Evaporation characteristics of water droplets in Cassie, Wenzel, and mixed states on superhydrophobic pillared Si surface

Abstract

Evaporation characteristics of various droplets on superhydrophobized Si micro-pillar (SiμP) substrate have been investigated experimentally. Especially, the unique characteristics of evaporating mixed drop have been elucidated for the first time, to the best of the authors’ knowledge. Processes for forming different droplets, i.e., Cassie, Wenzel, and mixed ones, are developed. Exposure to humidifier mist has enabled the formation of Cassie droplets, droplets sitting on pillars top with entrapped air underneath. Water vapor condensation has led to the formation of mixed droplets, that is, Cassie droplet with impaled plugs. The mixed droplets have been converted into Wenzel ones by simply adding additional water and applying pressure to them. These droplets have shown different evaporation characteristics. As expectedly, mixed droplets have shown evaporation time in-between of those for Cassie and Wenzel ones. Further, compared to the well-known constant contact angle (CCA) mode of evaporation for Cassie and constant contact radius (CCR) mode of evaporation for Wenzel droplets, respectively, the evaporation of the mixed droplet has been found to be pseudo-CCA mode, where the contact angle decreases/recovers with stepwise decrease in contact radius. This evaporation behavior is quite similar to the stick-slip motion of droplet evaporation on superhydrophobic surface. Finally, spontaneous droplet movement, depending on the droplet type, has been traced during the evaporation. While the Cassie drop moves rather randomly during evaporation, the mixed droplet showed predictable movement depending on the size and location of Wenzel plugs. The evaporation characteristics of different droplets can be a valuable help for a wide variety of applications such as self-cleaning, dewing, fog-basking, dropwise/filmwise condensation, etc.