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

Systematic study of soluble surfactant effect on thermocapillary flow that artificially provoked by a steady point heat source at a gas/liquid interfa ce was performed experimentally. Surface velocity and temperature profiles were measured at three Delta Ts (temperature difference between heated sour ce and bulk solution) for pure water and SDS solutions of five concentratio ns. Experimental results show that small amounts of SDS in water retard sig nificantly the thermocapillary flow and consequently alter the surface velo city and temperature profiles. The dependence of the average interfacial Re ynolds 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 satisfact orily represented by proposed empirical correlations. Moreover, mechanism o f 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 surf actant solutions, that of the diffusocapillary, on the other hand, is mainl y due to the reduction in the driving force that triggers the diffusocapill ary flow. (C) 2000 Elsevier Science Ltd.