Where does a cohesive granular heap break?

In this paper, we consider the effect of cohesion on the stability of a granular heap. We first briefly review literature results on the cohesion force between two rough granular beads and specifically consider the dependence of the adhesion force on the normal load. We then compute the dependence of the maximum angle of stability of the heap as a function of the cohesion. We point out that the dependence of the cohesive forces on the external normal load between grains is a key point in determining the localization of the failure plane. While for a constant cohesive force, slip occurs deep inside the heap, surface failure is obtained for a linear dependence of the cohesion on the normal stress.Comment: 6 pages, 6 figures. Submitted to Phys. Rev.

Slow Kinetics of Capillary Condensation in Confined Geometry: Experiment and Theory

When two solid surfaces are brought in contact, water vapor present in the ambient air may condense in the region of the contact to form a liquid bridge connecting the two surfaces : this is the so-called capillary condensation. This phenomenon has drastic consequences on the contact between solids, modifying the macroscopic adhesion and friction properties. In this paper, we present a survey of the work we have performed both experimentally and theoretically to understand the microscopic foundations of the kinetics of capillary condensation. From the theoretical point of view, we have computed the free energy barrier associated with the condensation of the liquid from the gas in a confined system. These calculations allow to understand the existence of very large hysteresis, which is often associated with capillary condensation. This results are compatible with experimental results obtained with a surface forces apparatus in a vapor atmosphere, showing a large hysteris of the surface energy of two parallel planes as a function of their distance. In the second part, we present some experiments on the influence of humidity on the avalanche angle of granular media. We show that the ageing in time of this avalanche angle can be explained by the slow kinetics of capillary condensation in a random confined geometry.Comment: Special Volume of Colloids and Surfaces A,Proceedings of Nanocapillarity: Wetting of Heterogeneous Surfaces and Porous Solids,June 25-27, 2001, TRI/Princeton International Workshop, Editor: Alexander V. Neimar

Hydrodynamic drag-force measurement and slip length on microstructured surfaces

International audienceIn this paper, a drainage experiment of water between a borosilicate sphere and a microstructured surface constituted by regularly spaced pillars is presented. The microstructured surface has two parts: on one part the liquid forms a Cassie interface and on the second it forms a Wenzel interface. The measured hydrodynamic drag force is larger on the Cassie part compared to the Wenzel part. Furthermore, for the Cassie part, from the hydrodynamic drag force measurements on a pillar and between pillars the corresponding local slip lengths have been extracted. The area average slip length on the surface is in agreement with the value expected by Philip's equation

A new capacitive sensor for displacement measurement in a surface force apparatus

We present a new capacitive sensor for displacement measurement in a Surface Forces Apparatus (SFA) which allows dynamical measurements in the range of 0-100 Hz. This sensor measures the relative displacement between two macroscopic opaque surfaces over periods of time ranging from milliseconds to in principle an indefinite period, at a very low price and down to atomic resolution. It consists of a plane capacitor, a high frequency oscillator, and a high sensitivity frequency to voltage conversion. We use this sensor to study the nanorheological properties of dodecane confined between glass surfaces.Comment: 7 pages, 8 figure

Low Friction Flows of Liquids at Nanopatterned Interfaces

With the recent important development of microfluidic systems, miniaturization of flow devices has become a real challenge. Microchannels, however, are characterized by a large surface to volume ratio, so that surface properties strongly affect flow resistance in submicrometric devices. We present here results showing that the concerted effect of wetting . properties and surface roughness may considerably reduce friction of the fluid past the boundaries. The slippage of the fluid at the channel boundaries is shown to be drastically increased by using surfaces that are patterned at the nanometer scale. This effect occurs in the regime where the surface pattern is partially dewetted, in the spirit of the 'superhydrophobic' effects that have been recently discovered at the macroscopic scales. Our results show for the first time that, in contrast to the common belief, surface friction may be reduced by surface roughness. They also open the possibility of a controlled realization of the 'nanobubbles' that have long been suspected to play a role in interfacial slippag

Aging in humid granular media

Aging behavior is an important effect in the friction properties of solid surfaces. In this paper we investigate the temporal evolution of the static properties of a granular medium by studying the aging over time of the maximum stability angle of submillimetric glass beads. We report the effect of several parameters on these aging properties, such as the wear on the beads, the stress during the resting period, and the humidity content of the atmosphere. Aging effects in an ethanol atmosphere are also studied. These experimental results are discussed at the end of the paper.Comment: 7 pages, 9 figure

Dynamics of a harmonically driven fluid interface in a capillary

We use an harmonic technique to probe the dynamics of a liquid-liquid interface in a capillary. The capillary pressure drop induced by a harmonic displacement of the fluids is recorded as a function of the excitation amplitude and frequency. At low frequency there is an amplitude threshold above which the contact line depins and slides, and this threshold is related to the static hysteresis of the contact angle. When the contact line depins, the damping due to its motion is measured from the component of the capillary pressure in phase with the fluids velocity. At frequencies higher than one hertz we observe a phenomenon of dynamic pinning. The behaviour of the damping suggests that the variation of the contact angle with the three-phase line velocity is frequency-dependent.Nous utilisons une technique harmonique pour étudier les propriétés dynamiques d'une interface fluide dans un capillaire. On mesure en fonction de l'amplitude et de la fréquence d'excitation la chute de pression capillaire causée par le déplacement des fluides. Pour des déplacement de basse fréquence, il existe un seuil de l'amplitude au-delà duquel la ligne de contact se désancre et glisse ; ce seuil est lié à l'hystérésis statique de l'angle de contact. Lorsqu'on augmente la fréquence à amplitude constante, on observe que la ligne se réancre progressivement. L'armortissement mesuré suggère que l'angle de contact dynamique dépend non seulement de la vitesse de la ligne de contact mais aussi de la fréquence d'excitation

Hydrodynamic drag-force measurement and slip length on microstructured surfaces

In this paper, a drainage experiment of water between a borosilicate sphere and a microstructured surface constituted by regularly spaced pillars is presented. The microstructured surface has two parts: on one part the liquid forms a Cassie interface and on the second it forms a Wenzel interface. The measured hydrodynamic drag force is larger on the Cassie part compared to the Wenzel part. Furthermore, for the Cassie part, from the hydrodynamic drag force measurements on a pillar and between pillars the corresponding local slip lengths have been extracted. The area average slip length on the surface is in agreement with the value expected by Philip's equation