Investigating the Influence of Monolayer Compressibility on Interfacial Fluid Dynamics with Combined Simulation and Experiment

The viscoelastic properties of monolayers of amphiphilic molecules at a fluid-fluid interface determine, in part, the stability of the foams and emulsions found in cosmetics, pharmaceutical formulations, food stuffs, and petroleum refining. In addition to foam and emulsion stability, the mechanical properties of fluid-fluid interfaces are relevant for the study of biological membranes, especially lipid bilayers, where viscoelastic properties affect the transport of molecules tangentially through the membrane, and mechanical properties provide biological cells with requisite structural integrity. Unlike most bulk fluids, fluid-fluid interfaces of this type are highly compressible and display a rich variety of viscoelastic behavior that depends strongly on the area-density of the interfacial molecules. Although these interfaces are quite clearly compressible, the most successful hydrodynamic models of probes translating through a fluid-fluid interface make the simplifying assumption that the interface is incompressible on the time scale of the movement of the probe. Recent experimental work has indicated that in certain interfaces the validity of this assumption is questionable. Understanding the role that dilatational effects play in fluid-fluid interfaces is especially important for the implementation of microrheological methods, and the movement of molecules within biological membranes. We will present our findings from simulation and experiment on the influence of compressibility on a probe translating through a fluid-fluid interface.