Gravity-driven evaporative flux profiles of a sessile droplet*

Droplet evaporation is a universal and important problem for various industrial fields such as inkjet printing, agriculture, spray cooling, and DNA microarray. It is well-known that the evaporative flux is non-uniform along the droplet surface and the flux is divergent at the contact line, which induces a coffee-ring flow. However, it is still difficult to directly measure the evaporative flux of the drying droplet. Hence, most of researchers typically investigated how the internal flow patterns look like when the droplet evaporated in various situations. Due to this reason, vapor distribution around an evaporating droplet has rarely been studied. In this work, we directly visualized the evaporated vapors of highly volatile liquid using a laser interferometry. We found that the evaporative flux and speed are dramatically suppressed or enhanced depending on the droplet position, e.g., on-plate and upside-down. We confirmed that the vapor distribution plays an important role to determine the total evaporation time.