(670a) Untangling the Emergence of Inhomogeneous Structure and Mobility in Confined Fluids

Confined fluids emerge in a diverse array natural and technological contexts, and relating their equilibrium structural and relaxation properties to one another and to the physics of bulk fluids facilitates the inverse engineering of nano- and microfluidic systems. While static effects due to confinement, such as one-body density variations that develop due to particle layering, are well-understood, many open questions remain in terms of characterizing and predicting how particle motions are coupled to (or, possibly, decoupled from) such density modulations. Using computer simulations together with techniques including a Fokker-Planck equation-based method, we examine the position-dependent and average diffusivities of tracer particles situated within (1) highly confined hard-sphere fluids designed to mimic real colloidal thin films and (2) systems that incorporate only the dynamic constraints of confinement while preserving homogeneous fluid structure. Based on the former, we show that local measurements of diffusive mobility cannot be naively extrapolated even qualitatively based on bulk fluid behaviors; this is evidenced by, e.g., a reversal of local structure-mobility correlations that emerges upon supercooling the thin film at high average packing fractions. Using the latter type of system, we further demonstrate that position-dependent dynamic signatures can emerge in the absence of structural inhomogeneity entirely.