where both Y002 and silicon oil are fully mixed. When cooling down to the smectic phase at a rate of 10 o C/min, the intensity profile associated to the silicon oil gets reversed, but the intensity values become ~8% larger than in the heating experiment, indicating that a small amount of red dye remains in the silicon oil after the phase transition. This color change of the host medium with temperature shows that the miscibility between Y002 and silicon oil is intrinsically poor, except when Y002 is in the isotropic phase (> 200 o C). Such a specific miscibility of the LC and silicon oil depending on the LC phase allows for rapid conversion between the emulsion and mixed states, facilitating the switching behavior of the emulsion.

The molecular structures associated to the different droplet morphologies are sketched in Figure 1b. In the SmA phase, the Y002 molecules are perpendicularly anchored to the interface with silicon oil, which prescribes homeotropic boundary conditions. [25] When the liquid crystal is confined to a spherical volume, these boundary conditions typically result in an onion-like organization of the smectic layers, as represented in Figure 1b (i). This concentric configuration assures the compliance of both boundary conditions and equidistance between smectic layers. The Maltese cross texture appearing when placing the sample between cross polarizers, together with the blue and yellow colors arising when adding a wave retardation plate (λ=530nm), confirms the concentric structure of the spherical droplets, as shown in the inset of Figure 2c and in Figure S1 in the supporting information). In their elongated shape, the Y002 droplets take the shape of spherocylinders, where two spherical caps are connected by a long cylinder. The molecular organization within the cylinder is shown in Figure 1b (iii), where the smectic layers are organized in a coaxial fashion.

Because of the curvature of the smectic layers, both the spherical and cylindrical structures hold topological defects, i. e. singular regions where the smectic order is ill-defined. A concentric structure of spherical smectic layers produces a point defect at the center of the structure, highlighted in red in Figure 1b (i). In contrast, a coaxial structure of cylindrical smectic layers leads to the formation of a defect line or disclination along the symmetry axis of the structure, highlighted in red in Figure 1b (ii). As one approaches the center of the concentric/coaxial structure, curvature gradients and thus elastic energy density dramatically increase. In nematics, the cylindrical structure can release this elastic energy by letting the molecules escape into the third dimension at the core of the cylinder. However, smectics cannot escape, so they release energy by locally melting those regions that localize curvature. [26,[START_REF] Kleman | Soft Matter Physics An Introduction[END_REF] This is confirmed by the POM images, which reveal an extinction (dark) line along the axis of the filament (inset Figure 2b), indicating the existence of an isotropic phase at the core of the cylinder. This core-shell structure becomes even more evident when examining the sample through scanning electron microscopy (SEM), which allows for threedimensional visualization of the structure. In these measurements, the sphere and filament structures were quenched, collected from the silicon oil by filtering, and coated by a 5nm platinum film to enhance image contrast. The resulting images are shown in Figure 2f, for the spherical geometry, and Figure 2e for the cylindrical one. Although the surface of each structure does not show any remarkable texture, the cross section of the filament, obtained by freeze fracture transmittance electron microscopy (FF-TEM), shows two distinct phases: a disordered core of about ~500nm surrounded by a thick shell of concentric smectic layers (inset of Figure 2e).

The formation of both spherical droplets and smectic filaments results from an interesting process where kinetics of growth and elastic energy interplay. The filaments are a nonequilibrium transient forms raised by fast cooling from the isotropic phase. As previously reported, [START_REF] Adamczyk | [END_REF] the formation of filaments may be preceded by the nucleation of two dimensional membranes of Y002 in the silicon oil, which are unstable and roll up into cylinders due to the anisotropic surface tension of the LC sheet. The elongation of the filaments are accompanied of buckling, which is driven by the internal force (load) produced by the growth of the smectic phase. [29] The load decreases as the filaments increase in length, making bucking less important when the filaments become long. Although spherical nucleii are also formed when temperature decreases below the critical one, the growth of smectic filaments is kinetically favored with respect to their spherical counterparts (Video S1). Indeed, isotropic growth is sustained by the local uptake of molecules, while longitudinal growth makes the structure travel through the sample, having fast access to molecules that are far from the initial nucleation spot. As in typical crystallization processes, the presence of a growing structure prevents the local formation of other ones by the uptake of neighboring molecules, this is why the formation of filaments is favored near the isotropic / smectic phase transition temperature (Video S1).

