Capillary Condensation in Confined Media

We review here the physics of capillary condensation of liquids in confined media, with a special regard to the application in nanotechnologies. The thermodynamics of capillary condensation and thin film adsorption are first exposed along with all the relevant notions. The focus is then shifted to the modelling of capillary forces, to their measurements techniques (including SFA, AFM and crack tips) and to their influence on AFM imaging techniques as well as on the static and dynamic friction properties of solids (including granular heaps and sliding nanocontacts). A great attention is spent in investigating the delicate role of the surface roughness and all the difficulties involved in the reduction of the probe size to nanometric dimensions. Another major consequence of capillary condensation in nanosystems is the activation of several chemical and corrosive processes that can significantly alter the surface properties, such as dissolution/redeposition of solid materials and stress-corrosion crack propagation.Comment: 28 pages - To appear in 2010 in the Handbook of Nanophysics - Vol 1 - Edited by Klaus Sattler - CRC Pres

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

Giant osmotic pressure in the forced wetting of hydrophobic nanopores

The forced intrusion of water in hydrophobic nanoporous pulverulent material is of interest for quick storage of energy. With nanometric pores the energy storage capacity is controlled by interfacial phenomena. With subnanometric pores, we demonstrate that a breakdown occurs with the emergence of molecular exclusion as a leading contribution. This bulk exclusion effect leads to an osmotic contribution to the pressure that can reach levels never previously sustained. We illustrate on various electrolytes and different microporous materials, that a simple osmotic pressure law accounts quantitatively for the enhancement of the intrusion and extrusion pressures governing the forced wetting and spontaneous drying of the nanopores. Using electrolyte solutions, energy storage and power capacities can be widely enhanced

Wall slip of complex fluids: interfacial friction or slip length?

Using a dynamic Surface Force Apparatus, we demonstrate that the notion of slip length used to describe the boundary flow of simple liquids, is not appropriate for viscoelastic liquids. Rather, the appropriate description lies in the original Navier's partial slip boundary condition, formulated in terms of an interfacial friction coefficient. We establish an exact analytical expression to extract the interfacial friction coefficient from oscillatory drainage forces between a sphere and a plane, suitable for dynamic SFA or Atomic Force Microscopy non-contact measurements. We use this model to investigate the boundary friction of viscoelastic polymer solutions over 5 decades of film thicknesses and one decade in frequency. The proper use of the original Navier's condition describes accurately the complex hydrodynamic force up to scales of tens of micrometers, with a simple "Newtonian-like" friction coefficient, not frequency dependent, and reflecting closely the dynamics of an interfacial depletion layer at the solution/solid interface.Comment: 7 pages, 5 figure

Nanorheology : an Investigation of the Boundary Condition at Hydrophobic and Hydrophilic Interfaces

t 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.Comment: 8 pages, 7 figures, 2 tables. Accepted for European Physical Journal E - Sofr Mate

Out of equilibrium anomalous elastic response of a water nano-meniscus

We report the observation of a transition in the dynamical properties of water nano-menicus which dramatically change when probed at different time scales. Using a AFM mode that we name Force Feedback Microscopy, we observe this change in the simultaneous measurements, at different frequencies, of the stiffness G'(N/m), the dissipative coefficient G''(kg/sec) together with the static force. At low frequency we observe a negative stiffness as expected for capillary forces. As the measuring time approaches the microsecond, the dynamic response exhibits a transition toward a very large positive stiffness. When evaporation and condensation gradually lose efficiency, the contact line progressively becomes immobile. This transition is essentially controlled by variations of Laplace pressure