J.-M. di Meglio

Deformation and Flow of a Two-Dimensional Foam under Continuous Shear

We investigate the flow properties of a 2D foam (a confined monolayer of jammed bubbles) submitted to a continuous shear in a Couette geometry. A strong localization of the flow at the moving inner wall is evidenced. Moreover, velocity fluctuations measurements reveal self-similar dynamical structures consisting of clusters of bubbles moving coherently. A stochastic model is proposed where bubbles rearrangements are activated by local stress fluctuations produced by the shearing wheel. This model gives a complete description of our observations and is also consistent with available data on granular shear bands.

Contact Angle Hysteresis and Interacting Surface Defects

We present model experiments which point out the importance of the interactions of surface defects in the phenomenon of contact angle hysteresis. In particular, i) the hysteresis does not increase linearly with the density of defects and ii) the Fourier analysis of the force reveals a regime of avalanches.

An elasto-hydrodynamical model of friction for the locomotion of Caenorhabditis elegans

Caenorhabditis elegans (C. elegans) is one of the most studied organisms by biologists. Composed of around one thousand cells, easy to culture and to modify genetically, it is a good model system to address fundamental physiological questions and in particular to investigate neuromuscular processes. Many C. elegans mutants can be distinguished by their locomotion phenotype and it then important to understand the biomechanics of their locomotion and in particular the mechanics of their undulating crawling motion on agar aqueous gels where they are commonly grown and observed. In this article, we present a mechanical model of the friction of the worms on their substrate where we have included capillarity (which pins the worm of the gel), the hydrodynamics of the lubrication film (between worm and gel) and the substrate/body elasticity. We determine the ratio of the transverse to longitudinal friction coefficients of the worm body on the culture gel as a function of a control parameter which describes the relative role of the deformation of the gel and the viscous dissipation in the lubrication film. Experimentally this ratio is - for soft gels - larger than the maximal value predicted by our model (this maximum is equal to 2, the value for an infinite cylinder in bulk liquid) and we propose to include the plasticity of the gel (i.e. the dissipation of the deformation of the gel) for a better description of the worm/gel interaction.

Contact Angle Hysteresis: a First Analysis of the Noise of the Creeping Motion of the Contact Line

We study the force exerted on a fiber by a moving liquid interface. We show that the noise of this force should correspond to the pinning-depinning process (or stick and slip) of the contact line on defects (which can be physical defects as roughness or chemical heterogeneities). For fibers with a low concentration of defects, we find experimentally that the contact angle hysteresis γ(cos θr−cos θa) is proportional to −h2/S0, where h is the fluctuation of this force and S0 the average spreading parameter.

X-ray observation of micro-failures in granular piles approaching an avalanche

An X-ray imaging technique is used to probe the stability of 3-dimensional granular packs in a slowly rotating drum. Well before the surface reaches the avalanche angle, we observe intermittent plastic events associated with collective rearrangements of the grains located in the vicinity of the free surface. The energy released by these discrete events grows as the system approaches the avalanche threshold. By testing various preparation methods, we show that the pre-avalanche dynamics is not solely controlled by the difference between the free surface inclination and the avalanche angle. As a consequence, the measure of the pre-avalanche dynamics is unlikely to serve as a tool for predicting macroscopic avalanches.

Making van der Waals Films on Fibers

The liquid layer coating a solid fiber drawn at constant velocity V0 out of a bath of liquid is all the thicker since V0 is large. We discuss in this letter the limit of very low capillary numbers: if the liquid wets the fiber, the thickness e of the liquid layer should not depend any longer on V0 below a critical value of the velocity (V0 < 0) and should be equal to a2/3 b1/3, where a is a microscopic length related to the wetting interactions (which are supposed to be van der Waals ones) and b is the radius of the fiber. This thickness is in the 100 A range. Then, a new experiment is presented: it consists of making such very thin films of dodecane on polymeric fibers and measuring their average thicknesses by gas chromatrography titrations. The results are promising: this technique could be an interesting alternative for detecting or measuring very thin films on curved surfaces for which more usual methods like ellipsometry are not suited.

Equilibrium of a spherical particle at a curved liquid/liquid interface

Abstract We consider the equilibrium position of a solid spherical particle at a curved liquid/liquid interface. Our analysis is based on the total free energy of the system. It is shown in particular that (i) the contact angle at the three-phase line obeys the Young equation, and (ii) the particle is never ejected out of the droplet, whatever the curvature of the interface. We then study the equilibrium position of a “Janus bead,” that is, a spherical particle with hemispheres of different surface energy.

Irreversible Adsorption of a Polymer Melt on a Colloidal Particle

We study the irreversible adsorption of linear polymer chains from a melt onto a spherical particle of radius R. This study generalizes the recent model of Guiselin for the irreversible adsorption of linear chains onto a planar surface. Our main motivation is the major role played by adsorbed polymer layers in the stabilization of colloidal particles. We consider the following sequence of events: a) the particle is put in contact with the polymer melt, all the monomers touching the surface adsorbing instantaneously and irreversibly, b) the particle is then washed out with pure solvent. Using scaling argument, we derive the volume fraction profile (r) (where r is the distance from the centre), the external radius Rc and the free energy F of the adsorbed layer. We then estimate the force F(h) between two particles separated by a distance 2h. We find that irreversible adsorption might represent a more efficient way of protecting the particles against coalescence than grafting.

Adsorption of polyampholytes on charged surfaces

Abstract:We have studied the adsorption of neutral polyampholytes on model charged surfaces that have been characterized by contact angle and streaming current measurements. The loop size distributions of adsorbed polymer chains have been obtained using atomic-force microscopy (AFM) and compared to recent theoretical predictions. We find a qualitative agreement with theory; the higher the surface charge, the smaller the number of monomers in the adsorbed layer. We propose an original scenario for the adsorption of polyampholytes on surfaces covered with both neutral long-chain and charged short-chain thiols.

Two-dimensional rheology of soap films

We present an entirely new experimental device dedicated to the study of the mechanical behaviour of freely suspended soap films. We find that the measured surface viscosity is comparable to the surface viscosity of Gibbs monolayers, but surprisingly, we found that soap films have a small shear modulus.