(705a) Tuning Local Structure and Position-Dependent Mobility in Inhomogeneous Fluids

When fluids are subjected to static spatially-variant external fields, they become inhomogeneous; i.e. their structural properties including the one-body density and their dynamic properties (e.g. diffusivity tensor) develop dependencies on position. Given the ubiquity of inhomogeneous fluids in nature and in technological applications due to, e.g., the presence of multiphase interfaces, substrate patterns, applied fields, etc., there is considerable interest in understanding and modeling these position-dependent behaviors. We use molecular dynamics simulations and a stochastic method based on the Fokker-Planck formalism to examine the consequences of inhomogeneous density profiles on the thermodynamic and dynamic properties of the hard-sphere fluid and supercooled liquid water. Effects of scale and strength of the inhomogeneity are systematically considered via the imposition of periodic external fields with various wavelengths and amplitudes, with a focus on molecular-scale wavelengths. For sufficiently long-wavelength density variations, bulk-like relationships between local structure, thermodynamics, and diffusivity are observed as expected. However, for both hard-sphere and supercooled water systems, a crossover in behavior occurs as the inhomogeneity wavelength approaches the scale of a particle diameter, with qualitatively different correlations emerging between the local static and dynamic quantities. Irrespective of the characteristic wavelength of the external fields, we find that average diffusivities of the hard-sphere fluids in both inhomogeneous and homogeneous directions are coupled and approximately correlate with local available volume (i.e. volume not occupied by particles). This criterion highlights external field patterns that can induce density fluctuations which inherently maximize (or, conversely, minimize) average particle mobility.