The filaments become unstable and transform into spherical droplets by fast cooling with a rate of 10 o C/min. When decreasing T, the Y002 molecular interactions become stronger with respect to k B T, resulting in the increase of the elastic constants of the liquid crystal. [30,31] In the smectic A phase, the elastic energy per unit volume is mainly controlled by the splay term, which takes into account the mean curvature of the smectic layers: [32] (1

)
where K is the splay elastic constant and H is the mean curvature of the smectic layers (R 1 and R 2 are principal curvature radii). In order to understand the transformation of the filaments into spheres, let consider a long filament and a sphere with the same volume: because of the larger mean curvature of the filament, the associated elastic energy is also larger, making the structure more energetically costly than the spherical one. [33] Although the formation of filaments is kinetically favored, they become energetically unstable as elastic energy comes into play. Reaching the middle of the temperature range for the smectic phase (below ~ 185 o C), all the filaments are transformed into spherical droplets. Looking closely at the contraction dynamics of the filament, it was frequently observed that two spheres start to be formed at both ends of the filament at the initial stages of the process and then, as the two spheres grow, the filament connecting them becomes progressively stretched, until it eventually breaks (see video S3 and figure S2). Although it is extremely hard to measure a critical length or radius for the fracture of the filament, it seems obvious that the splitting of the filament is responsible for the rather uniform size of the spherical droplets obtained with this method. Upon further cooling to room temperature, the spheres adopt a smectic E phase and eventually a soft crystal phase. [START_REF] Yoon | Proc. Natl. Acad. Sci. U S A[END_REF] The LC spheres stop their growing when they become solidified, which limits the uptake of free molecules/aggregates and prevents Ostwald ripening in the solution. As a result, the size of the final LC spheres is rather uniform (Figure 3a).

In order to discuss the stability of the arrangement in concentric layers, we compare elastic energy, which tends to reduce the curvature of the layers, with anchoring energy, which forces the smectic layers to curve so that the Y002 molecules are perpendicular to the interface with the silicon oil. While the former scales as WR 2 , the latter scales as KR, where R is the radius of the droplet and W is the anchoring coefficient. [START_REF] Kleman | Soft Matter Physics An Introduction[END_REF] For large values of the radius, R>K/W, the anchoring energy penalty is significantly larger than the elastic cost of the concentric packing, favoring the concentric structure within the droplet. Here, it is extremely hard to measure the exact value of the surface anchoring energy between Y002 and the silicone oil at the isotropic-smectic phase transition because of the high temperature at which the transition takes place (~190 o C). Yet, we can get an estimation of W by measuring the smallest radius of the concentric smectic layers, R S , ~1μm, appearing when quenching the sample to room temperature from the isotropic-smectic phase transition temperature, where smectic layers starts forming (Figure S3). We assume the characteristic length K/W is approximately equal to R S , thus, W≈K/R S ≈3×10 -5 N/m, considering the average value of the Frank elastic constant K ~ 30 pN. [START_REF] Kleman | Soft Matter Physics An Introduction[END_REF][START_REF] Lee | [END_REF] Thus, for W=3×10 -5 N/m and the elastic constant K=30pN, the concentric layering with circular curvature should be valid as long as R>K/W, that is, practically for all scales exceeding 1μm. Interestingly, the central defects observed in our concentric layered structures have anomaly large cores, ~ 500nm, compared to those observed in conventional thermotropic LCs, ~10nm. [START_REF] Kleman | Soft Matter Physics An Introduction[END_REF] Such large disordered defect cores have been frequently observed in spherical droplets of lyotropic LCs, where the liquid crystalline molecules are dispersed in a solvent. The origin of such large defect cores has been attributed to a larger concentration of solvent, which can even lead to a complete phase separation, at the defect core . [36,37] Since our system also has two different components, Y002 and silicon oil, it seems reasonable to make the assumption that the defect core may also contain silicon oil. To prove this hypothesis, we designed an experiment where a fluorescent dye (Coumarin 153, Sigma-Aldrich), soluble in silicon oil but not in the LC, was used to analyze the composition of the defect cores. The marked sample was quenched to room temperature from the isotropicsmectic phase transition temperature. As shown in the fluorescent confocal microscope images (Figure S4), the droplets contain silicon oil in their cores, as expected, explaining the larger size of these defects in comparison with standard (mono-component) thermotropic liquid crystals.

