(260g) Supression of Capillary Waves in a Dipolar Fluid

A well-known aspect of liquid/vapor systems is the presence of capillary waves at an interface. Capillary waves are density fluctuations that result of a balance between thermal fluctuations and the surface tension. Because the liquid/vapor interface is two-dimensional, the amplitude of the capillary wave diverges logarithmically with the interfacial area. In the absence of an external field, the amplitude of the logarithmic divergence depends inversely on the liquid/vapor surface tension. We use molecular dynamic simulations to investigate the roles of both surface tension and an external electric field strength and direction on the capillary waves in a system of Lennard-Jones particles with fixed-point dipoles, commonly known as a Stockmayer fluid. In the presence of an electric field parallel to the interface, the amplitude of the capillary waves is suppressed. We show that while the surface tension increases with increasing strength for an external field parallel to the interface, the large decrease of the amplitude of the capillary waves is not solely a result of the increased surface tension. We show that field dependence at small field strength can be accurately treated by an additional prefactor to the logarithm that is linear in in the field strength. For an electric field perpendicular to the interface, the amplitude of the capillary waves initially increases with increasing field strength until the interface becomes unstable. The critical perpendicular field strength value is inversely proportional to the size of the liquid/vapor interface. This instability is well known in systems such as ferrofluids.