Effect of entrapped phase on the filling characteristics of closed-end nanopores.

We investigated the filling dynamics in closed-end capillaries of sub-micron length scale, in which the displacing phase advances at the expense of the entrapped phase. Contrary to common intuition, we reveal that the existence of a displaced phase in a closed-end nano-scale system does not necessarily retard the meniscus advancement over all temporal regimes, unlike what is observed in cases of macro-scale capillaries, but can also sometimes augment the local filling rates. We determined that the combined effect of surface wettability and the displaced phase molecules resulted from the pinning-depinning of the meniscus, and hence, from the local dynamics of capillary filling. We also employed a simple force balance-based model to capture the essential interfacial phenomena governing this behavior, and benchmarked the same with our molecular dynamics simulations. Our results suggest a possible mechanism for modifying the effective wettabilities of nano-scale capillaries without any modification of the surface architecture or chemical treatment of the surface.