Internal flow patterns of an evaporating multicomponent droplet on a flat surface

Evaporation processes of droplets with different ethanol/water compositions on a hydrophobic heated surface were investigated utilizing particle image velocimetry (PIV). It is found that the process of an evaporating two-component droplet can be divided into downward vortex stage, transition stage and upward vortex stage sequentially, while a pure water droplet only has upward vortices stage. The three stages were analyzed utilizing experimental results and dimensionless parameters, such as thermal Marangoni number (MaT) and Solutal Marangoni number (MaS). For downward vortex and transition stages, solutal and thermal Marangoni effects were dominating. The occurrence of transition stage indicated buoyancy effect becomes comparable to Marangoni effect as ethanol continuously evaporated, and the occurrence of upward vortex stage indicated most of ethanol gasified and buoyancy effect dominated. The influence of initial composition and heating power on the downward vortices and transition stages has also been investigated. With low input power, droplets with high ethanol concentration have shorter evaporation time in above two stages. On the contrary, however, droplets with high ethanol concentration have longer evaporation time in above two stages with high input power.