(527c) How Surface Features and Roughness Impact the Structure, Dynamics, and Thermodynamics of a Confined Fluid

It is well appreciated that placing a fluid under confinement drastically alters both its static and dynamic properties relative to a bulk, unconfined fluid. However, predicting these changes (even semi-quantitatively) remains a scientific challenge, and thus, there exist fundamental knowledge gaps in a number of areas where confined fluids play a central role. Examples include lubrication, friction, and adsorption. In an effort to understand the impact of confinement on fluid properties, recent work [1] has demonstrated that there exists a robust relationship between static fluid properties, such as excess entropy and insertion probabilities, and dynamic fluid properties, in particular, the self-diffusion coefficient, that is independent of the degree and geometry of confinement. Yet, this has been identified from studies of systems with straightforward confinement and it is unclear whether the characteristics of the confining medium, such as roughness will alter such a relationship. Here, we seek to test the relationship between static and dynamic properties more rigorously by investigating the influence of the physical characteristics of the confining surfaces. In this talk, we address this question via molecular simulations, of a model fluid confined in a slit-pore geometry that incorporates sinusoidal corrugations in on direction [2], i.e., a type of surface roughness. We systematically investigate the dependence of static and dynamic properties on the length scales associated with the wall corrugation and show that static thermodynamic properties, such as the excess entropy and solid-fluid interfacial tension, are insensitive to the degree of surface roughness. However, we find that the self-diffusion of particles in the rough direction varies drastically depending on the degree of surface roughness. Although this calls into question the link between static quantities and self-diffusion previously reported, we demonstrate that this discrepancy may be explained by a scaling law that governs dynamics in the limit of zero fluid density.

[1] J. Mittal et al. J. Phys. Chem. B 111 10054 (2007); G. Goel et al., J. Stat. Mech. 2009 P04006 (2009).

[2] D. J. Diestler and M. Schoen Phys. Rev. E 62 6615 (2000); F. Porcheron and M. Schoen and A. H. Fuchs J. Chem. Phys. 116 5816 (2002)