Charlotte Harrison

Nicola Binetti

Antoine Coutrot

Alan Johnston

Isabelle Mareschal

Personality Traits Do Not Predict How We Look at Faces

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

Gaze is a vital component of human interaction. In both infancy and adulthood, people prefer to look at faces compared to any other feature in visual scenes [START_REF] Birmingham | Saliency does not account for fixations to eyes within social scenes[END_REF][START_REF] Coutrot | How saliency, faces, and sound influence gaze in dynamic social scenes[END_REF]. In many contexts, the eyes are amongst the most salient facial features (Yarbus, 1976;[START_REF] Maurer | Developmental changes in the scanning of faces by young infants[END_REF]. The eyes are important in facilitating successful social interaction and when the eyes are occluded, face recognition [START_REF] Mckelvie | The role of eyes and mouth in the memory of a face[END_REF] and detection [START_REF] Lewis | Face detection: Mapping human performance[END_REF] performance drops.

Gaze also influences the perception of gender [START_REF] Schyns | Show me the features! Understanding recognition from the use of visual information[END_REF] and emotion [START_REF] Calder | Configural information in facial expression perception[END_REF]. Findings across experiments are constant and seemingly task-independent, suggesting that information from the eye region is critical for multiple aspects of facial processing, including recognition of identity, age, gender and expression [START_REF] Itier | Neural bases of eye and gaze processing: the core of social cognition[END_REF].

There are also known asymmetries in the processing of direct, compared to averted, gaze. The "eye contact effect" refers to any cognitive, perceptual or behavioural change that occurs because of the processing of direct gaze [START_REF] Senju | The eye contact effect: mechanisms and development[END_REF]. When compared to averted, direct gaze evokes stronger galvanic skin responses (GSRa proxy of arousal) [START_REF] Nichols | Eye gaze and the GSR[END_REF], facilitates face recognition (Hood, Macrae, Cole-Davies, & Dias, 2003;Vuilleumier, George, Lister, Armony, & Driver, 2005), and gender categorisation (Macrae, Hood, Milne, Rowe, & Mason, 2002). Direct gaze is also known to elicit avoidance responses. [START_REF] Ellsworth | The stare as a stimulus to flight in human subjects: a series of field experiments[END_REF] found that staring confederates on motorcycles caused motorists to depart more rapidly from traffic lights, with comparable results obtained with pedestrians. There have also been multiple findings that more submissive animals will avert gaze, reducing the likelihood of attack by another animal [START_REF] Argyle | Gaze and mutual gaze[END_REF]. These findings support the idea that prolonged direct gaze is an aversive and threatening signal in many animals [START_REF] Emery | The eyes have it: the neuroethology, function and evolution of social gaze[END_REF].

Gaze preference in humans is often linked to anxiety of the gazer, although the relationship between trait characteristics such as anxiety and neuroticism, and gaze, are mixed. Perlmann et al. (2009) found that participants who scored highly on neuroticism fixated longer in the eye-region of fearful faces compared to those with low levels of neuroticism. Similarly, an earlier study found that in an interview with 'awkward silences', interviewees with low self-concepts and high scores on neuroticism and trait anxiety reacted with increased gaze toward the interviewer [START_REF] Wiens | Personality correlates of nonverbal interview behavior[END_REF]. Conversely, [START_REF] Uusberg | Eye contact reveals a relationship between Neuroticism and anterior EEG asymmetry[END_REF] found that higher neuroticism scores were behaviourally correlated with direct gaze avoidance and subjective averted gaze preference, as well as avoidance-related, relative right-sided functional brain asymmetry. Other studies (e.g. [START_REF] Hobson | Anxiety and gaze aversion in dyadic encounters[END_REF] found no significant differences.

A study by Roelofts, Putman, Schouten, Lange, Volman & Rinck (2010) reported that gaze direction mediates processing of emotional faces in anxious individuals. When asked to 'approach' (push a joystick towards) or 'avoid' (push a joystick away) faces with varying expressions, participants with high social anxiety were faster in conditions avoiding angry faces. This avoidance tendency was only present when the perceived anger was directed towards the participant (direct gaze) rather than being directed elsewhere (averted gaze). However, the differences were only found in the 'withdrawal' aspect factor of neuroticism, which may explain previous mixed findings. Reciprocally gaze direction in emotional faces is also influenced by social anxiety, with anxious participants more likely to report gaze directed at them over a wider range of gaze deviations than controls [START_REF] Jun | Cone of direct gaze is a marker of social anxiety in males[END_REF].

Compared to other personality traits, individual differences in extraversion are typically expected to be most clearly evident in a broad range of self-reported, non-verbal behaviour; in comparison to introverts, extraverts report using more eye contact, vocal variety, gesticulation, and facial expression [START_REF] France | Is there empirical evidence for a nonverbal profile of extraversion?: a meta-analysis and critique of the literature[END_REF]. [START_REF] Exline | Explorations in the process of person perception: visual interaction in relation to competition, sex, and need for affiliation1[END_REF] reported that women with high scores on a measure of "need for affiliation" gazed more at other women in cooperative rather than competitive interactions whereas women with a low "need for affiliation" gazed more at other women in a competitive interaction. Need for affiliation scores were not related to gazing behaviours of men. In face-to-face situations, Mobbs (1968) found that extraverts invest more time looking at a person's eyes, measured by both total and average gaze duration compared to introverts, while [START_REF] Argyle | Gaze, mutual gaze, and proximity[END_REF] found evidence for correlated extraversion scores and mutual gaze in English, but not Swedish, subjects. Extraverts have been shown to look more frequently at faces than introverts, but there were no differences between the two groups in the proportions of time spent in looking at the others' face and eye contact, or in the mean length of looks. Another study of extraversion showed that extraverts were more likely than introverts to maintain longer gazes while listening during social interaction [START_REF] Iizuka | Eye contact in dating couples and unacquainted couples[END_REF]. However, more recent research shows that correlations between extraversion and nonverbal behaviour in the literature range between an r of .73 to -.36 [START_REF] France | Is there empirical evidence for a nonverbal profile of extraversion?: a meta-analysis and critique of the literature[END_REF]. When sample size and variance is accounted for, this correlation drops to .13. In addition, some of the earlier results may be influenced by the fact that extraverts are more likely to speak up in-group conversations and eye contact normally accompanies speech [START_REF] Rutter | Visual interaction in a group of introverts and extraverts[END_REF].

In additional to the direction of gaze, there are also reported links between personality and blinking habits, with increased blink rate associated with higher neuroticism scores (Barbato, Della Monica, Constanzo & De Padova, 2012). It has been suggested that blink rate may be modulated by muscle tension, and this may be the mediating link between blink rate and neuroticism (Martin, 1958). Other studies dispute this link, however, with one study only finding a link to the personality trait of psychoticism, and not to any other traits [START_REF] Colzato | Closing one's eyes to reality: Evidence for a dopaminergic basis of psychoticism from spontaneous eye blink rates[END_REF]. The above studies, while mixed, seem to suggest links between personality and gazing behaviour (either in direction of gaze, blink rates or eye contact), which warrants further investigation [START_REF] Kleinke | Gaze and eye contact: a research review[END_REF].

In our initial exploratory experiment conducted at the London Science Museum we measured the period of mutual gaze that individuals are comfortable with, and examined how that might differ across age, culture and personality. Participants completed a psychological task where they had to judge whether duration of gaze by an actor seen in a video clip felt too long or two short to be comfortable. A staircase procedure was used, and eye tracking-data were collected to also examine differences in scan-path behaviour. Afterwards participants answered a brief questionnaire with demographic information and a short form version of the Big Five Inventory [START_REF] Rammstedt | Measuring personality in one minute or less: A 10item short version of the Big Five Inventory in English and German[END_REF]. Our initial study found that preferred gaze duration was not dependent on gender, personality or attractiveness. However, pupil dilation correlated with the amount of eye contact individuals found comfortable; those who preferred longer durations of eye contact displayed faster increases in pupil size when viewing the face than those who preferred shorter durations. This suggested that physiological indices might play an involuntary role in modulating gaze behaviour. The overall preferred gaze duration was found to be 3.3 seconds (±.71).

While we previously failed to find a difference in preferred mutual gaze duration and personality, we did not look at other key indicators, such as fixation locations or durations [START_REF] Binetti | Pupil dilation as an index of preferred mutual gaze duration[END_REF]. Further, in our earlier study only a 10-item, short-form version of the Big Five Inventory [START_REF] Rammstedt | Measuring personality in one minute or less: A 10item short version of the Big Five Inventory in English and German[END_REF] was used to measure personality scores. This questionnaire might have been too coarse to pick up on individual differences, as traits were only being measured by two items each.

Here we followed-up on our exploratory approach and sought to investigate whether the use of more exhaustive personality inventory (the full Big Five Inventory) would reveal differences, both in terms of participants' preferred gaze duration with an actor on screen, and their own gaze behaviour. Based on mixed evidence in the literature as well as our earlier result, we hypothesise that personality traits are not strongly related to preferred gaze duration. However, we expect that blink count, average saccade amplitude and mean fixation duration may be linked to personality traits of extraversion and neuroticism.

