Imbibing drops of ethanol/water mixtures in model nanoporous networks with tunable pore structure: Deviation from square root to linear time regime imbibition kinetics

In this paper, we report on the imbibition of model hydrophobic nanoporous networks formed by polystyrene (PS) microbead assemblies. Spontaneous imbibition of these structures with sessile drops was achieved using water/ethanol mixtures of higher wettability than pure water. We used the high sensitivity of particle coalescence to tiny temperature variations around T(g) to finely adjust the topographical features, and thereafter the imbibition properties of the porous networks. Reconstructions of the networks led to distinct and quantifiable variations of the free volume, and pore sizes. These model hydrophobic nanoporous systems allowed addressing the important issue of the interplay between the topographical features of a porous medium, and the free-imbibition kinetics of a liquid mixture droplet over a wide range of concentration, surface tension and hence, of driving capillary pressure. Our results show deviations from the expected square root time-dependence characterizing the imbibition in the Darcy's regime. In particular, a linear time-regime was observed above a threshold ethanol concentration (capillary pressure), and below a threshold sintering temperature of the microbead assembly. While the latter arises naturally as a result of the modification of the pore structure (sintering and pore closure), and hence of the pellet's permeability following the annealing, the deviation related to the composition of the liquid mixture is a less trivial and a quite new behaviour. Here we propose a phenomenological picture which allows accounting for this concentration-driven acceleration of the spontaneous imbibition, based on the singular properties of the liquid mixture which dynamically modify the local structure and conditions around the confined imbibition front, especially at high ethanol content. This work may thus contribute understanding imbibition behaviour of liquid mixtures and of complex fluid in general in nanoporous networks, an issue that is of technological relevance in current material science and engineering. (C) 2011 Elsevier B.V. All rights reserved.