EXPERIMENTAL-EVIDENCE OF THE EFFECT OF EVAPORATION-CONDENSATION ON THERMAL MARANGONI FLOWS IN AQUEOUS FATTY ALCOHOL-SOLUTIONS

The static surface tension (sigma(e)) of aqueous solutions of a fatty alcohol versus temperature exhibits a minimum. Thus, a temperature gra dient which is created at the free surface of such a solution at tempe ratures higher than that of the minimum (sigma(e)) should induce a sur face flow from the cold area to the hot one. This was indeed observed even at temperatures much lower than that of the minimum. A possible e xplanation is the evaporation of alcohol at the hot area where the sur face tension increases, and its condensation on the cold area where th e surface tension decreases, the alcohol being tranported by the gas p hase. The thickness of the gas phase over the liquid could play a role in the observed flows. A device was built in which the thickness of t he gas phase could be adjusted. Experiments performed with a 6.2 x 10( -3) m solution of n-heptanol indicate a lowering of the surface veloci ty where the thickness of the gas phase is reduced, (C) 1998 Academic Press.