Method

Participants

Our original participant sample consisted of 498 visitors (224 male, 56 nationalities) to the London Science Museum, (M = 29.9, SD = 12.3, range = 11-79 years). Written consent was obtained from participants prior to the experiment. 105 participants responded to the follow-up questionnaire that was sent via email; 15 were excluded from the analysis as their behavioural data for preferred gaze duration could not be reliably fitted to a psychometric function, and a further 13 were removed for having no available or poor eye-tracking data.

This left a sample of 77 participants, 37 of which were male (age M = 33.1, SD = 13.6, range = 16-72 years). The experiment was approved by the UCL Research Ethics Committee and by the London Science Museum.

Experimental setup

Behavioural task. Full details are available at [START_REF] Binetti | Pupil dilation as an index of preferred mutual gaze duration[END_REF]. In brief, participants were shown a series of 40 video clips of an actor who could be male or female (chosen randomly from a set of 8, between the ages of 20-33 years, Caucasian with neutral expressions) gazing directly at them for varying lengths of time. Participants indicated via keyboard response whether the actor's gaze felt too long or too short to feel 'comfortable' (see Figure 1). Video clips were edited in Premiere Pro (CS6, Adobe; www.adobe.com) so the eye region roughly occupied an equivalent area on the screen and the bridge of the nose (nasion) of all actors was aligned with the screen centre. There were 35 possible clips with a period of direct eye contact ranging from 100ms to 10,300ms (300ms increments, with ms control). The direct gaze was preceded and followed by 500ms averted downward gaze. Prior to each trial, the nasion position was cued by a black central fixation cross-presented on a grey background to ensure homogeneity in participants' first fixation. Participants gave their response via a key press response after which, a grey screen with the fixation point appeared for 1s prior to the next trial commencing. Clip duration was selected based on randomly perturbed estimates yielded by two interleaved QUEST adaptive staircase routines.

Results

Personality data. The relationship between participants' original BFI-10 scores and full BFI scores were examined. A significant positive correlation was found between all ratings: extraversion (r = .655, p < .001), conscientiousness (r = .600, p < .001), neuroticism (r = .712, p < .001), openness (r = .285, p = .006) and agreeableness (r = .539, p < .001), indicating that self-reported personality traits remained stable over time. See Figure 2 for probability density estimates of self-reported scores.

Figure 2. Probability density estimates for the five personality traits measured by the Big

Five Inventory in our participants. Behavioural data. Participants' "too long" responses were plotted as a function of the different gaze durations and a cumulative Gaussian was fit to the data to extract their preferred amount of eye contact, which was taken as the midpoint on the Gaussian function.

The original dataset (Binetti et al., 2017) had a mean preferred gaze duration (PGD) of 3,295ms ± 706ms, mode = 2,800ms. The current dataset had a mean PGD of 3,550ms ± 1,312ms, and mode of 3,100ms (Figure 3). We performed a multiple regression on the preferred gaze duration, using the 5 personality traits as predictors. Initial correlational analysis revealed that openness was the factor most likely to affect preferred gaze duration, therefore it was the first factor entered into the multiple regression. This was followed by extraversion, then agreeableness, conscientiousness and neuroticism. We found that openness only significantly (weakly) predicted preferred gaze duration F(1,76)=4.022, p<0.49. Given the exploratory nature of this study and the small magnitude of this effect, it must be considered with care.

Eye data. Eye position signals were parsed into fixations and saccades with a custom algorithm based on [START_REF] Nyström | An adaptive algorithm for fixation, saccade, and glissade detection in eyetracking data[END_REF]see Coutrot et al., 2016 for details). The range of samples lost, and the saccade velocity before and after loss of signal were used to determine blinks. Only blinks during the trials were counted. As the overall length of the experiment varied due to QUEST procedure, each participant's blink count was normalised by dividing the total number of blinks by the total duration of the trials. The average number of blinks for a participant was 15 over the entire experiment (average of 5 minutes). Time spent looking at the eye area was calculated as the proportion of fixations occurring within that region of interest. We performed a multiple regression to examine whether personality traits predicted mean fixation duration but none were significant. None of the personality traits predicted time spent in eye region either, however there was a significant effect of extraversion on blinks F(1,76)=7.16, p<0.009.

Discussion

The current experiment investigated whether there was a relationship between selfreported personality measures and preferred length of mutual gaze duration. We found a weakly significant effect of openness on preferred gaze duration. Further, participants' personality trait of neuroticism predicted their number of blinks. However, the exploratory nature of this study and the overall weakness of these effects make them vulnerable to type I error and should be interpreted with caution.

Past findings on gaze behaviour and personality have been mixed. It may be that the significant differences that have been found have been influenced by contextual variables.

For example, eye contact normally accompanies speech in dyads where extraversion scores correlate positively with amount of conversation (Argyle & Cook, 1967), and more frequent mutual gaze may be a consequence of this greater likelihood of conversation. Other factors than can affect preferred gaze duration, include conversation subject matter, facial expression and gender of those involved [START_REF] Argyle | Gaze and mutual gaze[END_REF]Thayer & Schiff, 1975). This suggests that gaze may be context-dependent rather than trait-dependent. However, it may be that personality variables have a mediating effect; for example, agreeableness may link to respect for social norms and regulate behaviour to act. It may also be that we use someone else's gaze behaviour as an indicator of their personality, even if it does not truly reflect it [START_REF] France | Is there empirical evidence for a nonverbal profile of extraversion?: a meta-analysis and critique of the literature[END_REF]. Further, it may be that our sample size of 77 was underpowered when trying to find significant differences in smaller effects, of below .30.

Our results are partially consistent with [START_REF] Perlman | Individual differences in personality predict how people look at faces[END_REF] who reported a significant correlation between amount of time spent in the eye region of fearful faces and higher neuroticism scores. Although we found no change in amount of time in eye region, we did find a significant effect of neuroticism on blinks (which they did not investigate). There are however some significant differences between our methodologies that may also account for this discrepancy with respect to amount of time in eye region. Notably, the only significant finding in [START_REF] Perlman | Individual differences in personality predict how people look at faces[END_REF] paper came from fearful faces, but not neutral faces (consistent with our result which is based only on neutral faces). Further, they made use of static images compared to our dynamic ones. It has been shown that we process static versus dynamic faces differently [START_REF] Pitcher | Differential selectivity for dynamic versus static information in face-selective cortical regions[END_REF]. Finally, our participants were given a task compared to the free viewing condition of the other study, which in itself may alter gaze behaviour (Yarbus, 1976). It has been suggested that the eye contact effects may only occur when the observer knows that he/she is being looked at by another "mind". This kind of mentalizing is probably reduced when facing an image or a non-living stimulus [START_REF] Myllyneva | There is more to eye contact than meets the eye[END_REF]. For instance, [START_REF] Pönkänen | The observer observed: Frontal EEG asymmetry and autonomic responses differentiate between another person's direct and averted gaze when the face is seen live[END_REF] showed in an ERP study that the N170 normally associated with face and gaze processing produces different signals for direct and averted gaze, but only in live situations and not virtual. Another recent study indicates that time is perceived as being expanded with direct but not indirect gaze, and only when interacting with real people and not virtual avatars [START_REF] Jarick | Eye contact affects attention more than arousal as revealed by prospective time estimation[END_REF], suggesting a modulatory role of arousal. However, avatars, along with photos and video recordings, have been successfully utilised in gaze and face perception literature [START_REF] Itier | Neural bases of eye and gaze processing: the core of social cognition[END_REF]. Research into the increasing use of technology-mediated conversation (such as video conferencing) and level of emotional engagement suggest that stimuli viewed on a screen can be arousing and socially engaging if it is reciprocal [START_REF] Mcclure | Infant emotional engagement in video mediated interactions[END_REF]. Therefore future research should explore whether robust gaze preference / personality trait relationships can be observed in a reciprocal gaze paradigm, whether face-to-face or via video conferencing.

The current study found that overall, personality traits do not influence face exploration, with the exception of openness weakly predicting preferred gaze duration. Future experiments should focus on contextual factors that may mediate personality traits and on how stimulusrealism and arousal may influence gaze behaviours of both the observer and observed. While gaze behaviour is a clearly important factor in clinical conditions such as autism spectrum disorders and schizophrenia [START_REF] Itier | Neural bases of eye and gaze processing: the core of social cognition[END_REF], the link between personality and gaze may be more complicated.

Figure 1 .

 1 Figure 1. On each trial, an actor directly gazes at the participant for a variable amount of time. Participants indicate at the end of the clip whether the actor's direct gaze was 'too short' or 'too long' to feel comfortable.

Figure 3 .