As the potential applications of our switching emulsion, we investigated the possibility of making a switchable laser. This application takes advantage of two properties of our system: the fast switching between the uniform and emulsified states and its capability of selectively trapping dyes inside the LC droplets. Figure 4a shows sequential fluorescent micrographs of LC droplets doped with Rhodamine 6G when varying temperature from 120 o C to 200 o C, clearly showing thermal phase transition from smectic to isotropic and vice versa. The behavior is analogous to that is shown in Figure 3, although here the dye is more strongly associated with the LC molecules. Clear fluorescence is observed only inside the LC droplets at the smectic phase, while a uniform fluorescent background is observed at the isotropic phase, where the LC and dye molecules are fully dispersed through the entire silicon oil medium.

To test the lasing ability of our doped LC droplets, they were optically pumped with an external light source (5 ns pulsed laser, optical parametric oscillator, 532 nm) through a confocal setup, while a lasing signal was simultaneously collected by a spectrometer. The optical index of the silicon oil (n ~ 1.4) is lower than that of conventional calamitic smectic LC (n o ~ 1.516), [38] thus, we expect the Y002 droplets to support whispering gallery modes (WGMs) to confine light. Since the LC spheres are doped with an organic laser dye, Rhodamine 6G, each individual LC droplet provides gain for lasing activities, and at the same time, serve as a laser microcavity. Figure 4b depicts a representative lasing spectrum for the smectic droplets at 120 o C, when the pump energy density is 3.1 mJ/mm 2 . The lasing emission has multi-mode lasing peaks due to the small spectral range of the WGMs in the ~13 sized LC droplets, as well as coexistence of different TM and TE modes along the equator of the droplet. To further verify whether this is a lasing phenomenon, we measured the spectrally integrated optical emission intensities as a function of the pump energy density. Figure 4c shows that the fluorescence signal is virtually negligible before reaching a threshold, over which it gets abruptly amplified. This nonlinear response of the output signal is a direct proof of lasing. Above the threshold, the laser intensity linearly increases with the pump energy density. The lasing threshold of our LC spherical laser system, estimated from the linear fit of the curve, is 0.54 mJ/mm 2 . This value is a few orders of magnitude higher than in other laser devices with similar R6G dye concentrations utilizing solid-state microcavities [39] due to a lower Q-factor. That mainly refers to small refractive index contrast between Y002 and the surrounding medium (silicon oil) and the small size of the droplets. Yet, the LC spheres provide enough feedback for substantial lasing, leading to reliable laser demonstration with our system. The Y002 molecules get dispersed into the silicon oil as temperature increases, and the entire medium exhibit fluorescence when externally pumped. Since light confinement by the LC sphere is absent, the medium simply shows typical fluorescence emission but not lasing, as indicated in Figure 4d, even though the system is pumped with same pump energy as in Figure 4b. Note that the nature of the R6G organic dye is not affected by the temperature change. The lasing behavior of our switchable emulsion can be reproduced with other dyes, as long as the association between the LC and the guest dye is successful. For instance, using BTBP instead of R6G as a dopant leads to similar results, as illustrated in Figure S5. The only difference is the relatively small output emission and increased lasing threshold compared to the former case of R6G, due to the lower quantum efficiency of the BTBP dye. These results demonstrate that our LC/oil emulsion platform enables producing a large number of microlasers that can be switched on and off simply by modifying temperature.

