Linking nonisothermal interfacial temperature and flow field measurements at an evaporating droplet

Abstract

Regular thermal patterns can be formed spontaneously at the evaporating interface of a sessile droplet. Our experimental investigations through the thermography and particle image velocimetry reveal the linkage of the nonuniform interfacial temperature and the flow field in an ethanol droplet which is designed to evaporate on a heated substrate with a constant contact line mode before the last phase of drying. It is suggested that the Bénard-Marangoni instability is responsible for the regular thermal patterns. For the present cases with a fixed substrate temperature, the evolution of deformed Bénard-Marangoni convection cells is solely dependent on the instant contact angle. The thermocapillary instabilities in the sessile droplet follow an evolution tendency consisting of three stages and two transition periods. As the contact angle decreases during evaporation, the dominant thermocapillary instabilities firstly transfer from Marangoni-capillary circulation to deformed Bénard-Marangoni cells at a growth rate of 7.9 per degree, and then to conventional Bénard-Marangoni cells. The decrease of deformed Bénard-Marangoni cells is almost at a constant rate of 1.5 per degree during the second stage. Details of flow fields and the corresponding interfacial temperature distributions are consistent with each other qualitatively and quantitatively. Two critical contact angles, 46∘ and 22∘, are found for the transitions of instabilities.