 3 Figure 3. Distribution of preferred Gaze Durations.

Figure 1 Figure 2 Figure 3

 123 Figure 1


P Maurice

J Čamernik

D Gorjan

B Schirrmeister

J Bornmann

L Tagliapietra

C Latella

D Pucci

L Fritzsche

S Ivaldi

J Babič

Evaluation of PAEXO, a novel passive exoskeleton for overhead work

Keywords: Physiological measures, workload assessment, technology acceptance, field-testing, upper-limb exoskeleton

Introduction

Work-related musculoskeletal disorders (WMSDs) are the first cause of occupational diseases in developed countries and represent a major health issue and an important cost for companies [START_REF] Parent-Thirion | Fifth european working conditions survey[END_REF]. WMSDs develop when biomechanical demands at work repeatedly exceed the worker's physical capacity (e.g., extreme postures, high efforts). Overhead work has been identified as a major risk factor for shoulder WMSDs [START_REF] Grieve | Overhead work: Identification of evidence-based exposure guidelines[END_REF]. Even without external load or force exertion, supporting the arms' weight imposes prolonged stress on shoulder muscles. Yet overhead work remains very common on assembly lines, especially in the automotive and aerospace industries. One solution to physically relieve workers while keeping them in control of the task execution is to assist them with an exoskeleton [START_REF] De Looze | Exoskeletons for industrial application and their potential effects on physical work load[END_REF]. Recently, several industrial prototypes providing arm support during overhead tasks have been developed and tested, and some are already commercialized [START_REF] Bornmann | Tragbare assistenzsysteme in der automobilmontage: Forschung und entwicklung innovativer orthetischer systeme zur physischen unterstützung wahrend der Überkopfarbeit[END_REF][START_REF] Butler | Exoskeleton technology: Making workers safer and more productive[END_REF][START_REF] Gillette | Emg assessment of a shoulder support exoskeleton during on-site job tasks[END_REF][START_REF] Spada | Analysis of exoskeleton introduction in industrial reality: Main issues and eaws risk assessment[END_REF][START_REF] Huysamen | Evaluation of a passive exoskeleton for static upper limb activities[END_REF][START_REF] Kim | Assessing the influence of a passive, upper extremity exoskeletal vest for tasks requiring arm elevation[END_REF][START_REF] Otten | Evaluation of a novel active exoskeleton for tasks at or above head level[END_REF][START_REF] Theurel | Physiological consequences of using an upper limb exoskeleton during manual handling tasks[END_REF][START_REF] Van Engelhoven | Evaluation of an adjustable support shoulder exoskeleton on static and dynamic overhead tasks[END_REF]. The majority of studies on those exoskeletons showed promising results, reporting reduction in shoulder muscle activity and/or increase in endurance and task productivity. A decrease in physical workload of the targeted limb and/or increased productivity are, however, not sufficient to demonstrate the benefit of an exoskeleton. Several other factors may affect the system's effectiveness. Exoskeleton-induced load transfer and movement modification or restriction may increase biomechanical strain elsewhere in the body [START_REF] Spada | Analysis of exoskeleton introduction in industrial reality: Main issues and eaws risk assessment[END_REF][START_REF] Kim | Assessing the influence of a passive, upper extremity exoskeletal vest for tasks requiring arm elevation[END_REF][START_REF] Otten | Evaluation of a novel active exoskeleton for tasks at or above head level[END_REF][START_REF] Theurel | Physiological consequences of using an upper limb exoskeleton during manual handling tasks[END_REF]). In addition, even when a beneficial biomechanical effect is proven, users may perceive otherwise and remain dissatisfied with the system [START_REF] Kim | Assessing the influence of a passive, upper extremity exoskeletal vest for tasks requiring arm elevation[END_REF]. In this work, we present an exhaustive assessment of the benefit provided by PAEXO, a novel passive exoskeleton for overhead work, and describe on-going field testing.

Methods

Evaluation criteria

In our view, a thorough evaluation should include objective performance measures of local effects and global effects on the user's posture, movement and effort, as well as subjective evaluations of the user's perception and acceptance of the system. We therefore propose the following assessment criteria:

 Impacted limb: Using the exoskeleton should physically relieve the joint or limb it is designed to support.  Side effects: Using the exoskeleton should not increase physical strain on other body parts.  Workload: Using the exoskeleton should reduce the global physical and cognitive workload.  Task performance: The exoskeleton should not degrade task performance or productivity.  Movement strategy: Potential modifications in users' movements due to the exoskeleton should be investigated to evaluate their consequences.  Acceptance: Users should feel better, physically and mentally, when using the exoskeleton.

Exoskeleton

The exoskeleton evaluated in this work is PAEXO developed by Ottobock SE & Co. KGaA together with Volkswagen AG commercialized [START_REF] Bornmann | Tragbare assistenzsysteme in der automobilmontage: Forschung und entwicklung innovativer orthetischer systeme zur physischen unterstützung wahrend der Überkopfarbeit[END_REF]. PAEXO is a lightweight (1.8 kg) passive exoskeleton that provides a support torque to the user's arms, by transferring an adjustable portion of the arm weight to a hip belt via a passive actuator (Fig. 1).

Figure 1: Description of the PAEXO exoskeleton.

Lab experiment

Twelve healthy college students performed an overhead pointing task with a portable tool, with and without wearing PAEXO (Fig. 2). The participants' physical and physiological state was monitored with whole-body inertial motion capture (Xsens Awinda system), ground reaction force (Kistler force plates), EMG on shoulder and back muscles (right anterior deltoid and right erector spinae longissimus), oxygen consumption, and heart rate.

The tool motion was recorded with optical motion capture to evaluate task completion time. The perceived workload was assessed with the NASA Task Load Index. Following the experiment, participants answered a questionnaire and a semi-directed interview was conducted to evaluate technology acceptance.

Field testing

Following validation with the lab study described above, PAEXO was tested with industrial workers in an automotive assembly factory. Four workers wore PAEXO during 20 consecutive workdays. Data were collected for 15 minutes at the beginning and end of each shift, during one week before starting using PAEXO (baseline), and during the first and last week of use. A simpler set of sensors was used to comply with the work requirements. Movements of workers were recorded with a regular camera, and body pose will be extracted using an image-processing library. Heart rate was also recorded during their shift. At the end of the shift, workers answered a technology acceptance questionnaire.

Results and discussion

Comparison of the two conditions in the lab study revealed that muscle activation of anterior deltoid, oxygen consumption and heart rate were significantly reduced when using the exoskeleton, respectively by 55%, 33% and 19%. These results suggest that PAEXO efficiently reduces physical strain and fatigue. Conversely, task performance -assessed by movement duration-, activation of erector spinae and center of pressure movements remained unaffected. Hence PAEXO has no negative side effects neither on the user nor on productivity. Importantly, NASA-TLX scores indicate that the reduction in workload observed with objective measurements was perceived as such by participants (21 % reduction in perceived workload with PAEXO). A modification of the arm movement was observed, with the arm being more abducted when using PAEXO. This modified posture however seems to come from a free choice of participants related to not having to sustain the arm weight anymore, rather than being imposed by the exoskeleton. Indeed, participants mentioned that they did not feel constrained in their movements. Eventually, acceptance score was high and participants all said that they would choose to use the exoskeleton again for such a task.

Conclusions

Assessment of physical, physiological, and psychological aspects in the lab study suggest that PAEXO is a promising solution to reduce shoulder WMSDs among overhead workers. An inverse dynamics analysis is being conducted to estimate joint torques from whole-body kinematics and ground reaction force to complement the present assessment. Data collected during field-testing with industrial workers are currently analyzed to evaluate the impact of PAEXO on real end-users.

Figure 2 :

 2 Figure 2: Experimental set-up and sensors used to evaluate the effects of PAEXO in a lab study.

Acknowledgements

This work has received funding from the EU's H2020 research and innovation program (No. 731540, An.Dy).


Xiangen Wu

Wanjun Lu
email: luwanjuncug@126.com

Wenjia Ou

Marie-Camille Caumon

Jean Dubessy

Temperature and salinity effects on the Raman scattering cross section of the water OH stretching vibration band in NaCl aqueous solutions from 0 to 300 o C

Keywords: quantitative Raman spectroscopy, water, NaCl, Raman scattering cross section, temperature

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

INTRODUCTION

As the most abundant liquid on the earth, water plays an essential role in countless chemical reactions occurring in living and nonliving systems because of its many unique properties. The structure and dynamics of water molecules is rather complex due to the many-body and cooperative effects related to hydrogen bonds and solute solvation. [1] Raman spectroscopy historically has been a powerful tool for studying the structural changes occurring in water caused by temperature, pressure and dissolved ions. Generally, breaking hydrogen bonds by increasing temperature or ion solvation results in a decrease of inter-molecular coupling of OH-stretching vibrations. [9] Many vibrational studies of aqueous alkali halide solutions revealed that the perturbations of the water structure are mainly caused by the halide ions, whereas the alkali cations have relatively negligible effect on the water OH Raman band shape. [10,12,21,[23][24][25][26] Moreover, neutron scattering studies [27] and timeresolved infrared (pump-probe) studies [28,29] indicated that perturbations of the water structure around both anions and cations are largely localized to the first hydration shell.