In conclusion, we have designed a switchable oil-in-oil emulsion using LC and silicon oil as guest and host materials, respectively, which is operated by simple thermal stimuli. By taking advantage of fluorochemistry, the LC, Y002, was designed and used as both guest-oil and emulsifier. Due to the liquid crystallinity of Y002 and semi-miscibility between Y002 and silicon oil, the transition between emulsification and demulsification is reversibly triggered by varying temperature. Interestingly, the droplets adopt a transient filament-like structure upon fast cooling from the isotropic to the smectic phase, which plays a significant role in determining the final diameter of the LC droplets, leading to a emulsifiable system with uniform droplet size and repeatable and reliable switching properties. This switching behavior enabled us to successfully apply our emulsion to get on-off lasing performance by confining organic laser dyes in the smectic phase.

To our best knowledge, our system is the first example of switchable oil-in-oil emulsion that does not require any surfactant or emulsifier, fulfilling the conditions required for an ideal switchable emulsion. It also provides an interesting example of switchable opto-electric device based on emulsions. With future efforts to better control the shape of the droplets and the reflective index matching, we envisage that our tunable laser system may be useful for real applications, e.g., as a smart thermal sensor.

Experimental Section

Materials

The LC material Y002 was synthesized by the procedure for the alkylation of methyl ethyl 4'hydroxy-4-biphenyl carboxylate with 1H,1H,2H,2H,3H,3H,4H,4Hperfluorododecyl bromide in DMF (Sigma Aldrich, 99.8%) at 70 o C under reflux with K 2 CO 3 . [22] Sample preparation 1wt% of crystalline powdered Y002 is added to silicon oil and vigorously stirred at 200 o C for 1 min. The mixture is infiltrated into a cell where two glass substrates are separated by 100μm spacers made of polyimide tape (3M).

For the red dyed sample, 0.1wt% of oil soluble red dye (PYLAM dyaes) was added to above emulsion mixture and vigorously stirred at 200 o C for 1 min. Then, the sample was diluted in pure silicon oil so that the red dye was kept only in the LC spheres (figure S6).

For the lasing test, the florescent LC spheres were prepared by adding 0.01wt% of rhodamine 6G (Sigma Aldrich) or N,N-Bis(2,5-di-tert-butylphenyl)-3,4,9,10-perylenedicarboximide (BTBP, Sigma Aldrich) to the mixture by following the same procedure as with the red dye, and diluted so that the red dye was kept only in the LC spheres (figure S7).

To control the morphological changes of the LC droplets with temperature, the sample was heated and cooled with a rate of 10 o C/min on a heating stage (LINKAM LTS350) controlled by a temperature controller (LINKAM TMS94).

Characterization

The LC samples were directly imaged using polarized optical microscopy (POM) (LV100POL, Nikon) and field-emission SEM (FE-SEM, Hitachi, S-4800). Fluorescent images of the optical textures were obtained by fluorescence confocal microscopy (C2 plus, Nikon) with a 488 nm light source (Coherent). For cross sectional images of LC sample, the samples were prepared by using freeze fracture (BalTec BAF060) and observed by using Transmission Electron Microscopy (TEM, Tecnai G2 F20). The images show that the fluorescent molecules specifically dope the LC spheres.

Supporting Information

Figure 1 .

 1 Figure 1. (a) Molecular structure and phase transition temperatures of Y002, where Cr, SmE, SmA and iso refer to the crystal, smectic E, smectic A and isotropic phase, respectively. The schematics show the molecular organization in the smectic A and smectic E phases, where the black dashed lines indicate plane of the biphenyl group of Y002. (b) Schematic illustration of the reversible emulsification process of the LC in the silicon oil medium induced by temperature. The fluorinated part of the LC molecule is oriented towards the silicon oil due to relatively lower surface tension it induces compared to the bi-phenyl part. The red dot and line in sphere and cylinder, respectively, indicate the presence of topological defects.

Figure 2 .

