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Center for Nonlinear Studies |
Unsaturated porous media are ubiquitous in agriculture, natural resources and industry (e.g. soils, oil reservoirs, fuel cells, porosimetry), thereby eliciting a fertile scientific literature. This talk discusses two challenges: (1) how can one predict the amount of fluid retained by a porous sample subject to an applied capillary suction; and (2) how does a drop of wetting fluid penetrate a porous wall? To address the first question, we outline a new statistical mechanics treatment that derives the fluid retention of a porous matrix from known surface energies and dry void geometry without empirical input. In the limit of vanishing inertial and viscous forces, the theory provides a framework for interpreting numerical simulations and X-ray tomographic data. It attributes retention hysteresis to collective first-order phase transitions in the frozen disorder of the porous network. We show that hysteresis strength grows with the mean specific neck cross-section and weakens as the distribution of specific pore area broadens. We suggest that abrupt phase transitions on the mesoscopic scale are the origin of intermittent spasms in the liquid phase called "Haines jumps". We turn to microgravity experiments to explore the second question. Here, our objective is to inform the recent lattice-Boltzmann numerical simulations of Frank and Perré [Phys. Fluids 2012, PRE 2015], who studied spreading and imbibition of a spherical drop of finite inertia into a wall featuring a bundle of capillary tubes. To that end, we develop a free-fall apparatus that can produce relatively large stable spheres of water. We present preliminary results and models of this phenomenon, which delays imbibition of liquids into porous media.
Host: Duan Z. Zhang