Surface viscosities at high pressure gas-liquid interfaces

Abstract

Coalescence times of bubbles of Ar, CH 4, C 2H 4, C 2H 6, H 2S, CO 2, and N 2O in n-hexane are reported. The bubbles were formed simultaneously on two adjacent nozzles in saturated n-hexane and the film lifetimes, or bubble coalescence times, were measured at pressures up to 3.5 MPa by the use of high-speed cinematography. The coalescence times, defined as the time from first touching of the bubbles until they break, were all very short and in no instance exceeded 5 msec. From these results and from previous results for gases in water, the surface viscosities of the solutions were calculated using a modification of the procedure developed by Hartland and Barber. To use this method, an estimate of the film thickness at the time of breaking is required, and this was supplied by Vrij's theory for the inherent breaking times of films. This theory requires a value for the surface viscosity and thus an iterative procedure was used to calculate the surface viscosity appropriate to the observed coalescence time. The surface viscosity obtained is the sum of the dilational and shear viscosities. Reasons are advanced for believing that in these high pressure gas systems the surface dilational viscosity is low. Thus the viscosities measured are largely surface shear viscosities. With water as a solvent, surface viscosities rise from less than 3 μg/sec to as much as 400 μg/sec as the saturation vapor pressure is approached, whereas with n-hexane as a solvent, the surface viscosities are all less than 3 μg/sec. Molecular interactions are proposed to explain this observed behavior. The ability of the gas to form adsorbed multilayers on the solvent surface is believed to promote an increase in surface viscosity.