Evaporating drops on patterned surfaces: Transition from pinned to moving triple line

Evaporation of sessile drops on micro-patterned surfaces is investigated over a range of heterogeneity length scales and solid area fractions. The surface topology is generated by a uniform arrangement of square pillars or square holes. The evaporation process is captured using high resolution imaging techniques and later post-processed for such information as contact angle, contact circle diameter and drop volume. It is observed that two distinct phases of evaporation existed for all substrate characteristics: pinned triple line (TL) phase and moving TL phase. In both phases, the process follows a linear decrease of surface area. The dimensionless evaporation rate constant is found to be higher during the moving TL phase in comparison with the pinned TL phase. In addition, it is found that the triple line topology has no effect on the evaporation rate constant. (C) 2009 Elsevier Inc. All rights reserved.