The OH stretching region of the Raman spectrum is a complex profile of overlapping bands and there is no general consensus in the number of underlying bands and their origin inside the complex OH profile. The OH band profile of non-polarized Raman spectra of liquid water at least consists of three main components, located at around 3260, 3450 and 3600 cm -1 , which indicate different types of hydrogen bonding: multiple bond, single bond and non-bond, respectively. [START_REF] Dubessy | Raman spectroscopy applied to earth sciences and cultural heritage[END_REF] Apart from application of Raman spectroscopy to molecular interactions, quantitative analytical application of Raman spectroscopy has been developed successfully in both laboratory and submarine environment at various temperature-salinity-pressure conditions because of its non-destructive and non-contact advantages. Indeed, the water stretching Raman band is often used as internal standard in the determination of aqueous dissolved species concentration in the molality scale. [24,[31][32][33][34][35][36][37][38][39][40][41][42][43] In our previous works, Ou et al. [39,40] found that the Raman peak area ratio (PAR= ACH4/AH2O) between the stretching bands of methane and water were affected by temperature and salinity, and the effect of dissolved NaCl on the PAR/mCH4 (mCH4 the concentration of CH4 by mol/kg•H2O) became more and more significant with increasing temperature. Nevertheless, the mechanism of the dependence of PAR/mCH4 on temperature and salinity remains unclear.

The area A of a Raman peak over a finite range of wavenumbers ( 1v to 2 v ) can be given by (modified from Wopenka and Pasteris [44] ):

2 2 0 0 1 1 d d v v i v i v i v v i w I v I v A M σ η σ ρ Ω = Ω ∝   (1)
Where 0 v I is the laser irradiance onto the sample, σi the relative differential Raman Scattering Cross Section of species i (which is a measure of the scattering efficiency of a particular vibrational mode of one molecule for given experimental geometry and polarization), η the molar density (mol.cm -3 ), wi the mass fraction of species i in the solution, Mi the molar mass, ρ the mass density of the solution (g.cm -3 ), and Ω the solid angle of collection of the scattered Raman radiations. For two or more species in the same phase of a fluid system, the irradiance and the solid angle of collection of light are the same for all species. [44] Obviously the dependence of the RSCS of the OH stretching vibration band of water on the temperature and dissolved ions is essential for quantifying the effect of temperature and salinity on the PAR/mCH4 when choosing water as internal standard.

Many researchers have attempted to quantify the effect of temperature and dissolved ions on the intensity of the OH stretching vibration band of water since 1960s. [9,15,16,18,19,21] Although having similar effect on the Raman band profile of the OH-stretching vibrations, the increase of temperature and the addition of dissolved halide alkali/calc-alkali salts lead to contrary changes of the intensity of the OH stretching vibration band of water.

The intensity of the OH stretching vibration band of water decreases with increasing temperature, [9,16,18] but increases with the presence of dissolved halide alkali/calc-alkali salts. [8,45,46] The band shape and the intensity of the OH stretching band are significantly affected by the intermolecular coupling [47] and Fermi resonance (FR). [8,9,48] Both Raman experiments [12,49] and quantum calculations [50,51] of ion-water clusters indicate that the RSCS of hydration water can be significantly modified by anions, but is little affected by alkali cations. [23] A study by Raman spectroscopy of 1 M KX(aq) solutions (X= F -, Cl -, Br -and I -) combined with multivariate Raman curve resolution indicates that the Raman cross section (relative to bulk water) of hydration shell water molecules around F -, Cl -, Br -and I -is multiplied by a factor of 0.70 ± 0.1, 1.54 ± 0.26, 1.63 ± 0.27, and 1.83 ± 0.32. [8] The charge transfer from Cl -, Br -and I -ions to the hydrating water also leads to the enhancement of the intensity of the OH stretching band of water. [45,46] However, Wu et al. [51] argues that the polarization effect is responsible for the increased Raman cross section of water in the presence of anions (F -, Cl -, Br -and I -), and that the hypothesis of charge transfer is not valid to explain the Raman intensity variation.

Although numerous studies have been dedicated to the effect of dissolved ions on the RSCS of OH stretching vibration band of water, the experimental conditions were mostly limited to ambient condition. In this study, the

Apparatus and sample preparation procedures

The capillary High-Pressure Optical Cell was used in this study (Fig. 1). [38] Experiments were conducted at ambient condition (20 o C and atmospheric pressure) and in a broad range of temperatures (0 to 300 o C) at 30 MPa.

For the ambient condition experiment, the capillary optical cell (with two open ends) was fixed on a Linkam CAP500 heating-cooling stage to ensure the same position of the laser focus for all analyses. For collecting Raman Spectra, NaCl solutions with different concentrations were injected into the capillary optical cell one by one with a syringe externally linked to one end of the capillary cell (Fig. 1a).

After the measurement of a sample, the capillary optical cell must be evacuated by an empty syringe and then washed at least three times by the next sample before its spectra were collected.

For the experiment at temperatures from 0 to 300 o C at 30 MPa, round cross-sectional flexible fused silica capillary tube was used. The sample preparation procedures were similar to those described in our previous studies, [36,40] including the following steps:

Spectra collection and analysis

Spectra were acquired using a JY/Horiba LabRam HR800 Raman system equipped with a frequency doubled Nd:YAG laser (532.06 nm). The output laser power is 14 mW. The pseudo back-scattering geometry of the experiments is defined in the following way: the laser excitation and Raman scattered radiations are respectively focused and collected with a 50 X long working distance Olympus objective with a 0.5 N.A. A 300 grooves.mm -1 grating together with the 800 mm focal distance of the spectrometer give a spectral resolution around 3 cm -1 .

Spectra were collected in the range of ~1200-4000 cm - 1 , with the same acquisition time (80 s) and 3

accumulations. For a given experimental conditions (NaCl concentration and temperature), the focal position was always optimized to get the highest signal-to-noise ratio and kept the same for the whole spectra collection.

Two or three spectra were collected for each experimental condition. The peak positions and peak area of the OH stretching vibration band of water were determined using software GRAMS/AI (Thermo Galactic).

Determination of the relative RSCS of the OH stretching vibration band of water

The relative RSCS of the OH stretching vibration band of water corresponding to this pseudo backscattering geometry were determined from the ratio between the peak area of the OH stretching bands of water. All the measured peak area should be normalized because Eq.

(1) contains factors other than σ which are specific and constant for our experimental conditions. The parameters such as the laser irradiance of the specimen and the complex geometry of the Raman radiations collection in the case of cylindrical capillary can be canceled after normalization. The molar density of the sample are temperature and salinity-dependent. So the measured peak area was corrected by the molar density of the solution. η was calculated from the ρ based on the model of Mao and Duan [52] by the factor wi/Mi from equation 1.

The relative RSCS of the water OH stretching vibration band of NaCl solutions at ambient conditions were determined by the following equation referencing to the pure water signal at 20 o C at atmospheric pressure:

( , 20 ( , 20 ) ( , 20 , ) , , , ) ) ( , 20 ( , , 20 ) ( , , 20 ) /
( , 20 , 30 ) ( , 20 , 30 ) ( , 20 ( , 30 ) ( , 20 ( , 20 , 30 ) , ( , 2 ) , , 0 , ) 30 )
( , ( , ) ( , 20 ) ( , , 30 ) , 30 , 30 , 30 , 30 ) ) ( , 20 ( , 0 20 , ) 3 ( /

RESULTS

The wavenumbers. This change in the band shape is similar to the effect of increasing temperature on the Raman spectra of water. [9,16,53] It is obvious that the decrease in intensity of the shoulder at around 3260 cm -1 (strongly H-bonded water OH oscillator) is significantly weaker than the increase of the main peak at around 3450 cm -1 (less strongly H-bonded water OH oscillator). It leads to an increase of the overall area of the OH stretching vibration band of water with the concentration of NaCl. where a = 0.000089 × mNaCl 1/2 -0.001165, R 2 = 0.991; b = 0.0355 × mNaCl + 0.9989, R 2 = 1.000;

DISCUSSION

The effect of temperature on the relative RSCS of the OH stretching vibration band of water

As shown in Figure 3, the behavior of the spectra in all series is very similar, which is a clear indication of the significant weakening of the H-bond in all solutions with the increase of temperature. [START_REF] Eisenberg | The structure and properties of water[END_REF] According to the Placzek theory, for freely rotating molecule, when the angle between the incident radiation and the direction of observation is 90 o or 180 o , the expression for the differential Raman cross section [START_REF] Nestor | [END_REF] is:

4 2 ' 2 ' 2 4
(2 ) g ( 45

( ) ( ) d d liquid vapor L d d σ σ = × Ω Ω (8)
The local field correction factor L can be given by equation ( 9) [56] :

2 2 4 0 0 0 ( 2) ( 2) ( 2) 81 81 s s n n n n L n + + + = ≈ (9)
where ns and no are the refractive index of the liquid sample at the wavelength of the Raman scattered radiation and at the wavelength of the exciting radiation.

The internal field factor explains the increase of the Raman intensity in liquid by density effects only. The refractive indexes of liquid water at λ=706.52 nm and λ=650.84 nm from 0 to 300 o C at 30 MPa were both used to calculated L. [57] The calculation shows that the effect of wavelength is weak, so the hypothesis ns=n0 is valid for all the wavenumber range of the profile of the OH stretching vibration. Consequently, in the absence of other processes, a decrease of density resulting from the increase of temperature at constant pressure should result in a decrease in intensity by the same factor for all the OH oscillators at the origin of the profile. The extent of this decrease is calculated in table 2 and is smaller than the experimental values. In addition, the field effect factor cannot account for the modification of the profile of the OH-stretching Raman band.