 2 Figure 2. (a,d) OM and POM images of the well-mixed state at isotropic temperature, 200 o C, where the LC and silicon oil molecules form a continuous phase. (b) OM image of a filament structure dispersed in silicon oil; the inset shows the filament POM image with a retardation plate. (c) OM image of LC spheres dispersed in silicon oil; the insets show a POM image with a retardation plate (right) revealing the radial configuration of the droplet (left). (e,f) SEM images of a LC filament and spheres, respectively. The inset of (e) shows the cross section of a LC filament obtained by FF-TEM.

Figure 3 .

 3 Figure 3. (a) Average size of LC spheres as a function of the number of transition cycles from isotropic (200 o C) to smectic phase (120 o C) (the number of spheres measured in each cycle is around 100) (b) Intensity profiles of the silicon oil during heating and cooling process when the LC is doped with red dye molecules, showing homogeneous dispersion of LC molecules upon demulsification in the isotropic phase (c) Sequential OM images of the reversible emulsification of the LC by thermal phase transition from smectic to isotropic phase.

Figure 4 .

 4 Figure 4. (a) FM images of the sample during the assembly and disassembly of the LC, dyed with R6G, into spheres by thermal phase transition from smectic to isotropic phase. All scale bars are 100μm. (b) Lasing spectrum of the dye-doped LC spheres at room temperature. The inset shows a cross sectional sketch of the LC droplet with florescent dyes. (c) Output intensity of the lasing emission as a function of the pumped energy. (d) Fluorescent spectrum in the fully mixed state including R6G dye at 200 o C.

Figure S2 .

 S2 Figure S2. (a-d) Sequential images showing how a long filament splits into two to form two spherical droplets. The images were taken during the cooling of the sample from the isotropic to the smectic transition temperature (~195oC). Scale bars are 20 m.

Figure S3 .

 S3 Figure S3. Cross section of a smectic filament showing the smectic layering nucleation

Figure S5 .

 S5 Figure S5. (a) Lasing spectrum in the WGM on the LC spheres at room temperature. (b)

Figure S6 .

 S6 Figure S6. OM images of silicon oil containing LC droplets and red dyes. The sample,

Figure S7 .

 S7 Figure S7. Fluorescent OM images of silicon oil containing LC droplets and BTBP. The

Acknowledgements

This work was supported by the National Research Foundation of Korea (NRF), grant funded by the Korean government (MSIT) 2017M3C1A3013923 and 2017R1E1A1A01072798, and the French National Research Agency (ANR), Grant No. 13-JS08-0006-01. D.S.K. and D.K.Y. designed the research. D.S.K. and W.L. performed the experiments. D.S.K., W.L., T.L.L. and D.K.Y. analyzed experimental data and wrote the manuscript. D.S.K and W.L. are equally contributed to this work and D.K.Y. and T.L.L. are cocorresponding authors.

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Supporting Information is available from the Wiley Online Library or from the author.

The table of contents entry should be 50-60 words long and should be written in the present tense and impersonal style (i.e., avoid we). The text should be different from the abstract text.


Rahaf Rahal

Giulia Matarese

Marco Gabiccini

Alessio Artoni

Domenico Prattichizzo

Paolo Robuffo Giordano

Claudio Pacchierotti

Haptic shared control for enhanced user comfort in robotic telemanipulation

I. INTRODUCTION

Shared control methods have been used in many applications related to robotic teleoperation, making it possible to share the degrees of freedom of the system between the human operator and the autonomous controller. Examples include mobile robotics [START_REF] Bruemmer | Shared understanding for collaborative control[END_REF], robot-assisted surgery [START_REF] Khademian | Dual-user teleoperation systems: New multilateral shared control architecture and kinesthetic performance measures[END_REF], and assistive robotics [START_REF] Selvaggio | Haptic-based shared-control methods for a dual-arm system[END_REF]. Haptic shared control is one such method, which utilizes haptic active constraints to share the control of the system between the user and the autonomous controller [START_REF] Abbink | Haptic shared control: smoothly shifting control authority?[END_REF]. Haptic shared control has been successfully used to perform different tasks, e.g., to guide the operator toward a reference position [START_REF] Mulder | Sharing control with haptics: Seamless driver support from manual to automatic control[END_REF], to avoid certain areas of the environment [START_REF] Bowyer | Active constraints/virtual fixtures: A survey[END_REF], to apply virtual nonholonomic constraints [START_REF] Rahal | Haptic shared-control methods for robotic cutting under nonholonomic constraints[END_REF] and for learning manual tasks [START_REF] Marchal-Crespo | The effect of haptic guidance on learning a hybrid rhythmic-discrete motor task[END_REF].

