Elie Wandersman

Probing locally the onset of slippage at a model multi-contact interface

We report on the multicontact frictional dynamics of model elastomer surfaces rubbed against bare glass slides. The surfaces consist of layers patterned with thousands of spherical caps distributed both spatially and in height, regularly or randomly. Use of spherical asperities yields circular microcontacts whose radii are a direct measure of the contact pressure distribution. Optical tracking of individual contacts provides the in-plane deformations of the tangentially loaded interface, yielding the shear force distribution. We then investigate the stick-slip frictional dynamics of a regular hexagonal array. For all stick phases, slip precursors are evidenced and found to propagate quasistatically, normally to the isopressure contours. A simple quasistatic model relying on the existence of interfacial stress gradients is derived and predicts qualitatively the position of slip precursors.

Probing heterogeneous dynamics of a repulsive colloidal glass by time resolved x-ray correlation spectroscopy

We present here experimental results obtained by time resolved x-ray correlation spectroscopy, revealing the heterogeneous nature of the dynamics down to the nanoscale in a dense dispersion of magnetic nanoparticles. The dynamical susceptibility, which quantifies the amplitude of the dynamical fluctuations, is investigated in a repulsive colloidal glass. Its value is an order of magnitude smaller than the ones found in gels and is length scale independent in the Q-range of the x-ray scattering experiment.

Soft Lithography Using Nectar Droplets

In spite of significant advances in replication technologies, methods to produce well-defined three-dimensional structures are still at its infancy. Such a limitation would be evident if we were to produce a large array of simple and, especially, compound convex lenses, also guaranteeing that their surfaces would be molecularly smooth. Here, we report a novel method to produce such structures by cloning the 3D shape of nectar drops, found widely in nature, using conventional soft lithography.The elementary process involves transfer of a thin patch of the sugar solution coated on a glass slide onto a hydrophobic substrate on which this patch evolves into a microdroplet. Upon the absorption of water vapor, such a microdroplet grows linearly with time, and its final size can be controlled by varying its exposure time to water vapor. At any stage of the evolution of the size of the drop, its shape can be cloned onto a soft elastomer by following the well-known methods of molding and cross-linking the same. A unique new science that emerges in our attempt to understand the transfer of the sugar patch and its evolution to a spherical drop is the elucidation of the mechanics underlying the contact of a deformable sphere against a solid support intervening a thin liquid film. A unique aspect of this work is to demonstrate that higher level structures can also be generated by transferring even smaller nucleation sites on the surface of the primary lenses and then allowing them to grow by absorption of water vapor. What results at the end is either a well-controlled distribution of smooth hemispherical lenses or compound structures that could have potential applications in the fundamental studies of contact mechanics, wettability, and even in optics.

Whisker Contact Detection of Rodents Based on Slow and Fast Mechanical Inputs

Rodents use their whiskers to locate nearby objects with an extreme precision. To perform such tasks, they need to detect whisker/object contacts with a high temporal accuracy. This contact detection is conveyed by classes of mechanoreceptors whose neural activity is sensitive to either slow or fast time varying mechanical stresses acting at the base of the whiskers. We developed a biomimetic approach to separate and characterize slow quasi-static and fast vibrational stress signals acting on a whisker base in realistic exploratory phases, using experiments on both real and artificial whiskers. Both slow and fast mechanical inputs are successfully captured using a mechanical model of the whisker. We present and discuss consequences of the whisking process in purely mechanical terms and hypothesize that free whisking in air sets a mechanical threshold for contact detection. The time resolution and robustness of the contact detection strategies based on either slow or fast stress signals are determined. Contact detection based on the vibrational signal is faster and more robust to exploratory conditions than the slow quasi-static component, although both slow/fast components allow localizing the object.

Repulsive and attractive ferroglasses: a SAXS and XPCS study

A dynamical freezing, which is the analogous of a glass transition, is observed at large concentrations in aqueous dispersions of maghemite nanoparticles. We study experimentally the structure and the dynamical properties of two dense ferroglass-formers in two very distinct states of interparticle interaction, either strongly repulsive or attractive. The static structure of the magnetic colloidal dispersions is probed by means of Small Angle X-ray Scattering both in and without the presence of an external magnetic field. Translational dynamics of the repulsive glassformer are investigated by X ray Photon Correlation Spectroscopy. Slow dynamics and aging properties, which both become anisotropic under an applied field, are here investigated.

Quasistatic Microdroplet Production in a Capillary Trap

We have developed a method to produce aqueous microdroplets in an oil phase, based on the periodic extraction of a pending droplet across the oil/air interface. This interface forms a capillary trap inside which a droplet can be captured and detached. This process is found to be capillary- based and quasi-static. The droplet size and emission rate are independently governed by the injected volume per cycle and the extraction frequency. We find that the minimum droplet diameter is close to the injection glass capillary diameter and that variations in surface tension moderately perturb the droplet size. A theoretical model based on surface energy minimization in the oil/water/air phases was derived and captures the experimental results. This method enables robust, versatile and tunable production of microdroplets at low production rates.