Surfactant-induced retardation of the thermocapillary flow at a gas/liquid interface

Systematic study of soluble surfactant effect on thermocapillary flow that artificially provoked by a steady point heat source at a gas/liquid interface was performed experimentally. Surface velocity and temperature profiles were measured at three Delta Ts (temperature difference between heated source and bulk solution) for pure water and SDS solutions of five concentrations. Experimental results show that small amounts of SDS in water retard significantly the thermocapillary flow and consequently alter the surface velocity and temperature profiles. The dependence of the average interfacial Reynolds number ratio, Re-S / Re-W, and average temperature difference ratio, (Theta(S)-Theta(C))/(Theta(W)-Theta(C)), on Delta T and C can be satisfactorily represented by proposed empirical correlations. Moreover, mechanism of retardation by surfactant for thermocapillary and diffusocapillary flows is found to be quite different. While surfactant-induced retardation of the thermocapillary flow is due to the interfacial elasticity property of surfactant solutions, that of the diffusocapillary, on the other hand, is mainly due to the reduction in the driving force that triggers the diffusocapillary flow. (C) 2000 Elsevier Science Ltd.