The statistical factor S, expressed by (1-exp(-hcv/kT) -1 , contains two parameters, the wavenumber of the vibration and temperature. First, this equation shows that the statistical factor is increasing with temperature for a given wavenumber value. Table 2 shows this variation is completely negligible in the range 20-300 °C (less than 0.0001%). In addition, the effect of the wavenumber is also insignificant (the difference for the relative S is less than 0.08% when the wavenumber ranges from 2800 to 3700 cm -1 ).

Therefore, the local field factor can only explain partially the variation of the RSCS of the OH-stretching vibration and of the band profile for pure water at 300 °C. It is reasonable to consider that this temperature effect in pure water solutions also applies to NaCl-bearing aqueous solutions.

According to equation (7) the RSCS also depends on the derivatives of the polarizability tensor invariants, a'j 2 and γ' 2 . The quantity gj(45a'j 2 +7γ' 2 ), referred to as the scattering activity, was found to be phase independent in most cases, except for the hydrogen bonded liquids. [START_REF] Nestor | [END_REF]58] Rull and de Saja [11] studied the isotropic and anisotropic components of the OH stretching band as a function of solute concentration (types AB and AB2, A=Li, Na, K, Ca;

B=Cl, Br, I, NO3) and temperature. They found that the isotropic part 45a'j 2 does not vary with the concentration of NaCl but the anisotropic part 3γ' 2 increases with NaCl concentration. Based on previous studies, [9,59,60] Rull and de Saja concluded that both the isotropic and anisotropic parts decrease with temperature. Abe and Ito [45] studied the effect of hydrogen bonding on the Raman intensity of methanol, ethanol and water. They attributed the observed intensity enhancement of the OH stretching band when changing from gas to liquid to the additional contribution of the charge transfer electronic excited state arising from hydrogen bond formation. But so far it is still impossible to calculate from ab initio models to what extent the degree of hydrogen bonding in the system affects the Raman intensity of the stretching band and its profile. Intra-and inter-molecular interaction and FR due to the hydrogen bond make it much more complex.

The effect of dissolved NaCl on the relative RSCS of the water OH stretching vibration band at different temperatures

The increase of the RSCS of the OH-stretching vibration band of water with salt concentration at ambient conditions was evidenced in many studies [8,12,23,46] and is confirmed in this study. The anisotropic environment of water molecules in an aqueous electrolyte is likely to reduce the intermolecular coupling and FR, which in turn would alter the RSCS of water. [61,62] Moreover, the RSCS of the OH-stretching vibration band of water increases with the size of the halide ions. [8,46] Mondal et al. [46] attribute the increased Raman intensity of water to the charge transfer from the halide ions to the combined antibonding orbitals of hydrating water. However, the study from Wu et al. [51] indicates that it is the polarization effect that makes the main contribution.

The influence of dissolved NaCl on the RSCS of the OHstretching vibration band of water at high temperature is also revealed in this study (Fig. 4). Figure 4 The molar density-and intensity-corrected Raman spectra of NaCl aqueous solutions with various NaCl concentrations at different temperatures may provide some important information for this phenomena (Fig. 5).

In spite of the extent of the ionic perturbation to the local structure still being controversial, the impact of ions on the water structure due to the interaction between the Na + and Cl -and water molecules in the ionic hydration shells has been confirmed by many studies. [63][64][65][66] However, Na + has only a negligible effect on the OH stretching vibration band of water and the spectral change of the water OH stretching vibration band of water was mainly attributed to the perturbation of water molecules in the hydration shell of Cl -. [10,12,21,23] As shown in Figure 5, at temperature below 160 o C, the intensity of the shoulder around 3260 cm -1 decreases with the addition of NaCl. That of the intensity maximum around 3450 cm -1 increases and slightly shifts towards higher wavenumbers. These variations are usually attributed to the conversation of strong H-bonded water to weakly H-bonded water by dissolved NaCl. [46] When the temperature increases at around 160 o Cthe position of the shoulder and main peak almost both remains the same with the addition of NaCl.

Then, a reverse trend occurs at temperatures above 160 °C (300 °C is selected for an example): with the addition of NaCl, the intensity of the shoulder increases obviously and that of the main peak varies little.

Meanwhile, the position of the main peak shifts to lower wavenumbers with increasing NaCl concentration. This reverse trend indicates that the H-bonding energy in NaCl aqueous solutions is higher than that in pure water at higher temperatures.

These observations may result from changes in dissolved ions speciation with temperature (Na + and Cl -solvation and NaCl° ion pair formation), modifying the structure of water. Raising both the concentration of NaCl [65] and the temperature [67] will promote the formation of NaCl° ion pair. So the ion-ion interactions may play an important role in determining the structure and properties of electrolyte solutions at high temperatures. [68,69] Bondarenko et al. [67] found that the number of O-H As an example, we have chosen the NaCl-CH4-water system to study the effect of temperature-and salinity-dependent RSCS of the OH stretching vibration band of water on the PAR/mCH4 from Ou et al. [39,40] PAR/mCH4 can be represented by the following equation: [39,40] resulting from the underestimation of the value of ACH4 (Fig. 6). So the area difference between the spectrum of methane-bearing solution from Ou et al. [39,40] and the spectrum of the solution without methane at the same temperature and salinity from this study were employed to calculated the ACH4. And then the new PAR/mCH4 were obtained in this study for further comparison and discussion. al. [39,40] is still valuable only if the same integration method of CH4 peak is taken with them.

SUMMARY AND CONCLUSIONS

We

  Temperature and salinity effects on the Raman scattering cross section of the water OH-stretching vibration band in NaCl aqueous solutions from 0 to 300 °C Xiangen Wu, Wanjun Lu, Wenjia Ou, Marie-Camille Caumon, Jean Dubessy To cite this version: Xiangen Wu, Wanjun Lu, Wenjia Ou, Marie-Camille Caumon, Jean Dubessy. Temperature and salinity effects on the Raman scattering cross section of the water OH-stretching vibration band in NaCl aqueous solutions from 0 to 300 °C. Journal of Raman Spectroscopy, 2017, 48 (2), pp.314-322. 10.1002/jrs.5039. hal-02347839

  quantitative temperature and NaCl dependence of the relative RSCS of OH stretching vibration band of water in a wide temperature range from 0 to 300 o C at 30 MPa was established for the first time. The new relative RSCS of OH stretching vibration band of water was then used to quantify the dependence on temperature and NaCl concentration of PAR/mCH4 in the NaCl-CH4-H2O system. EXPERIMENTAL Materials Water was ultrapurified in the laboratory with resistivity of 18.24 MΩ.cm. Aqueous solutions of different NaCl concentrations (0, 1, 2, 3 and 5 mol/kg•H2O) were prepared. All the reagents were analytical grade without further purification.

( 1 )

 1 load an aqueous solution column (15 mm long) into a large capillary tube (665 μm OD and 300 μm ID for pressure less than or equal to 50 MPa) with one end closed; (2) insert a small tube (200 μm OD and 75 μm ID) to a certain length (10 mm) away from the end of the large tube to inject a mercury column (approximately 5 mm in length) into the aqueous solution. (3) The capillary highpressure optical cell containing a NaCl solution at a specific concentration was connected to a high pressure line (Fig. 1b) and fixed at the experiment table to ensure that the capillary optical cell does not move during measurements at different temperatures. The pressure in the cell was maintained by water in the line and adjusted by a pressure generator, and read from a Seta 204D digital pressure transducer equipped with a Datum 2000 TM manometer (69 MPa full scale; accurate to 0.14%). The temperature of the solution in the cell was maintained by a Linkam CAP500 heating-cooling stage with an accuracy of ± 0.3 K.

  the relative RSCS of the OH stretching vibration band of water at temperature from 0 to 300 o C at 30 MPa can be calculated by the following equation, and the condition T = 20 o C at 30 MPa for pure water was chosen as a reference in this study:

Figure 2

 2 Figure 2 displays the molar density-corrected spectra of the OH stretching vibration band of water for different NaCl solutions at 20 o C at atmospheric pressure. With increasing concentration of NaCl, the intensity of the shoulder around 3260 cm -1 decreases, the intensity of the main peak around 3450 cm -1 increases. Dissolved NaCl reduces the width of the OH stretching band and causes the band position shift toward higher Raman shift

