Interplay between viscosity and elasticity in freely expanding liquid sheets

We investigate the dynamics of freely expanding liquid sheets prepared with fluids with different rheological properties, (i) viscous fluids with a zero-shear viscosity $\eta_0$ in the range $(1-1000) \, \mathrm{mPa.s}$ and (ii) viscoelastic Maxwell fluids whose elastic modulus, $G_0$, characteristic relaxation time, $\tau$, and zero-shear viscosity, $\eta_0=G_0 \tau$, can be tuned over several orders of magnitude. The sheets are produced by impacting a drop of fluid on a small cylindrical solid target. For viscoelastic fluids, we show that, when $\tau$ is shorter than the typical lifetime of the sheet ($\sim 10$ ms), the dynamics of the sheet is similar to that of Newtonian viscous liquids with equal zero-shear viscosity. In that case, for little viscous samples ($\eta_0 <\sim 30$ mPa.s), the maximal expansion of the sheet, $d_{\rm{max}}$, is independent of $\eta_0$, whereas for more viscous samples, $d_{\rm{max}}$ decreases as $\eta_0$ increases. We provide a simple model to account for the dependence of the maximal expansion of the sheet with the viscosity that is based on an energy balance between inertia, surface tension and viscous shear dissipation on the solid target, and which accounts well for our experimental data. By contrast, when $\tau$ is longer than the typical lifetime of the sheet, the behavior drastically differs. The sheet expansion is strongly enhanced as compared to that of viscous samples with comparable zero-shear viscosity, but is heterogeneous with the occurrence of cracks, revealing the elastic nature of the viscoelastic fluid.Comment: 10 pages, 9 figure

Gouttes, perles et filaments de gel sous déformations extrêmes

We investigate the behavior of transient self-assembled and permanently crosslinked gels subjected to extreme mechanical stresses. On the one hand, we study the dynamics of freely expanding sheets produced by impacting a (Newtonian or viscoelastic) liquid drop or a bead of polymeric gel on solid surfaces in minimal dissipation conditions, achieved using either a small solid target or a repellent surface. Upon impact, the drop or bead is transformed into a thin sheet that expands and retracts due to elasticity. For viscoelastic fluids with a relaxation time smaller than the typical lifetime of the sheet, the dynamics of the viscoelastic sheet is similar to that of Newtonian liquids with equal viscosity. The maximal expansion of the sheet decreases with the viscosity and is quantitatively modeled using an energy balance between inertia, surface tension and viscous shear dissipation on the solid target that can be accounted by measuring an effective velocity of the expanding sheet at short time scale. We further show that the shear dissipation can be substantially eliminated by performing impact experiments on a solid surface based on an inverse Leidenfrost effect. Experiments performed using elastic beads of various elastic moduli, viscoelastic or liquid drops of various surface tensions reveal a universal scaling behavior of the maximum expansion with the impact velocity, with a dynamics that mimics that of a conventional spring-mass system. We furthermore show that, for drops impacting a solid target, a similar scaling holds once the viscous dissipation is accounted by replacing the impact velocity with the effective velocity. Another fascinating behavior of viscoelastic fluids is a heterogeneous expansion of the sheet with the occurrence of cracks, revealing the elastic nature of the viscoelastic fluid. On the other hand, we study the uniaxial deformation and the fracture of reversible double transient networks by coupling extensional rheology to fast imaging. We provide a state diagram that delineates the regime of fracture without necking of the filament, when it is stretched at a rate larger than the inverse of the slowest relaxation time of the networks. We quantitatively demonstrate that dissipation processes are not relevant in our experimental conditions and that, depending on the density of nodes in the networks, fracture occurs in the linear elastic regime, or in a non-linear elastic regime preceded by a considerable strain hardening. In addition, analysis of the crack opening profiles indicates, for weakly connected networks, deviations from a parabolic shape close to the crack tip, which is expected for the linear elasticity of a brittle fracture. We demonstrate a direct correlation between the amplitude of the deviation from the parabolic shape and the amount of non-linear elasticity.Nous étudions le comportement de gels, transitoires auto-assemblés et réticulés permanents, soumis à des contraintes mécaniques extrêmes. D'une part, nous étudions la déformation biaxiale de nappes libres produites par impact d'une goutte liquide (Newtonienne ou viscoélastique) ou d’une perle de gel polymère sur des surfaces solides dans des conditions de dissipation minimale, obtenues soit avec une petite cible solide, soit avec une surface répulsive. Lors de l'impact, la goutte ou perle est transformée en une nappe mince qui s’étend et se rétracte sous l’action de forces de rappel élastiques. Pour les fluides viscoélastiques avec un temps de relaxation plus petit que la durée de vie typique de la nappe, la dynamique de la nappe viscoélastique est similaire à d’un liquide visqueux Newtonien de même viscosité. L’expansion maximale de la nappe diminue avec la viscosité et est modélisée quantitativement en utilisant un bilan énergétique entre l'inertie, la tension superficielle et la dissipation du cisaillement visqueux sur la cible solide, qui peut être prise en compte en mesurant une vitesse effective de la nappe à temps court, en sortie de cible. Nous montrons en outre que la dissipation visqueuse peut être sensiblement éliminée en utilisant une surface solide sur la base d'un effet de Leidenfrost inverse. Les expériences effectuées à l'aide de perles élastiques de modules élastiques variables, de gouttes liquides de tensions superficielles variables et de gouttes viscoélastiques révèlent un comportement universel pour l’expansion maximale de la nappe avec la vitesse d'impact, avec une dynamique analogue à celle d'un système ressort-masse conventionnel. Nous montrons en outre que, pour les gouttes qui impactent une petite cible solide, une description similaire peut être utilisée une fois que la dissipation visqueuse est prise en compte en remplaçant la vitesse d'impact par la vitesse effective. Un autre comportement fascinant des fluides viscoélastiques est l’expansion hétérogène de la nappe associée à l’apparition de fissures, révélant la nature élastique du fluide viscoélastique. D’autre part, nous étudions la déformation uniaxiale et la facture de filaments de doubles réseaux transitoires en couplant rhéométrie extensionnelle et imagerie rapide des filaments étirés. Nous établissons un diagramme d'état qui délimite le régime de du filament, lorsqu'il est étiré à un taux supérieur à l'inverse du temps de relaxation du plus lent des deux réseaux. Nous démontrons quantitativement que les processus de dissipation ne sont pas pertinents dans nos conditions expérimentales et que, suivant la densité des nœuds dans les réseaux, la rupture se produit dans le régime élastique linéaire, ou non linéaire précédé d'une augmentation considérable de la viscosité extensionnelle. L'analyse des profils d'ouverture des fissures indique, pour des réseaux faiblement connectés, des écarts par rapport à une forme parabolique en pointe de fissure, caractéristique d’une fracture fragile en régime linéaire. Nous montrons une corrélation directe entre l'amplitude de la déviation de la forme parabolique et le caractère non linéaire de la déformation élastique