However, researchers have rarely focused on designing methods accounting for the user's comfort during robotic telemanipulation. While a few examples are available in the field of human robot interaction (HRI), e.g. [START_REF] Busch | Postural optimization for an ergonomic human-robot interaction[END_REF], [START_REF] Peternel | Towards ergonomic control of human-robot co-manipulation and handover[END_REF], haptic shared control has never been, to the best of our knowledge, designed for this specific purpose. This is nonetheless a very important issue for operators in many high-impact applications, using robotic teleoperation systems for long periods of time.

This paper introduces a haptic shared control technique minimizing the user's workload during robotic teleoperation. Using an inverse kinematic model of the human arm and an online implementation of the Rapid Upper Limb Assessment (RULA) tool [START_REF] Mcatamney | Rula: a survey method for the investigation of work-related upper limb disorders[END_REF], the proposed approach starts by estimating the current user's discomfort at runtime. Then, this metric is combined with some knowledge of the target task and system (e.g., direction to follow, target position to reach, effort demanded to the robot) to generate dynamic active constraints guiding the user towards a successful completion of the task along directions that require a reduced workload.

II. SYSTEM DETAILS

In this work, we consider a bilateral teleoperation system composed of a 6-DoF master interface and a velocity con-R. Rahal and G. Matarese are with Univ Rennes, Inria, CNRS, IRISA, Rennes, France, {rahaf.rahal, giulia.matarese}@irisa.fr. These two authors contributed equally to the manuscript.

M. Gabiccini and A. Artoni are with the Dip. Ingegneria Civile e Industriale, Univ. Pisa, Pisa, Italy, {m.gabiccini,a.artoni}@ing.unipi.it. M. Gabiccini is also with the Centro di Ricerca "E. Piaggio", Univ. Pisa, Pisa, Italy.

D. Prattichizzo is with the Dip. Ingegneria dell'Informazione e Scienze Matematiche, Univ. Siena, Siena, Italy and with the Dept. Advanced Robotics, Istituto Italiano di Tecnologia, Genova, Italy, prattichizzo@diism.unisi.it. P. Robuffo Giordano, and C. Pacchierotti are with CNRS, Univ Rennes, Inria, IRISA, Rennes, France, {prg, claudio.pacchierotti}@irisa.fr.

The authors want to thank Charles Pontonnier for his help in defining the human effort cost function. As a first step, we devise an innovative approach to estimate the user's muscular comfort during the task, using an inverse kinematic model of the human arm and the popular RULA tool. We model the human arm as a 7-DoF robotic arm, with a spherical joint for representing the shoulder and the wrist, and a revolute joint for the elbow. Using the pose of the hand registered by the grounded haptic interface, and without the need of any additional sensor, we calculate, at each time step, q s the shoulder angles, q e the elbow angle, and q w the wrist angles. More details on the inverse kinematics algorithm and solving the redundancy of the arm are presented in [START_REF] Rahal | Caring about the human operator: haptic shared control for enhanced user comfort in robotic telemanipulation[END_REF].

The workload W is then defined as the sum of the squared differences between the angles and their rest positions (π/2 for the elbow angle, and 0 for the others).

W = q T s q s + (q e -π/2) 2 + q T w q w . (1)
Although rather simple and fast to compute, this estimation has shown a good correlation and prediction capability with respect to the NASA TLX [START_REF] Hart | Development of nasa-tlx (task load index): Results of empirical and theoretical research[END_REF] results compiled by the users at the end of the task, proving its effectiveness and viability in this scenario.