20 oC 5 )Figure 3

 2053 Figure 3 displays the molar density-and intensitycorrected spectra of the OH stretching vibration band of water for different NaCl solutions from 0 to 300 o C at 30 MPa.Similar to the effect of dissolved NaCl under ambient condition, the position of the shoulder at around 3260 cm - 1 remains unchanged and the intensity decreases with increasing temperature. The temperature dependence of the maximum peak above 3450 cm -1 is much different from that of the shoulder (Fig.3). First of all, the position of the peak maximum shifts to higher wavenumbers with increasing temperature. Secondly, the variation of its intensity with temperature is much more complicated than that of the shoulder. Specifically, for pure water, the maximal intensity of the band remains almost unchanged up to 140 o C and then increases unambiguously with rising temperature. For the 3 m NaCl solution, the maximal intensity of the band remains almost the same within the whole temperature range. The maximal intensity of the band for the 5 m NaCl solution decreases quite slightly up to 240 o C and then increases slightly from 240 to 300 o C. After normalization by molar density and by the intensity correction factor β, the relative RSCS of the OH stretching vibration band of water based on the reference condition T = 20 o C for pure water at 30 MPa (σ(mNaCl, T, 30 MPa)/σ(Pure water, 20 o C, 30 MPa)) is determined for this Raman scattering geometry (Fig. 4). Three conclusions can be drawn: (1) At constant NaCl concentration, the relative RSCS of the OH stretching vibration band of water decreases linearly with increasing temperature; (2) At constant temperature, the RSCS of the OH stretching vibration band of water increases with the dissolved NaCl concentration within the whole temperature range; (3) The enhancement factor of the RSCS of the OH stretching vibration band of water by dissolved NaCl increases with rising temperature. σ(mNaCl, T, 30 MPa)/σ(Pure water, 20 o C, 30 MPa) can be represented as a function of temperature (T in °C) and mNaCl (mol/kg•H2O): σ(mNaCl, T, 30 MPa)/σ(Pure water, 20 o C, 30 MPa) = f (T, mNaCl) = a(T-20) + b (6)

  Where bj2 is the zero point amplitude of the jth vibrational mode, gj the vibrational degeneracy, the derived polarizability tensor, vo the wavenumber of the exciting radiation in m -1 , vj the wavenumber in m -1 of the vibrational mode j, and h, c, k, and T the Plank constant, the speed of light, the Boltzmann constant, and absolute temperature (K) respectively. Although the Placzek theory has been applied nearly universally, experimental data show an increase of the RSCS by a factor L from low density vapor to liquid density for which molecules are not randomly oriented and freely rotating: this intensity increase is named the internal field effect:

  shows that the amplitude of the enhancement of the RSCS of water by dissolved NaCl increases with temperature. The RSCS (relative to pure water) of the OH stretching vibration band of water for 1 m, 2 m, 3 m, 5 m NaCl solutions were 1.04, 1.07, 1.11 and 1.18 20 o C and then increase to 1.08, 1.13,1.21 and 1.34 at 300 o C, respectively.

  •••Cl - bonds decreases partially because of ion pairing at increasing temperature. So the water structure in the solutions become closer to those of pure water at high temperature, looking like a mixture of water and NaCl° dipoles. Our result also shows that the perturbation by NaCl of the local structure of water decreases with increasing temperature. As a result, with the decreasing of the Cl -concentration as a consequence of NaCl° ion pair formation with increasing temperature, the effect of charge transfer should be weakened. This is contrary to the increase of the enhancement factor of the RSCS of water by dissolved NaCl with increasing temperature. The implication of the variation of the RSCS of the OH stretching vibration band of water for temperature and salinity dependence of Raman quantitative factors

Figure 7

 7 Figure 7 shows the experimental data of PAR*/mCH4 and the σ(mNaCl, 20 o C, 30 MPa)/σ(mNaCl, T, 30 MPa) calculated by the equation (6) from this study. Apparently the variation trend of PAR*/mCH4 is identical to that of σ(mNaCl, 20 o C, 30 MPa)/σ(mNaCl, T, 30 MPa) ratio, increasing with temperature and decreasing with the presence of NaCl. This implies that the dependence of the PAR*/mCH4 on temperature and NaCl concentration is mainly caused by the variation of the RSCS of the OH stretching vibration band of water with increasing temperature and the addition of NaCl salts. The RSCS of the symmetric vibration band of dissolved methane is almost independent on temperature and dissolved NaCl concentration. The PAR with constant dissolved CH4 concentration can be increased by about 47%, 34% and 29% by the decreasing RSCS of the OH stretching vibration band of water when the temperature

  investigated temperature-and salinity-dependence of the RSCS of the OH stretching vibration band of water, and found it linearly decreases with increasing temperature whereas the addition of NaCl at constant temperature increases its value. The influence of dissolved NaCl on the RSCS of the OH stretching vibration band of water becomes more and more significant with rising temperature. The temperature-and salinity-dependence of the relative RSCS of the OH stretching vibration band of water mainly accounts for the dependence on temperature and salinity of the PAR/mCH4 in CH4 homogeneous solutions. When temperature increases from 20 to 300 o C, the RSCS of the OH stretching vibration band of water would induce about 47%, 34% and 29% error for the determined concentration of dissolved CH4 (in mol/kg•H2O) by Raman spectroscopy using water as internal standard for pure water, 3 m NaCl and 5 m NaCl aqueous system, respectively. The concentration error of dissolved CH4 brought about by the addition of dissolved NaCl becomes more and more obvious with rising temperature. Equation (6) can be used to quantify such effects of temperature and NaCl concentration.The study of the role of the derivatives of the polarizability tensor invariants, a'j 2 and γ' 2 , which vary with the variation of hydrogen bonding with temperature and salt ions through molecular interactions, is essential for the interpretation of the temperature and dissolved ions dependence of the RSCS of the OH stretching vibration band of water. The temperature-and salinitydependences of Raman spectra of isotopically diluted water which is free from intramolecular and intermolecular coupling and FR may be promising for studying the degree of the impact of these factors on the RSCS of the OH stretching vibration band of water.

Figure 1 .

 1 Figure 1. Schematic diagram of the capillary optical cell system for collecting the Raman spectra of NaCl solutions.

Figure 2 .

 2 Figure 2. The molar density-corrected spectra of the OH stretching vibration band of water for different NaCl solutions at 20 o C at atmospheric pressure.

Figure 3 .

 3 Figure 3. Molar density-and intensity-normalized spectra of the OH stretching vibration band of water for different NaCl solutions from 0 to 300 o C at 30 MPa.

Figure 4 .

 4 Figure 4. The relative RSCS of the OH stretching vibration band of water (σ(mNaCl, T, 30 MPa)/σ(Pure water, 20 o C, 30 MPa)) at 30 MPa as a function of temperature and NaCl concentration.

Figure 5 .

 5 Figure 5. Comparison of the Raman spectra for different NaCl solutions at different temperatures.

Figure 6 .

 6 Figure 6. Variations of the PAR*/mCH4 and the σ(mNaCl, 20 o C, 30 MPa)/σ(mNaCl, T, 30 MPa) ratio as a function of temperature and NaCl concentration. The blank symbols represent PAR*/mCH4. The solid symbols are the σ(mNaCl, 20 o C, 30 MPa)/σ(mNaCl, T, 30 MPa). The color of back, red and blue represent pure water, 3 m NaCl and 5 m NaCl aqueous system, respectively.

Figure 5 13Figure 6

 56 Figure 5

  )

	σ	OH σ	OH mNaCl Pure water	T	o	C	MPa MPa	β = ×	A η	mNaCl T mNaCl T	MPa MPa	A η	Pure water Pure water	o o	C C	MPa MPa

Table 1

 1

	is the measured peak area
	and the determined relative RSCS of OH stretching
	vibration band of water based on the reference condition
	for pure water at 20

o C at atmospheric pressure (σ(mNaCl,

Table 1 .

 1 The A, η and σ(mNaCl, 20 o C, Atm)/σ(Pure water, 20 o C, Atm) for different NaCl solutions at 20 o C at atmospheric pressure

		A (Arbitr. Units)	η (mol/cm 3 )	A/η	σ(mNaCl, 20 o C, Atm) /σ(Pure water, 20 o C, Atm)
	Pure water	14500000	0.0555	261468028	1.000
	1 m NaCl	14712500	0.0545	270070777	1.0329
	2 m NaCl	14950000	0.0534	279788054	1.0701
	3 m NaCl	15160000	0.0524	289506620	1.1072
	5 m NaCl	15450000	0.0502	307532726	1.1762

Table 2

 2 variation of the relative RSCS of the water OH stretching vibration band derived from experiment calculation and theoretical calculation.

	T ( o C)	LT/L20 o C	ST/S20 o C	LST/LS20 o C	σ(Pure water, T, 30 MPa)/σ(Pure water, 20 o C, 30 MPa)
	0	1.0015962	1.0000000	1.0015962	1.0331945
	20	1.0000000	1.0000000	1.0000000	1.0000000
	40	0.9968851	1.0000000	0.9968851	0.9749414
	60	0.9927034	1.0000000	0.9927034	0.9465141
	80	0.9876791	1.0000000	0.9876791	0.9247645
	100	0.9819755	1.0000000	0.9819755	0.8959325
	120	0.9756725	1.0000000	0.9756725	0.8760990
	140	0.9688376	1.0000001	0.9688376	0.8369854
	160	0.9614911	1.0000002	0.9614911	0.8251367
	180	0.9536307	1.0000004	0.9536307	0.8029974
	200	0.9452436	1.0000007	0.9452436	0.7816258
	220	0.9362839	1.0000013	0.9362839	0.7530525
	240	0.9266968	1.0000022	0.9266968	0.7306089
	260	0.9163743	1.0000036	0.9163743	0.7223460
	280	0.9051473	1.0000057	0.9051473	0.7025627
	300	0.8927894	1.0000087	0.8927894	0.6803223

ACKNOWLEDGEMENTS

We thank Dr. I-Ming Chou and Dr. R.C. Burruss for their kind help and guidance for many years on the Raman spectroscopic research on Geo-fluids. This work was partly supported by the National Sciences Foundation of China (No. 41102154, 41176047), the Key Project of Chinese Ministry of Education (No. 109108), and the Programme of Introducing Talents of Discipline to Universities (No. B14031) by Ministry of Education. This study was also partly supported by CNRS, Université de Lorraine and ANR Program (SEED, CGS Microlab).


