Bridging Microscopic Dynamics and Hydraulic Permeability in Mechanically-Deformed Nanoporous Materials.

In the field of nanoconfined fluids, there are striking examples of deformation/transport coupling in which mechanical solicitation of the confining solid and dynamics of the confined fluid impact each other. While this intriguing behavior can be harnessed for applications (e.g., energy storage, phase separation, catalysis), the underlying mechanisms remain to be understood. Here, using molecular simulations, we investigate fluid flow in deformable nanoporous materials subjected to external mechanical stresses. We show that the pore mechanical properties significantly affect fluid flow as they lead to significant pore deformations and different fluid organization at the solid surface. Despite such mechanical effects, we show that the fluid thermodynamic properties (i.e., adsorption) can be linked consistently to Darcy's law for the permeability by invoking a pore size definition based on the concept of Gibbs' dividing surface. In particular, regardless of the solid stiffness and applied external stress, all data can be rationalized by accounting for the fluid viscosity and slippage at the solid surface (independently of a specific pore size definition). Using such a formalism, we establish that the intimate relation─derived using the linear response theory─between collective diffusivity and hydraulic permeability remains valid. This allows linking consistently microscopic dynamics experiments and macroscopic permeability experiments on fluid flow in deformable nanoporous materials.