The surface viscosity of soluble films

Abstract

The surface viscosity of soluble films can be determined by measuring the velocity, v_m, of an element of surface moving along the centre-line of a rectangular trough under the influence of a constant bulk flow, V, of solution along the trough. An equation is derived in which the ratio, V/v_m, is described accurately in terms of the dimensions of the trough, the viscosity of the solution, and the surface viscosity of the film adsorbed at the air-water interface. This equation is based on the assumption that the surface pressure gradient along the trough is zero. When the surface pressure gradient is not zero, but is constant and of known magnitude, a second equation makes it possible to determine the surface viscosity.       Measurements of surface viscosity have been made with water, electrolyte solutions, and solutions of sodium dodecyl sulphate and of n-octanol.