Drying by Cavitation and Poroelastic Relaxations in Porous Media with Macroscopic Pores Connected by Nanoscale Throats

Abstract

We investigate the drying dynamics of porous media with two pore diameters separated by several orders of magnitude. Nanometer-sized pores at the edge of our samples prevent air entry, while drying proceeds by heterogeneous nucleation of vapor bubbles -cavitation -in the liquid in micrometer-sized voids within the sample. We show that the dynamics of cavitation and drying are set by the interplay of the deterministic poroelastic mass transport in the porous medium and the stochastic nucleation process. Spatio-temporal patterns emerge in this unusual reaction-diffusion system, with temporal oscillations in the drying rate and variable roughness of the drying front.The desorption, or drying, of liquids from porous media is important in a variety of contexts, both in nature (e.g., the movement of water in plants [1]) and in technology (e.g., the synthesis [2] and characterization [3] of advanced materials). Studies of desorption have focused on the dynamics and pattern formation associated with drying [4-6] and the thermodynamics that define the shapes of desorption isotherms [7,8]. Despite this attention, uncertainties remain regarding the physical processes that govern desorption. It has been proposed that, rather than by receding of the liquid phase from the edges of the material, drying from porous media could occur by cavitation, i.e. the spontaneous formation of vapor bubbles either when the liquid tensile strength is attained [9] or by thermally activated nucleation in the metastable pore liquid (Fig. 1a) [10]. This process has been observed in simulations [8,11] and has been proposed on several occasions to explain the shape of desorption isotherms in nano-scale porous media [8,12,13], or the apparent emergence of drying events far from the evaporation front [14,15]. Yet, we are unaware of direct optical observation of desorption by cavitation or of an investigation of its effect on drying dynamics.In this Letter, we present a tailored porous medium formed of microfabricated voids coupled to each other and the outside via a nanoporous substrate (Figs. 1b-1c). This extreme ink-bottle structure -large pore bodies connected via narrow throats [16] -has allowed us to observe the nucleation and growth of cavitation bubbles during drying (Fig. 1d) and to show that this process gives rise to interesting coupled drying dynamics that is tunable with geometry.Drying occurs when a saturated porous medium is placed in a sub-saturated atmosphere (relative humidity p v /p sat < 1, where p v and p sat are the vapor pressure and its saturation value). At high relative humidity, evaporation results in the formation of menisci in the pores at the surface of the material, until local mechanical and andarXiv:1402.6776v2 [cond-mat.soft]

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