Jérôme Crassous

Moisture-induced ageing in granular media and the kinetics of capillary condensation

In 1773 Coulomb recognized that the static properties of granular systems can be discussed in terms of the frictional properties between different layers, leading to his relationship between the angle of repose of a granular pile (θ0) and the coefficient of static friction µ s: tanu2009θ0 =µs. Two centuries later, solid friction and granular media still present many puzzles. One such is that the coefficient of static friction depends on the time during which the solids remain in contact before the measurement. Here we show that this ageing effect is manifested too in the angle of repose of granular media and originates from capillary condensation of water vapour between the packed particles, leading to the formation of water bridges. By assuming that the kinetics of this process are governed by the thermally activated nucleation of bridges, we can reproduce both the time- and humidity-dependence of the ageing behaviour. Our results also clarify the kinetics of adsorption in porous media more generally.

Nanorheology: An investigation of the boundary condition at hydrophobic and hydrophilic interfaces

Abstract:It has been shown that the flow of a simple liquid over a solid surface can violate the so-called no-slip boundary condition. We investigate the flow of polar liquids, water and glycerol, on a hydrophilic Pyrex surface and a hydrophobic surface made of a Self-Assembled Monolayer of OTS (octadecyltrichlorosilane) on Pyrex. We use a Dynamic Surface Force Apparatus (DSFA) which allows one to study the flow of a liquid film confined between two surfaces with a nanometer resolution. No-slip boundary conditions are found for both fluids on hydrophilic surfaces only. Significant slip is found on the hydrophobic surfaces, with a typical length of one hundred nanometers.

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

We present a new capacitive sensor for displacement measurement in a surface-force apparatus which allows dynamical measurements in the range 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 converter. We use this sensor to study the nanorheological properties of dodecane confined between glass surfaces.

Sphere penetration by impact in a granular medium: A collisional process

The penetration by a gravity-driven impact of a solid sphere into a granular medium is studied by two-dimensional simulations. The scaling laws observed experimentally for both the final penetration depth and the stopping time with the relevant physical parameters are here recovered numerically without the consideration of any microscopic solid friction but with dissipative collisions only. Dissipative collisional processes are thus found as essential in catching the penetration dynamics in granular matter whereas microscopic frictional processes can only be considered as secondary effects.

Capillary Condensation between High-Energy Surfaces. An Experimental Study with a Surface Force Apparatus

We study the capillary condensation of an undersaturated vapour between high-energy surfaces with a surface force apparatus. The adhesion force shows a large hysteresis cycle when the surfaces are quasi-statistically brought to contact and moved apart, revealing an asymmetry between the condensation and evaporation processes. The condensation of a liquid bridge between the surfaces is associated with a force discontinuity, and occurs at distances which increase with the vapour pressure. The adhesion force is consistent with the classical theory of capillarity, and reveals the presence of wetting films. A mechanism for condensation is proposed based on the instability of the wetting films under their mutual attraction. We predict a condensation distance which grows linearly with the wetting film thickness, in good agreement with the experimental observation.

A new surface forces apparatus for nanorheology

We present an original surface forces apparatus which enables us to measure the interaction forces between any solid surfaces such as, e.g., metallic surfaces, opaque surfaces, or rough surfaces. The relative displacement of the surfaces is measured with a capacitive sensor. The forces are measured by a stiff and highly sensitive interferometric sensor. The measurements are performed in a dc to 100 Hz bandwidth. This feature allows us to study the mechanical response of a nanometric confined medium to rapid strain variations in the linear regime. An example of nanorheological measurement of dodecane confined in a nanometric gap is given at the end of this article.

Experimental study of a creeping granular flow at very low velocity

We report an experimental study of the surface flow and the creeping flow of glass spheres on a heap. We characterize the dynamics of the flow with particle tracking velocimetry and dynamic light scattering measurements. We observe a creeping flow with a dynamics which slows down exponentially with the depth. The characteristic distance for the decay is one bead diameter. A striking observation is that the exponential decay for the mean velocity holds over more than six orders of magnitude.

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 understanding of the existence of very large hysteresis, which is often associated with capillary condensation. These results are compatible with experimental results obtained with a surface forces apparatus in a vapor atmosphere, showing a large hysteresis 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 aging in time of this avalanche angle can be explained by the slow kinetics of capillary condensation in a random confined geometry.

Contact Angle Hysteresis on a Heterogeneous Surface: Solution in the Limit of a Weakly Distorted Contact Line

In the case of a weakly distorted contact line, we construct a one-variable equation giving the metastable configurations of a fluid interface in contact with a heterogeneous solid surface. In this limit it is possible to study the contact angle hysteresis created by semi-concentrated assemblies of individual defects. The results exhibit many collective effects, some of which can be described by a convenient renormalization of the hysteresis threshold and of the defect density. They are consistent with available experimental data.

Adhesion forces between wetted solid surfaces

We investigate in this paper the influence of wetting films on the adhesion forces between macroscopic solid surfaces connected by a liquid bridge. We show that the capillary forces are dependent on the interactions governing the wetting layers, and that those interactions may be determined from the measurement of the capillary force in the presence of a condensable vapor. We illustrate those results with a surface force apparatus experiment where the capillary force between high-energy surfaces is measured for different liquid pressures.