Effect of Hydrothermal Waves on Evaporation Distribution during Drop Evaporation

The objective of this study is to clarify physical mechanisms involved in the evaporation of small (a few microliters) sessile drops. We aim to understand the relation between local thermal information at the solid-liquid interface and overall evaporation. An infrared (IR) camera and a charge-coupled device (CCD) camera were used to determine the temperature and heat flux distribution at the solid-liquid interface and the profile of the evaporating drop, respectively. The temperature distribution at the solid-liquid interface was determined using a multilayer substrate consisting of a silicon wafer coated with a thin thermal insulator that is partially transparent to IR. The liquids used were water and FC-72. The evaporation rate of water drops was found to occur mostly at the contact line. However, the heat transfer distribution at the liquid-solid interface was relatively uniform, indicating the heat transferred from the wall must be transported within the drop to the contact line. The mechanisms by which this occurs have yet to be determined. In contrast, the evaporation rate of FC-72 drops where hydrothermal waves were present was found to be proportional to the liquid-vapor interface area rather than the circumference of the drop, indicating a more uniform distribution of evaporation.