Christophe Poulard

Elasticity and wrinkled morphology of Bacillus subtilis pellicles

Wrinkled morphology is a distinctive phenotype observed in mature biofilms produced by a great number of bacteria. Here we study the formation of macroscopic structures (wrinkles and folds) observed during the maturation of Bacillus subtilis pellicles in relation to their mechanical response. We show how the mechanical buckling instability can explain their formation. By performing simple tests, we highlight the role of confining geometry and growth in determining the symmetry of wrinkles. We also experimentally demonstrate that the pellicles are soft elastic materials for small deformations induced by a tensile device. The wrinkled structures are then described by using the equations of elastic plates, which include the growth process as a simple parameter representing biomass production. This growth controls buckling instability, which triggers the formation of wrinkles. We also describe how the structure of ripples is modified when capillary effects are dominant. Finally, the experiments performed on a mutant strain indicate that the presence of an extracellular matrix is required to maintain a connective and elastic pellicle.

Diffusion-driven evaporation of sessile drops

The evaporation of wetting drops deposited on a substrate at thermal equilibrium under normal atmosphere is discussed. The evaporation rate appears to be controlled by the stationary diffusion of vapour molecules in the gas phase. Experiments with alkanes and water drops are fairly well accounted for by an isothermal model, taking into account the specific properties of thin films.

Mechanical tuning of adhesion through micro-patterning of elastic surfaces

We present an investigation of the role of micropatterning on adhesion properties at soft deformable polydimethylsiloxane (PDMS)/acrylic adhesive interfaces. Contrary to what has been observed for low aspect ratio rigid patterns, where the adhesion enhancement was found to only result from the increase of the interfacial area due to patterning, we show that for soft elastic arrays of cylindrical pillars, the elastic deformation of the patterns can lead to a noticeable extra adhesion increase. The effect of the geometrical characteristics of the patterning for hexagonal arrays of PDMS micropillars on the adhesion energy is presented. We show that varying the size of the pattern allows one to tune the adhesion energy, and that this adhesion enhancement saturates when the pillars become too close to each other, due a coupling of the elastic deformation fields inside the underlying substrate. A mechanical model has been developed and found in good quantitative agreement with experimental data, with a unique fitting parameter, the rupture criteria for the adhesive on the top of the pillars. Such a rupture criterion can thus be extracted from systematic experiments on controlled patterned surfaces. This criterion remains sensitive to the chemistry of the surfaces.

Sliding friction at soft micropatterned elastomer interfaces

In this paper, we present an experimental study of the friction between a smooth elastomer lens and an elastomer substrate micropatterned with hexagonal arrays of cylindrical pillars. Depending on the normal load, the surfaces can be in top or mixed contact. The friction force can be interpreted in terms of friction stresses in the full contact and top contact zones. The latter is higher than that on smooth surfaces evidencing the role of the elastic deformations of the surfaces in the dissipation processes.

Incidence of the molecular organization on friction at soft polymer interfaces

Polymer molecules strongly anchored to a solid substrate and interdigitated into bulk crosslinked elastomer have been shown recently to efficiently promote adhesion and friction between substrate and elastomer. Concerning friction, the regime of low surface coverage in surface anchored chains has been fully and quantitatively accounted for by the pull off mechanisms, where individual chains are dynamically extracted from the elastomer. Then, the stretching energy of these chains dominates the friction losses. We focus here on the dense surface coverage regime. We present systematic experiments performed on the polydimethylsiloxane (PDMS) – silica system, and determine molecular weight and sliding velocity dependences of the friction stress. We show that the friction is dominated by the shear thinning of the grafted layer confined between the elastomer and the substrate, and responding to the shear solicitation like a melt, but with very long relaxation times. We also show that the friction stress appears highly sensitive to the molecular organization inside the surface anchored polymer layer, comparing end grafted and strongly adsorbed layers having otherwise the same molecular characteristics (molecular weight of the chains, and thickness of the surface anchored layer).

