Drying regimes in homogeneous porous media from macro- to nanoscale

Abstract

MRI visualization down to nanometric liquid films in model porous media with pore size from micro-to nano-meter enables to fully characterize the physical mechanisms of drying. In larger pores, we identify an initial constant drying rate period, probing homogeneous desaturation, followed by a falling drying rate period. This second period is associated with the development of a gradient in saturation underneath the sample free surface that initiates the inward recession of the contact line. During this latter stage, the drying rate varies in accordance with vapor diffusion through the dry porous region; possibly affected by Knudsen effect for small pore size. However, we show that for sufficiently small pore size and/or saturation the drying rate is increasingly reduced by the Kelvin effect. Subsequently, we demonstrate that this effect governs the kinetics of evaporation in nanopores as a homogeneous desaturation occurs. Eventually, under our experimental conditions, we show that the saturation unceasingly decreases in a homogeneous manner throughout the wet regions of the medium regardless of pore size or drying regime considered. This finding suggests the existence of continuous liquid flow towards the interface of higher evaporation, down to very low saturation or very small pore size. Paradoxically, even if this net flow is unidirectional and capillary driven, it corresponds to a series of diffused local capillary equilibrations over the full height of the sample, which might explain that a simple Darcy's law model does not predict the scaling of the net flow rate on the pore size observed in our tests.

Convective drying of a porous medium with a paste cover

“…Under this assumption, after some transient stage, the drying rate would be controlled by vapor diffusion from the wet front to the free surface of the sample. This leads to a mass loss varying with the square root of time (see below), in agreement with some observations [13,15,[18][19][20].…”

“…Let us start by reviewing typical observations for the drying of a bead packing directly submitted to an air flux along its top surface. In that aim we may refer to the detailed liquid distributions in time provided by [18] for a series of bead packings with bead size ranging from a few nanometers to a few tens of microns. For bead size larger than 80 nm the set of liquid profiles in times have similar qualitative characteristics [18].…”

“…In that aim we may refer to the detailed liquid distributions in time provided by [18] for a series of bead packings with bead size ranging from a few nanometers to a few tens of microns. For bead size larger than 80 nm the set of liquid profiles in times have similar qualitative characteristics [18]. During a first period the saturation profiles remain parallel to each other and almost horizontal, which means that the saturation decreases while remaining uniform throughout the sample.…”

Strengthening and drying rate of a drying emulsion layer

“…During this period, the drying rate becomes much less dependent on the external air flux. Such a scheme remains qualitatively valid down to nanometric pore size [18].…”

“…Thus, during a first period, a soft colloidal solid shrinks and the drying rate remains constant as liquid is still surrounding the solid phase [34]. In a second stage, the particles cannot approach closer anymore, so that the liquid has to find a way through the rigid network now formed; air penetrates the pores and the drying rate decreases [18], [34]. Finally, by analogy with these materials, we expect a constant drying rate for emulsions.…”