Contribution of thermocapillary convection to liquid evaporation

Abstract

A series of 12 steady-state evaporation experiments were conducted in which water evaporated into a controlled environment from one of either two funnels that had similar geometry but were constructed of PMMA or stainless steel. We examined the energy flux by thermal conduction and thermocapillary convection. We compared it with total energy required to evaporate water at the rate measured with a syringe pump. The results indicate that for the PMMA funnel, thermal conduction fully supplied the energy required for evaporation at the observed rate, and that thermocapillary convection was negligible. However, for the stainless steel funnel, the measurements showed that thermal conduction supplied approximately 50% of the energy. Once the thermocapillary transport of energy along the interface using the previously reported value of surface thermal capacity was accounted for, conservation of energy was fully satisfied meaning that thermocapillary convection contributed approximately 50%. The results support the validity of the proposed surface thermal capacity for water and highlights the necessity of including the thermocapillary transport. We also evaluated the possibility of a sharp temperature variation in the vapor phase near the interface and found that it led to an over estimating the energy required for evaporation.