Vapor flow induced and regulated spontaneous drop movement in Marangoni condensation of water-ethanol mixtures

Abstract

The spontaneous movement of condensate drops on heat transfer surfaces during Marangoni condensation has been attributed to surface tension gradients induced by temperature differences. While previous studies have explored this mechanism, a unique phenomenon is newly reported in this work: condensate drops exhibit spontaneous movement solely driven by the effect of vapor inflow on a horizontal heat transfer surface, even in the absence of a bulk temperature gradient. This previously unrecognized factor challenges and expands current understanding of Marangoni condensation dynamics. This study systematically investigates this vapor flow-induced drop movement and further analyzes the characteristics of spontaneous drop movement under the coexistence of both vapor flow and surface temperature gradients. Experimental analyses were performed across various parameters, including surface subcooling and vapor concentration, to elucidate the interplay between these two major factors. Results show that the direction and velocity of drop movement are determined by the dominance of either the vapor flow or the temperature gradient, with the vapor flow effect becoming more prominent with increasing surface subcooling. These findings offer critical new insights into the mechanisms governing Marangoni condensation, particularly emphasizing the significant, and often overlooked, role of vapor flow in driving and modulating droplet motion.