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Drying regimes in homogeneous porous media from macro- to nanoscale

J. Thiery1,2, S. Rodts2, D. A. Weitz1, and P. Coussot2,*

  • 1Experimental Soft Condensed Matter Group, School of Engineering and Applied Sciences, Harvard University, Cambridge, Massachusetts 02138, USA
  • 2Université Paris-Est, Laboratoire Navier (ENPC-IFSTTAR-CNRS), Champs sur Marne 77420, France

  • *philippe.coussot@ifsttar.fr
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Phys. Rev. Fluids 2, 074201 – Published 25 July, 2017

DOI: https://doi.org/10.1103/PhysRevFluids.2.074201

Abstract

Magnetic resonance imaging visualization down to nanometric liquid films in model porous media with pore sizes from micro- to nanometers enables one to fully characterize the physical mechanisms of drying. For pore size larger than a few tens of nanometers, 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 the 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 effect of scaling of the net flow rate on the pore size observed in our tests.

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Phys. Rev. Fluids 2, 074201– Published 25 July, 2017
  • Received 10 February 2017

©2017 American Physical Society

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