Contact Angle and Contact Angle Hysteresis Measurements Using the Capillary Bridge Technique

A new experimental technique is proposed to easily measure both advancing and receding contact angles of a liquid on a solid surface, with unprecedented accuracy. The technique is based on the analysis of the evolution of a capillary bridge formed between a liquid bath and a solid surface (which needs to be spherical) when the distance between the surface and the liquid bath is slowly varied. The feasibility of the technique is demonstrated using a low-energy perfluorinated surface with two different test liquids (water and hexadecane). A detailed description of both experimental procedures and computational modeling are given, allowing one to determine contact angle values. It is shown that the origin of the high accuracy of this technique relies on the fact that the contact angles are automatically averaged over the whole periphery of the contact. This method appears to be particularly adapted to the characterization of surfaces with very low contact angle hysteresis.

The dynamics of evaporating sessile droplets

Experiments on sessile drops evaporating in a normal atmosphere without an applied thermal gradient are reported and compared with an available theoretical model. The liquids used are alkanes; water; and, more recently, polydimethylsiloxane oligomers. The substrates are silicon wafers, completely wetted by the liquid. Experiments with hanging drops allow us first to discard any influence of convection in the gas phase on the drop dynamics. The model assumes the process to be controlled by the stationary diffusion of the evaporating molecules in the gas phase. For alkanes and water, and in a limited range of drop sizes where gravity can be ignored, the model accounts very well for the dynamics of the drop radius, and rather well for the contact angle. This is no longer the case with the polydimethylsiloxane oligomers, where the very small contact angles require a more elaborated analysis of the drop edge.

Convection-enhanced water evaporation

Water vapor is lighter than air; this can enhance water evaporation by triggering vapor convection but there is little evidence. We directly visualize evaporation of nanoliter (2 to 700 nL) water droplets resting on silicon wafer in calm air using a high-resolution dual X-ray imaging method. Temporal evolutions of contact radius and contact angle reveal that evaporation rate linearly changes with surface area, indicating convective (instead of diffusive) evaporation in nanoliter water droplets. This suggests that convection of water vapor would enhance water evaporation at nanoliter scales, for instance, on microdroplets or inside nanochannels.

Thin films in wetting and spreading

Abstract Thin films differ from bulk phases both from a thermodynamic point of view, i.e. the chemical potential of a molecule in a film depends on the film thickness, and in their dynamical response, because relaxation times may become large in these confined media. Examples of slow structural relaxation in thin films of liquid crystals, and their consequences on the wetting properties of the systems are presented first. Then, we illustrate the thermodynamic specificity of thin films when evaporation is considered. The consequences on the wetting dynamics of macroscopic evaporating droplets are presented in the last part of the paper.

Cassie-Wenzel–like transition in patterned soft elastomer adhesive contacts

In this paper, we presented an experimental and theoretical analysis of the formation of the contact between a smooth elastomer lens and an elastomer substrate micropatterned with hexagonal arrays of cylindrical pillars. We show using a JKR model coupled with a full description of the deformation of the substrate between the pillars that the transition between the top to the full contact is obtain when the normal load is increased above a well predicted threshold. We have also shown that above the onset of full contact, the evolution of the area of full contact was obeying a simple scaling.We present an experimental and theoretical analysis of the transition from top to mixed top and full contacts between a smooth elastomer sphere and an elastomer substrate micropatterned with hexagonal arrays of cylindrical pillars. We show that surprisingly the overall behavior of the apparent radius of contact vs. the applied load obeys JKR contact mechanics, whatever the nature of the contact (top or mixed). This allows us to propose a mechanical description predicting quantitatively the evolution of the critical load for the onset of full contact with the pattern geometry and qualitatively that of the area of full contact above this threshold. We emphasize the role of the mechanical coupling between the pillars induced by the deformation of the substrate for large enough densities of pillars.