Interplay of Laplace and Bangham effects during imbibition in nanopores.

We present water imbibition experiments in mesoporous silica glass (Vycor) using optical imaging and high-resolution dilatometry techniques. We find that a competition between surface stress release (Bangham effect) and tensile Laplace pressures acting on the nanopore walls during water imbibition results in two distinct regimes of deformation dynamics. During capillary filling, the imbibition front advancement is governed by the Lucas-Washburn law, and we observe continuous expansion of the mesoporous matrix following analogous square-root-of-the-time dynamics. We show as well how upon complete filling of the pores, the subsequent disappearance of the Laplace pressure leads to an additional sudden expansion of the porous matrix. This behavior, supported by single-pore molecular dynamics simulations, can be quantitatively described by a continuum (fluid) mechanical model that accounts for the interplay of expansive Bangham and contractile Laplace pressure effects in an isotropic 3D network of cylindrical pores.

Finally, we further investigate the Laplace pressure contribution to the strain. By performing imbibition experiments on unsealed porous monoliths under low humidity conditions, the porous sample is exposed to both inward (imbibition) and outward (evaporation) flows simultaneously. The imbibition flow rate decreases with time while the drying flow rate increases with the wetted volume. When the in- and out-flow rates match, a dynamic equilibrium scenario occurs, resulting in the arresting of the filling front. This state, known as the artificial tree configuration, is characterized by highly curved menisci at the filled-empty (liquid-gas) interface. Therefore, a linear negative pressure gradient exists between the top and bottom of the saturated region, which subjects this wetted portion of the pore space to tensile contraction. As a result, there is no observable disappearance of the Laplace pressure during the dilatometry experiments.