Deformation dynamics of nanopores upon water imbibition

Capillarity-driven transport in nanoporous solids is ubiquitous in nature and
is of increasing importance for the functionality of modern liquid-infused
engineering materials. During imbibition, highly curved menisci are driven by
negative Laplace pressures of several hundred atmospheres, exerting an enormous
contractile load on an increasing portion of the porous matrix. Due to the
challenge of simultaneously monitoring imbibition and deformation with high
spatial resolution, the resulting coupling of solid elasticity to liquid
capillarity has remained largely unexplored. Here, we study water imbibition in
mesoporous silica using optical imaging, gravimetry, and high-resolution
dilatometry. In contrast to an expected Laplace pressure-induced contraction,
we find a square-root-of-time expansion and an additional abrupt length
increase when the menisci reach the top surface. The final expansion is absent
when we stop the imbibition front inside the porous medium in a dynamic
imbibition-evaporation equilibrium, as is typical for water transport and
transpiration in plants. These peculiar deformation behaviors are validated by
single-nanopore molecular dynamics simulations and described by a continuum
model that highlights the importance of expansive surface stresses at the pore
walls (Bangham effect) and the buildup or release of contractile Laplace
pressures as nanoscale menisci collectively advance, arrest, or disappear. Our
model predicts that these observations are valid not only for water imbibition
in silica, but for any imbibition process in nanopores, regardless of the
liquid/solid combination. This also suggests that simple deformation
measurements can be used to quantify surface stresses and Laplace pressures or
transport in a wide variety of natural and artificial porous media.