Solutal and Gravitational Effects during Binary Mixture Droplets Evaporation

Abstract

For small droplets undergoing phase change, gravity is generally considered negligible. In the case of binary droplets evaporation, convective flows can be induced due to various mechanisms, such as continuity, buoyancy and/or selective evaporation of one of the components. Convection can also be induced by surface tension gradients resulting from concentration variations along the interface. This study presents experimental results of evaporation for binary mixture droplets. We concurrently investigate sessile and pendant droplets to assess gravity’s impact on binary droplet evaporation. We examine compositions including, pure butanol, pure methanol, pure water, and 50% per volume mixtures of water-butanol and water-methanol, evaporating in a controlled atmosphere. In the case of water-butanol mixtures, the drops contact line ‘depins’ during the evaporation process whereas the case of water-methanol mixture, the contact line of the drops remains pinned most of the lifetimes. The analysis of the evaporation dynamics reveals differences in the evaporation of these two mixtures and the effect of orientation (gravity). For water-butanol mixtures the evaporation occurs in four stages linked to preferential evaporation of the more volatile component and the ensuing surface tension gradients. In the case of water-methanol mixtures, contact lines tend to be pinned during most of the lifetimes of drops. The evaporation rate of the mixture is found to be between the ones of the pure components, i.e. water and methanol. The case of sessile drops exhibits a slight enhancement in evaporation rate in the case of the sessile configuration compared to the pendant one for pure water and mixture cases, which is explained by density differences and buoyancy driven flows. Solutal Marangoni flows in the case of water-methanol mixtures are deemed weaker compared to water-butanol ones. The use of the two mixtures allowed to have a good comparison between two cases where solutal-Marangoni effect can be strong (water-butanol) and weak (water- methanol) influence. The densities of the two organic liquids also highlighted gravitational effect due to the large difference in vapor densities.