Experimental and numerical study of anomalous thermocapillary convection in liquid gallium

Thermocapillary Marangoni convection of liquid gallium was studied experime ntally and numerically. A specially designed experimental setup ensured an oxide-free surface of the liquid gallium for a very long time. The convecti ve flow at the free surface was found to be directed opposite to both buoya ncy-driven and ordinary thermocapillary convection. The anomalous direction of the thermocapillary flow was explained by the presence of a small amoun t of a surface-active contaminant-lead adsorbed at the free surface. Two di fferent approaches were used to describe the observed phenomenon. First, th e flow was treated as a pure thermocapillary convection with a modified dep endence of the surface tension on temperature so that to reproduce the meas ured velocity distribution. Second, a novel physical model was devised for the flow driven by the gradient of the surface tension induced by the tempe rature dependence of the concentration of the adsorbed layer of contaminant . In contrast to the ordinary thermocapillary convection in low-Prandtl-num ber liquids, there is a strong coupling between the flow and the driving fo rce in the proposed model resulting in velocity profiles very similar to th ose observed in the experiment. (C) 1999 American Institute of Physics. [S1 070-6631(99)02811-1].