Using state-of-the-art computer simulation methods, UB chemical engineers are developing a better understanding of the behavior of fluids in the presence of one or more surfaces. Such knowledge is important from both scientific and industrial perspectives.

In the broadest sense our research focuses on the investigation of the structure, dynamics, and phase behavior of complex liquids, amorphous solids, and biological materials from a microscopic perspective. Essentially, we attempt to understand the macroscopic behavior of a system in terms of its underlying molecular-level details. The primary tool used to study these systems is molecular simulation. Within this general area, we employ a wide range of both molecular dynamics and Monte Carlo techniques.

One of our projects involves investigating the phase behavior of fluids in the presence of one or more surfaces. Under these conditions fluids exhibit a rich variety of phase transitions that are absent in bulk fluids. Examples include prewetting, layering, and capillary condensation transitions. Even the simplest of systems display a broad range of phase behavior, with surface phase diagrams depending qualitatively on the relative strengths and ranges of the fluid-fluid and fluid-substrate interactions. A fundamental understanding of these surface phenomena is important from both a scientific and industrial perspective.

When a fluid weakly adsorbs onto a solid substrate a system may exhibit a wetting transition. This transition is associated with a wetting temperature, which indicates the point at which fluid adsorption switches from partial to complete wetting. Below the wetting temperature, the thickness of an adsorbed film remains finite for all pressures below the bulk vapor-liquid saturation pressure. Above this temperature, the film thickness diverges as the saturation pressure is approached. The system may also exhibit prewetting transitions at temperatures above the wetting temperature, characterized by a transition between a thin and thick adsorbed film. Akin to bulk vapor-liquid saturation lines, prewetting saturation lines terminate at critical end points. The figure below provides an example of thin and thick coexisting absorbed films at a temperature approximately 20% below the system's prewetting critical temperature.

We have recently used state-of-the-art simulation methods to investigate the prewetting behavior of a model system that was originally designed to describe the adsorption of argon onto a solid carbon dioxide surface. Future work will focus on further understanding wetting phase behavior. For example, we are interested in quantitatively probing how the relative strength and range of surface-fluid and fluid-fluid interactions influence a system's prewetting phase behavior. Precise location of prewetting critical points and examination of the nature of prewetting criticality are also of interest. We are also interested in using molecular simulation to interrogate phase transitions that occur within confined systems. For example, we would like to better understand the competition between layering, prewetting, and capillary condensation transitions that occur within a slit pore.