Effects of evaporation on thermocapillary flows in a liquid thin layer

A series of experiments was conducted to investigate the thermocapillary flows in an evaporating liquid thin layer. Ethyl alcohol and freon-113 were used as the test liquids for their typical evaporation and convection characteristics. Both evaporating in open air and in a closed chamber were conducted. Visualization of the thermocapillary flow in the layers was performed through direct video recording in which aluminum powder was used as a tracer. Temperature profiles across Phe evaporating layers were determined by simultaneous temperature measurements through thermocouples deployed at different levels in the layer. The visualization shows that the evaporation condition of the layer obviously affects the thermocapillary flow. For example, once the evaporation is inhibited the convection dies out quickly. Benard cells were observed and recorded when the layer evaporate in open air at room temperature both with and without bottom cooling from below. Brisk Benard cells were observed in a freon-113 thin layer with a nonlinear temperature profile when it is cooled from below. More interesting results were obtained from a evaporating alcohol thin layer. Active Benard cells can be also observed in the layer with zero and even positive temperature gradient. In view of this experimental finding, the traditional explanations for Marangoni-Benard convection may not applicable to the case of evaporating layers. A new expression of Marangoni number dealing with evaporation rate and latent heat is suggested. A possible mechanism of Marangoni-Benard convection in an evaporating liquid thin layer which is called surface-tension-driven convection fueled by evaporation is presented and described briefly.