Internal flow near the triple line in sessile droplets of binary mixtures during evaporation at different ambient temperatures

The internal flow of an evaporating droplet is mainly influenced by capillary flow and Marangoni flow, which are affected by many factors including ambient temperature in the gas phase. For binary droplets, the internal flow near the triple line is of critical importance for understanding their evaporation characteristics. In this work, the internal flow near the triple line in evaporative sessile droplets of binary mixtures, including methanol, ethanol, 1-propanol, 1-butanol, 1-pentanol, 1-hexanol, and 1-heptanol of the same mass which were added to deionized water, at different ambient temperatures was experimentally investigated. An evanescent-wave based fluorescence microscope which has a depth of illumination field of about 360 nm was used to obtain the depinning times and near-wall average velocities of the samples. The relationship between the near-wall velocity V and the time t was described by V similar to (t(f)-t)(-1), for droplets without vortex, and changed to quadratic-functions due to the coexistence of the vortex and depinning phenomena. For droplets without vortex, an equation was derived for quantitatively describing the relationship between the near-wall velocity and the ambient temperature. (C) 2019 Elsevier Ltd. All rights reserved.