Light scattering study of collective effects during evaporation and condensation in a disordered porous material

We combine high-resolution isotherms measurements and light scattering technics to study over a broad temperature range collective effects during the condensation and evaporation of helium from Vycor, a prototypic disordered porous material. For evaporation, our results provide the first direct evidence for a crossover from a percolation collective mechanism at low temperature to a local cavitation mechanism at high temperature. No long-range collective effects are detected during condensation. We compare these results to recent theoretical predictions emphasizing the specific role of disorder, and discuss their relevance for determining pores sizes distributions in disordered porous materials. Copyright c © EPLA, 2013 Introduction. – Confining a fluid inside a wetting porous material results in a hysteresis between condensation and evaporation and a shift of the corresponding hysteresis loop below the bulk saturated vapor P0 [1]. The part played in this hysteresis by pore coupling and/or disorder has been studied by recent theoretical approaches [2–6] motivated both by the interest of this question in relation to statistical physics, and the need to understand how these effects affect the widely used Barrett-JoynerHalenda (BJH) method of characterization of porous materials [7], which assumes independent pores. Up to now, these different approaches have been mostly probed by analyzing the shape of experimental adsorbed amount vs. pressure isotherms. In this paper, we combine highresolution isotherms and light scattering measurements to directly investigate the collective effects involved in the condensation and evaporation processes in a disordered porous material, providing a new benchmark against which to test the applicability of the current theories of hysteresis to disordered porous materials. In independent, cylindrical, pores, the situation assumed by the BJH method, evaporation occurs at equilibrium by recession of a nearly hemispheric meniscus from the pore ends, while condensation is delayed with respect to this equilibrium situation, occuring by a spinodal instability of the (metastable) adsorbed film [8]. † Deceased. BJH then use the Kelvin relation between the equilibrium pressure and the pore radius, modified to account for the presence of an adsorbed film [1], to determinate pore sizes distributions from the desorption isotherm. However, connections between pores of different radii [2], or modulations of the pore radius along its length [9,10], are expected to modify this scheme, affecting the validity of the BJH approach. On adsorption, a pore can fill at equilibrium if one of its neighbors is already filled. Reversely, a pore will not empty at equilibrium if it has no free access to the vapor, i.e. if it is surrounded by smaller pores filled with liquid. Evaporation will be delayed until the neighbors empty (the so-called pore-blocking effect), or until thermally activated nucleation (cavitation) of a bubble occurs within the pore. In all cases, the pore size distribution deduced from the BJH approach would be wrong. In the past decade, progresses in chemical synthesis have allowed to produce ordered porous materials with controlled geometry, and to evidence these different mechanismes. The absence of pore-blocking in patterned silicon wafers has been interpreted in terms of cavitation [11,12]. A transition from the pore-blocking to the cavitation regime when increasing the temperature has been reported in cage-like porous materials constituted of large cavities separated by small constrictions [13,14]. In such materials, it has been shown that the cavity size is much better predicted from the adsorption isotherm,