Pauline Maurice

Vincent Padois

Yvan Measson

Philippe Bidaud

Assessing and improving human movements using sensitivity analysis and digital human simulation

published or not. The documents may come

Introduction

Enhancing the performance of technical postures or movements at work, in sports or in rehabilitation is of great concern for humans, and aims both at improving operational results and at reducing biomechanical demands on the body. Advances in human biomechanics and modeling tools allow to evaluate human performance with more and more details using digital human models [1]. However, the reliability of these force-related biomechanical measurements is questionable because most mappings of motion capture data onto a digital model do not ensure the dynamic consistency of the resulting motion [2]. Then, once an existing movement is evaluated, finding the right modifications to improve the performance is still addressed with extensive trial-and-error processes.

Methods

We propose a framework to easily and reliably assess the performance of a technical movement, and automatically provide recommendations to improve its performance. An optimization-based whole-body controller is used to track motion capture data in operational space while imposing dynamic and biomechanical constraints. Existing movements can thus be evaluated. Our method guarantees the dynamic consistency of the resulting motion (i.e. that the motion and force respect the laws of physics), without requiring the ground reaction force (GRF) measurement as an input. Digital human simulations are then automatically created and run (without motion capture) to estimate the performance when the movement is performed in many alternative ways. Sensitivity indices are thereby computed to quantify the influence of postural parameters on the performance. Critical parameters can thus be identified and tuned, using only little input data. Based on these results, recommendations for posture improvement are provided.

Results

The proposed method was validated on a drilling activity. 5 participants performed 10 trials of a drilling movement while their motion, drilling force and GRF were recorded. The tacking error of the replayed motion was smaller than 3 cm and the 6 components of the GRF computed in simulation closely matched their experimental counterparts (Pearson's correlation coefficient between 0.72 and 0.98). 9 postural parameters were varied to create 11601 different simulations of the drilling movement. The sensitivity analysis identified 3 parameters as crucial for the performance, and determined their optimal values. Compared to the original movement, the optimized movement significantly improved 5 performance indicators out of 6, while 1 was only slightly worsened.

Discussion

The good match between the experimental and replayed motion and GRF confirms the relevance of the dynamic replay method. Moreover, the significant increase in performance of the optimized movement shows the usefulness of the digital human simulation-based sensitivity analysis for quickly and easily identifying ways to improve a technical movement.


Neville Hogan
email: neville@mit.edu

Dagmar Sternad
email: d.sternad@northeastern.edu

Predictability, Effort and (Anti-)Resonance in Complex Object Control

Pauline Maurice1 and Neville Hogan 2 and Dagmar Sternad 3

I. INTRODUCTION

Manipulation of complex objects as in tool use is ubiquitous in everyday life and has given humans an evolutionary advantage. Yet there has been little research on complex interactive skills and control principles for such actions remain elusive. To gain insight into the control of complex objects, the present study examined the strategies that humans choose when rhythmically manipulating an object with internal dynamics, such as carrying a cup of coffee. The dynamics of the object can render the temporal evolution of the system complex, possibly even chaotic, and hence difficult to predict.

II. METHOD

A cart-and-pendulum model mimicking coffee sloshing in a cup was implemented in a virtual environment with a haptic interface. Ten participants rhythmically manipulated a virtual cup containing a ball; they were free to choose the frequency of oscillation, while the amplitude was imposed. To evaluate the strategies that humans adopted we mathematically examined the cart-and-pendulum system coupled to a model of hand impedance (Fig 1). The equations of motion of the model are

(m c + m p ) Ẍ = m p d θ2 sinθ -θcosθ + F inter θ = - Ẍ d cosθ - g d sinθ F inter = F input -K X -X des -B Ẋ -Ẋd es (1)
with X the cart position and m c its mass, θ the pendulum angle, m p its mass and d its length, K and B the hand stiffness and damping, F inter the interaction force between the cart and the human hand, and F input the force required to follow a desired trajectory (X des , Ẋdes ). This task-based approach allowed us to evaluate alternative execution strategies, i.e. different values of frequency and hand impedance that could be used to perform the task. This study examined two hypotheses: 1) humans increase predictability of the object dynamics, measured by the mutual information between the interaction force and the object dynamics, 2) humans decrease interaction forces between hand and object. Predictability of the object dynamics was characterized by the mutual information between the input and the output of the system, i.e. the cart trajectory φ(t) = arctan Ẋ/(2πf X) and the interaction force

F inter M I(φ, F inter ) = p (φ, F inter ) ln p (φ, F inter ) p (φ) p (F inter ) dφ dF inter
The force required to perform the task was estimated by the root mean square of the continuous interaction force

RM SF (F inter ) = 1 T T 0 F 2 inter (t) dt
where T is the duration of the task.

III. RESULTS

First analysis revealed that humans chose one of two distinct strategies, either with low or high frequency, respectively. Mathematical analyses revealed that both strategies adopted by participants generated highly predictable behavior, but did not reduce interaction forces. Fig 2 shows a 2D contour map of the result variables, plotted for a constant value of hand damping B = 10N.s/m. The result space for MI contains one area of very low predictability for frequencies around 0.8 Hz. This area coincides with an area where the interaction force is low; therefore the two hypotheses are exclusive. Reciprocally, for frequencies around 0.64 Hz and higher that 1.20 Hz, predictability is high, but interaction force is high as well. Participants' strategies are largely clustered in the pink areas, consistent with Hypothesis 1. The same set of points plotted in panel B did not lie in the green areas, counter to Hypothesis 2. Fig. 2. 2D maps of the mutual information MI between the cart trajectory and interaction force (A) and of the root mean squared interaction force RMSF (B) in the result space spanned by two of the execution variables: f and K. The hand damping B was fixed at 10N.s/m. The blue dots represent the strategies (f, K) adopted by participants in the behavioral experiment. The trials represented are those where the hand damping was 8 < B < 12N.s/m. The darker dots correspond to trials for which the impedance fit was good; the lighter dots are trials where the impedance fit is not very good.

IV. CONCLUSION

These results demonstrate that in complex object manipulation, humans do not prioritize effort, but rather seek strategies where interactions are predictable. In addition, predictability was found to be closely related to the resonance and anti-resonance structure of the system. This finding highlights that physical interactions with complex objects, present in numerous daily activities, introduce new challenges and engage different control principles than identified in the manipulation of rigid objects or in unconstrained movements.

Fig. 1 .

 1 Fig. 1. Model used to analyze the dynamics of the task in simulation: Forward dynamics of the cart-and-pendulum system coupled to a model of hand impedance.

Pauline Maurice is with the

ACKNOWLEDGMENT Dagmar Sternad was supported by the National Institutes of Health R01-HD087089, R01-HD081346, and R21-DC013095 and the National Science Foundation NSF-NRI 1637854 and NSF-EAGER-1548514. Neville Hogan was supported by NIH R01-HD087089, NSF-NRI 1637814, NSF-EAGER-1548501 and by the Eric P. and Evelyn E. Newman fund.


Pauline Maurice

Jernej Camernik

Dasa Gorjan

Benjamin Schirrmeister

Jonas Bornmann

Luca Tagliapietra

Daniele Pucci

Serena Ivaldi

Jan Babič

Objective and Subjective Effects of Passive Exoskeleton on Overhead Work

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

Objective and Subjective Effects of Passive Exoskeleton on Overhead Work

Pauline Maurice1 * , Jernej Čamernik2 * , Daša Gorjan 2 , Benjamin Schirrmeister 3 , Jonas Bornmann 3 , Luca Tagliapietra 4 , Daniele Pucci 4 , Serena Ivaldi 1 , Jan Babič 2

I. INTRODUCTION

Work-related musculoskeletal disorders (MSDs) are the first cause of occupational disease in developed countries and therefore represent a major health issue [START_REF] Parent-Thirion | Fifth european working conditions survey, publications office of the european union[END_REF]. MSDs develop when biomechanical demands exceed the worker's physical capacity. In this regard overhead work is often cited as a MSDs risk factor [START_REF] Grieve | Overhead work: Identification of evidence-based exposure guidelines[END_REF][START_REF] Sluiter | Criteria document for evaluating the work-relatedness of upper-extremity musculoskeletal disorders[END_REF]. Overhead work yet remains very common on assembly lines, especially in the automotive industry. Indeed many complex tasks cannot be fully automatized because they still require human cognitive skills. One solution to relieve workers while keeping them in control of the task execution is then to assist them with an exoskeleton [START_REF] De Looze | Exoskeletons for industrial application and their potential effects on physical work load[END_REF].

