Hydrodynamics of capillary imbibition under nanoconfinement.

Understanding fluid flow in nanoconfined geometries is crucial for a broad range of scientific problems relevant to the behavior of porous materials in biology, nanotechnology, and the built environment. Because of the dominant importance of surface effects at the nanoscale, long-standing assumptions that are valid for macroscopic systems must be revisited when modeling nanoconfined fluids, because boundary conditions and the confined behavior of liquids are challenging to discern from experiments. To address this issue, here we present a novel coarse-grained model that combines parameters calibrated for water with a dissipative particle dynamics thermostat for the purpose of investigating hydrodynamics under confinement at scales exceeding current capabilities with all-atomistic simulations. Conditions pertaining to slip boundary conditions and confinement emerge naturally from particle interactions, with no need for assumptions a priori. The model is used to systematically investigate the imbibition dynamics of water into cylindrical nanopores of different diameters. Interestingly, we find that the dynamic contact angle depends on the size of the nanopore in a way that cannot be explained through a relationship between contact line velocity and dynamic contact angle, suggesting nonlocal effects of the flow field may be important. Additionally, a size-dependent characteristic time scale for imbibition is found, which could be useful for the interpretation of experiments and design of novel nanofluidic devices. We present the first systematic study that explains how contact angle dynamics and imbibition dynamics vary with nanopore radius. Our modeling approach lays the foundation for broader investigations on the dynamics of fluids in nanoporous materials in conjunction with experimental efforts.