In a second step, we combine this workload measure with a cost function related to the task at hand or the status of the robotic system. As an example, in our experiments, we considered a cost function H r = [p r,gp r , r (θu) r,g ][p r,gp r , r (θu) r,g ] T , indicating the distance between the current pose of the robot {r} and its goal position {r, g}. However, the proposed framework supports any other task-related cost function (e.g., trajectory minimizing the energy consumed, displacement, risks of encountering singularities). From the combination of the two cost functions H W = W and H r , we then generate a dynamic active constraint guiding the user towards a successful completion of the task along directions maximizing the user comfort. (c) A40 Fig. 2. User trajectory for a reaching movement for the three weightings A0 (blue), A20 (red), and A40 (green). In A0, only Hr is considered, and the user trajectory is therefore almost horizontal. As the contribution of H W to the haptic feedback increases, the trajectory tries to minimize the user discomfort by moving the arm to a downward (more comfortable) position.

We design the haptic feedback to guide the user during teleoperation, such that both task-and human-comfortinspired cost functions are minimized. We thus divide our haptic feedback force into two components: a human-related component, f W , and a robot/task-related one, f r . The forces applied at the master end effector are defined as

f m = αf W + βf r , (2)
where f W is the force vector instantaneously guiding the user towards the position with the highest comfort (minimizing H W ), and vector f r is the force minimizing the task cost function H r , which in our case is related to the distance from the target release position. α and β are weights to be tuned depending on the importance to be given to each cost function. More details on the system architecture and the haptic feedback implementation are given in [START_REF] Rahal | Caring about the human operator: haptic shared control for enhanced user comfort in robotic telemanipulation[END_REF] and in the video at https://youtu.be/DodGI4wMRFA. Fig. 2 shows the effect of the weighting schemes in a simple reaching movement between two fixed points.

III. EXPERIMENTS AND RESULTS

To evaluate the effectiveness of our method, we designed a user study with 15 participants. The task consisted of pick and place trials on three different objects: a box (B), 2 cubes (C), and a wooden letter (H). We consider three weighting schemes for the contribution of each force component in eq. ( 2): α = 0, β = 1 (A0, the human-centered metric is disregarded), α = 0.2, β = 0.8 (A20, weak human-centered guidance), and α = 0.4, β = 0.6 (A40, strong humancentered guidance).

To evaluate the effectiveness of the proposed humancentered shared control approach, we recorded (i) the completion time, (ii) the error in placing the objects at the target, (iii) the maximum H W registered, and (iv) the NASA Task Load Index (NASA-TLX). Results of the statistical analysis performed are summarized in Fig. 3.

Completion time showed a significant degradation when adding our human-centered guidance (A20, A40) vs. standard task-centered guided teleoperation (A0). However, this (small) performance degradation is compensated by a significant reduction of the estimated muscular discomfort (H W ) and measured workload (NASA TLX). In fact, while the completion time and placing error degrade by 14% and 10%, respectively, the NASA TLX value is improved by 30%. IV. CONCLUSION In this paper, we presented a haptic shared control for robotic teleoperation that combines human-centered and task-centered cost functions. We propose a metric to estimate the user discomfort at runtime, and combine it with some knowledge of the target task to generate active constraints guiding the user towards a successful completion of the task along directions that require a reduced workload. Studies with human subjects show the effectiveness of the proposed approach, yielding a 30% perceived reduction of effort with respect to standard guided human-in-the-loop teleoperation.

Fig. 1 .

 1 Fig. 1. The experimental setup for the pick and place task. On the master side, a Haption Virtuose 6-DoF haptic device, on the slave side a 7-DoF Franka Emika Panda robot. The user shoulder is assumed to be fixed.

  H W value (rad 2 ).

Fig. 3 .

 3 Human subjects experiment. Mean and standard error of the mean of (a) completion time, (b) error in placing the objects, (c) maximum H W , and (d) NASA TLX load index for the three control conditions (A0, A20, A40) and the three target objects (B, L, C).