Recently several industrial exoskeletons have been developed to support arms and/or tool weight during overhead tasks [START_REF] Sylla | Ergonomic contribution of able exoskeleton in automotive industry[END_REF][START_REF] Rashedi | Ergonomic evaluation of a wearable assistive device for overhead work[END_REF][START_REF] Spada | Analysis of exoskeleton introduction in industrial reality: Main issues and eaws risk assessment[END_REF][START_REF] Theurel | Physiological consequences of using an upper limb exoskeleton during manual handling tasks[END_REF][START_REF] Otten | Evaluation of a novel active exoskeleton for tasks at or above head level[END_REF][START_REF] Huysamen | Evaluation of a passive exoskeleton for static upper limb activities[END_REF][START_REF] Weston | Biomechanical evaluation of exoskeleton use on loading of the lumbar spine[END_REF]. Many of them showed promising results regarding the reduction of physical workload. However, these studies present only partial assessments of the benefit provided by the exoskeletons. They exclude some important aspects like side-effects, adaptation, or user acceptance. In this work we present an exhaustive assessment of a novel passive exoskeleton for overhead work.

II. METHOD

The benefit provided by the use of an exoskeleton cannot be assessed solely based on the reduction of effort in the targeted limb. An exoskeleton is a wearable device, therefore its use might disrupt human movements or require additional effort. Supplemental effort can be caused by the weight of the device or, with passive exoskeletons, by the transfer of force from one joint to another. In addition, users' opinion of the device also affects its effectiveness. An exoskeleton that is ill-perceived by the user might remain unused, or cause psychological stress if use is imposed. Therefore we propose an assessment process that addresses the following aspects:

• Task performance: The task performance should be at least as good with the exoskeleton as without it.

• Fatigue: The exoskeleton should reduce metabolic demand and delay the apparition of fatigue.

• Physical effort: The exoskeleton should relieve the limb that is directly impacted.

• Side-effects: The exoskeleton should not significantly increase effort in limbs that are not directly impacted, nor cause bad postures.

• Adaptation: Using the exoskeleton should not require a long training nor cause after-effects at removal.

• Acceptance: Users should feel better when using the exoskeleton compared to when not using it.

A. Exoskeleton Description

Within the European project AnDy [START_REF] Ivaldi | Anticipatory models of human movements and dynamics: the roadmap of the andy project[END_REF], the provided exoskeleton prototype is an upper-limb passive exoskeleton intended for supporting the weight of the arms, and possibly of manipulated tools, while the user is working overhead. This exoskeleton does not enhance the human's strength, but renders his/her arms virtually weightless, thereby relieving the shoulder joint. Being passive, hence without motors, the exoskeleton is light, not bulky, and easy to wear.

B. Experiment

Twelve participants performed an overhead pointing task with a portable tool, with and without the exoskeleton (Fig. 1). The participants' physical and physiological state was monitored with whole-body inertial motion capture, ground reaction force, EMG on shoulder and back muscles (right anterior deltoid and right erector spinae longissimus), oxygen consumption, and heart rate. The tool motion was recorded with optical motion capture to evaluate accuracy and completion time. Following the experiment, the perceived workload was assessed with the NASA Task Load Index (NASA-TLX) [START_REF] Hart | Development of nasa-tlx (task load index): Results of empirical and theoretical research[END_REF]. In addition, participants answered a questionnaire and a semi-directed interview was conducted to evaluate technology acceptance.

C. Measures a) Task performance: Task performance was assessed with the movement accuracy and completion time.

b) Fatigue: Oxygen consumption and heart rate were used to evaluate objective metabolic demand and fatigue, while the NASA-TLX indicated subjective fatigue. Evolution of task performance over time was used as an additional indicator for fatigue. c) Physical Effort: Given that the exoskeleton aimed at supporting the arms weight, the shoulder joint was directly impacted by the use of the exoskeleton. Therefore, activation of the anterior deltoid and estimated shoulder torque were used to assess the physical demand on the impacted limb. Joint torques were computed with inverse dynamics based on the recorded whole-body kinematics and ground reaction force [START_REF] Latella | Whole-body human inverse dynamics with distributed microaccelerometers, gyros and force sensing[END_REF]. d) Side-effects: Activation of erector spinae and back and hip torques were used to assess potential increase in effort in non-directly impacted limbs. Joint angles obtained by whole-body kinematics served to evaluate postural changes. e) Adaptation: The tool 3D trajectory as well as trajectories of the shoulder, elbow and back in joint space were used to compare movement strategy with and without the exoskeleton. Evolution of task performance over time was used to detect learning and after-effects. f) Acceptance: Score obtained in the technology acceptance questionnaire was used to quantitatively assessed acceptance of the exoskeleton, while opinions expressed during the interview served to shed light on some of the questionnaire answers.

III. RESULTS

Comparison of the two conditions with and without exoskeleton revealed that muscle activation, oxygen consumption and heart rate were significantly reduced when using the exoskeleton. Conversely, task performance was affected neither positively nor negatively. Importantly, the reduction in overall workload observed with objective measurements was also observed in subjective measurements: the task not only was, but also felt, less demanding when wearing the exoskeleton. Eventually, acceptance score was high and participants all said that they would choose to use the exoskeleton again for such a task.

IV. CONCLUSION

Future work will be directed towards evaluating the exoskeleton on different tasks, including bending, crouching and walking to assess its transparency and potential disturbances of the users movements. Experiments on industrial sites are also planned. Furthermore, results from the evaluation will serve to guide the development of an intuitive adaptation of the level of support provided by the exoskeleton.

Fig. 1 .

 1 Fig. 1. Experimental set-up and sensors used to assess the exoskeleton.

Université de Lorraine, CNRS, Inria, LORIA, F-54000 Nancy, France pauline.maurice@inria.fr

Jožef Stefan Institute, Jamova cesta

39, SI-1000 Ljubljana, Slovenia 3 Otto Bock SE & Co. KGaA, Duderstadt, Germany 4 Fondazione Istituto Italiano di Tecnologia, via Morego 30, Genova, Italy * P. Maurice and J. Čamernik contributed equally to this work.

ACKNOWLEDGMENT

This work has received funding from the European Union's Horizon 2020 research and innovation programme under grant agreement No. 731540 (An.Dy). The content of this publication is the sole responsibility of the authors. The European Commission or its services cannot be held responsible for any use that may be made of the information it contains.


Adrien Malaisé

Pauline Maurice

Francis Colas

Serena Ivaldi

Online Human Activity Recognition for Ergonomics Assessment

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

Online Human Activity Recognition for Ergonomics Assessment

Adrien Malaisé1 , Pauline Maurice 1 , Francis Colas 1 and Serena Ivaldi [START_REF] Schneider | Osh in figures: occupational safety and health in the transport sectoran overview[END_REF] We address the problem of recognizing the current activity performed by a human worker, providing an information useful for automatic ergonomic evaluation of workstations for industrial applications.Traditional ergonomic assessment methods rely on pen-and-paper worksheet, such as the Ergonomic Assessment Worksheet (EAWS). Nowadays, there exists no tool to automatically estimate the ergonomics score from sensors (external cameras or wearable sensors).

As the ergonomic evaluation depends of the activity that is being performed, the first step towards a fully automatic ergonomic assessment is to automatically identify the different activities within an industrial task. To address this problem, we propose a method based on wearable sensors and supervised learning based on Hidden Markov Model (HMM). The activity recognition module works in two steps. First, the parameters of the model are learned offline from observation based on both sensors, then in a second stage, the model can be used to recognize the activity offline and online.

We apply our method to recognize the current activity of a worker during a series of tasks typical of the manufacturing industry. We recorded 6 participants performing a sequence of tasks using wearable sensors.Two systems were used: the MVN Link suit from Xsens and the e-glove from Emphasis Telematics (See Fig. 1). The first consists of 17 wireless inertial sensors embedded in a lycra suit, and is used to track the whole-body motion. The second is a glove that includes pressure sensors on fingertips, and finger flexion sensors. The motion capture data are combined with the one from the glove and fed to our activity recognition model.

The tasks were designed to involve elements of EAWS such as load handling, screwing and manipulating objects while in different static postures. The data are labeled following the EAWS categories such as "standing bent forward", "overhead work" or "kneeling".

In terms of performances, the model is able to recognize the activities related to EAWS with 91% of precision by using a small subset of features such as the vertical position of the center of mass, the velocity of the center of mass and the angle of the L5S1 joint. We presented this work at ACHI 2018 (The Eleventh International Conference on Advances in Computer-Human Interactions).

Université de Lorraine, CNRS, Inria, LORIA, F-54000 Nancy, France adrien.malaise@inria.fr a. b.Fig. 1. Wearable sensors used in the experiment: (a) XSens MVN suit; (b) e-glove from Emphasis Telematics.