Dynamic interactions of monomolecular films with imposed electric fields

Abstract

The electric surface force density caused by imposed electric fields and their concomitant surface charges is the basis for studying monomolecular films on liquid interfaces. The film is modeled as a compressible viscous two-dimensional fluid characterized by an elasticity ℰ, a surface dilatational viscosity, a surface shear viscosity, surface diffusion, and chemical equilibrium with film molecules diffusing through a liquid substrate. In the first of three configurations considered, the static rise in film surface pressure is experimentally shown to equal the integrated static shear stress. Steady “second-order” film circulations and film rupture under high electric stress, are discussed. In the second configuration, temporally and spatially periodic electric stresses are used to study the dilatational film dynamics in an “imposed ω-k” configuration having angular frequency ω and wavenumber k. Experiments demonstrate the theoretically predicted dilatational resonance at ω = (E0k2/√2ρη)2/3, where ρ and η are, respectively, the mass density and viscosity of the liquid bulk. Effects of surface and bulk diffusion, as well as dilatational viscosity, are shown each to have a characteristic effect on the frequency response. In the third configuration, surface shearing of a film is modeled and sensitivity to surface shearing viscosity predicted.