Granular aqueous suspensions with controlled interparticular friction and adhesion.

We present a simple route to obtain large quantities of suspensions of non-Brownian particles with stimuli-responsive surface properties to study the relation between their flow and interparticle interactions. We perform an alkaline hydrolysis reaction on poly(methyl methacrylate) (PMMA) particles to obtain poly(sodium methacrylate) (PMAA-Na) particles. We characterize the quasi-static macroscopic frictional response of their aqueous suspensions using a rotating drum. The suspensions are frictionless when the particles are dispersed in pure water. We relate this state to the presence of electrosteric repulsion between the charged surfaces of the ionized PMAA-Na particles in water. Then we add monovalent and multivalent ions (Na+, Ca2+, La3+) and we observe that the suspensions become frictional whatever the valency. For divalent and trivalent ions, the quasi-static avalanche angle θc at large ionic strength is greater than that of frictional PMMA particles in water, suggesting the presence of adhesion. Finally, a decrease in the pH of the suspending solution leads to a transition between a frictionless plateau and a frictional one. We perform atomic force microscopy (AFM) to relate our macroscopic observations to the surface features of the particles. In particular, we show that the increase in friction in the presence of multivalent ions or under acidic conditions is driven by a nanoscopic phase separation and the bundling of polyelectrolyte chains at the surface of the particle. Our results highlight the importance of surface interactions in the rheology of granular suspensions. Our particles provide a simple, yet flexible platform to study frictional suspension flows.

Friction through molecular adsorption at the sliding interface of hydrogels: theory and experiments.

We report on the frictional properties of thin (≈µm) poly(dimethylacrylamide) hydrogel films within contacts with spherical silica probes. In order to focus on the contribution to friction of interfacial dissipation, a dedicated rotational setup is designed which allows to suppress poroelastic flows while ensuring a uniform velocity field at the sliding interface. The physical-chemistry of the interface is varied from the grafting of various silanes on the silica probes. Remarkably, we identify a velocity range in which the average frictional stress systematically varies with the logarithm of the sliding velocity. This dependency is found to be sensitive to the physical-chemistry of the silica surfaces. Experimental observations are discussed in the light of a molecular model where friction arises from thermally activated adsorption of polymer chains at the sliding interface, their elastic stretching and subsequent desorption. From this theoretical description, our experimental data provide us with adhesion energies and characteristic times for molecular adsorption that are found consistent with the physico-chemistry of the chemically-modified silica surfaces.

Optical control of surface forces and instabilities in foam films using photosurfactants.

Molecular interactions in thin liquid films, such as the disjoining pressure, are involved in interfacial phenomena such as emulsion and foam stabilization. In this article we show that through light stimulation we can control remotely the disjoining pressure in a thin liquid film stabilized by a photosurfactant. We stabilize a horizontal thin liquid film using a cationic photosurfactant, AzoTAB, bearing an azobenzene moiety on the hydrophobic tail which can switch from a trans to a cis conformation upon light stimulation. As the film is illuminated at specific wavelengths the AzoTAB molecules switch continuously their conformation and consequently their interface affinity. The main consequence of stimulating the film with light is increasing the ratio of cis in the film. This provokes a desorption flux, and an increase in the concentration of free surfactants, as the CMC of the cis isomer is higher than that of the trans isomer. Therefore the electrostatic repulsion between the surfactant layers that stabilize the film decreases, inducing an instability in the film thickness. For films with a thickness between 20 nm and 60 nm, we observe the formation of spherical caps up to 100 µm wide, whose shape is controlled by the competition between surface tension and disjoining pressure. The motion of these caps in the film is restrained by the surface viscosity of the surfactant layers. In addition, for thicknesses below 40 nm and depending on light intensity, we can observe flat stratified islands up to 100 µm wide, with thickness steps corresponding to the size of a surfactant micelle. We suggest that this second instability is due to the oscillation of the disjoining pressure isotherm under light.

Friction and shear fracture of an adhesive contact under torsion.

The shear failure or stiction of an adhesive contact between a poly(dimethylsiloxane) (PDMS) rubber and a glass lens has been investigated using a torsional contact configuration. As compared to linear sliding, torsion presents the advantage of inducing a shear failure under a pure mode III condition, while preserving the cylindrical symmetry of the contact. The surface of the transparent PDMS substrate was marked using a network of dots in order to monitor continuously the in-plane surface displacements during the stiction process. Using a previously developed inversion procedure (A. Chateauminois and C. Fretigny, Eur. Phys. J. E 27, 221 (2008)), the corresponding surface shear stress distributions were obtained from the displacement fields. Stiction was found to involve the progressive shrinkage of a central adhesive zone surrounded by an annular microslip region. Adhesion effects were especially evidenced from a stress overshoot at the boundary of the adhesive zone. The experimental data were analysis using an extension to torsional contact of the Maugis-Dugdale approach's to adhesive contacts which takes into account frictional effects. This model allowed to extract an effective adhesion energy in the presence of friction, which dependence on kinetics effect is briefly discussed.