Measurement of an Evaporating Drop on a Reflective Substrate

The figure depicts an apparatus that simultaneously records magnified ordinary top-view video images and laser shadowgraph video images of a sessile drop on a flat, horizontal substrate that can be opaque or translucent and is at least partially specularly reflective. A similar apparatus for recording such images of a drop on a transparent substrate was described in Measuring Contact Angles of a Sessile Drop and Imaging Convection Within It" (LEW-17075), NASA Tech Briefs, Vol. 25, No. 3 (March 2001), page 50. As in the case of the previously reported apparatus, the diameter, contact angle, and rate of evaporation of the drop as functions of time can be calculated from the apparent diameters of the drop in sequences of the images acquired at known time intervals, and the shadowgrams that contain flow patterns indicative of thermocapillary convection (if any) within the drop. These time-dependent parameters and flow patterns are important for understanding the physical processes involved in the spreading and evaporation of drops.