Emotion profiling using deep learning

Individuals diagnosed with distress disorders exhibit heightened levels of negative affective factors that stem from underlying motivational systems associated with threat, safety, and loss of rewards. Emotion Regulation Therapy (ERT) is a psychotherapeutic approach that seeks to enhance the patient’s awareness of their motivational processes, develop their emotional regulation capacities, and facilitate new contextual learning. Neurofeedback training usingEEGhas showna potential to be employed as a therapeutic tool for emotion regulation and thereby improve the efficiency of Emotion Regulation Therapy. This technique involves providing real-time feedback to individuals about their brainwave activity while they engage in specific emotion regulation tasks. By using this feedback, individuals can learn to regulate their brainwave activity and, in turn, their emotional responses. Neurofeedback training relies on Emotion Recognition Systems for identifying the brainwave patterns associated with different emotional states and studying the neural mechanisms involved in emotion regulation. Motivated by the utility of emotion recognition in the treatment of mental health issues, this project aims to explore how emotion regulation therapy or music therapy can be made more effective with the help of EEG. The project involves the development of an emotion recognition system for studying how brain activity, recorded using EEG, varies with emotions. This is achieved by collecting data through experiments that attempted to induce different states of emotions using various music stimuli. This data is then analyzed to ensure emotions are induced and brain activities corresponding to different stimuli are distinguishable. Emotion states are then classified by a CNN-based deep neural network and a maximum accuracy of 69% is achieved.Bachelor of Engineering (Computer Science

Don’t fake it if you can’t make it: driver misconduct in last-mile delivery

In the last two decades, last-mile delivery (LMD) firms have seen immense growth fueled by the success of e-commerce, leading to faster and cheaper deliveries. Operating on thin margins, LMD firms strive for successful first-time deliveries to avoid the financial and reputational costs of reattempts. Delivery agents (DAs) are integral to LMD efficiency, influencing customer experience, delivery success, and productivity. However, most LMD performance enhancement research focuses on process, technology, and incentives, which presume workers will conform to procedures and monitoring tools will function flawlessly. Nevertheless, in practice, DAs deviate from expected behaviors, that is, indulge in misconduct, negatively affecting delivery efficiency, often resulting in returned parcels. One of the major forms of misconduct is entering fake remarks about deliveries, wherein DAs intentionally do not deliver the parcels and provide fake reasons for it. For instance, even without reaching a delivery address, a DA remarks “customer unavailable” and records a delivery failure. In this study, we collaborated with a leading Indian LMD firm and, using instrumental variable regression, found that such misconduct leads to a spillover productivity loss. This effect reduces the next day’s successful deliveries by 1.60% and first-time-right deliveries by 1.86%. We discuss misconduct’s correlation with factors such as task complexity and offer novel insights into how opportunistic circumstances can influence worker behavior

India\u27s COVID-19 Burdens, 2020

The purpose of this article is two pronged; first, to identify and report public health implications of the ongoing coronavirus (COVID-19) pandemic, and second, to report challenges uniquely faced by the citizens of India from a population health perspective. We have done both while closely examining epidemiological data that is accessible via SMAART\u27s RAPID Tracker. This policy informatics platform is a live database aimed to track the geospatial spread of the COVID-19 outbreak and policy actions globally and is administered collaboratively by CUNY\u27s Graduate School of Public Health and Health Policy and a global, non-profit public health incubator. Infectivity, incidence, and recovery rates were computed and graphical representations of epidemiological datasets were studied. We have discussed a plausible conceptual framework based on the principles of population health informatics for countries with similar characteristics to build a stronger public and community health foundation in order to safeguard populations during a health emergency in the future

Brittel facture of Polymer Networks

International audiencenon

Brittel facture of Polymer Networks

International audiencenon