Evaporation-triggered wetting transition for water droplets upon hydrophobic microstructures.
When placed on rough hydrophobic surfaces, water droplets of diameter larger than a few millimeters can easily form pearls, as they are in the Cassie-Baxter state with air pockets trapped underneath the droplet. Intriguingly, a natural evaporating process can drive such a Fakir drop into a completely wetting (Wenzel) state. Our microscopic observations with simultaneous side and bottom views of evaporating droplets upon transparent hydrophobic microstructures elucidate the water-filling dynamics and suggest the mechanism of this evaporation-triggered transition. For the present material the wetting transition occurs when the water droplet size decreases to a few hundreds of micrometers in radius. We present a general global energy argument which estimates the interfacial energies depending on the drop size and can account for the critical radius for the transition.
Tsai
Peichun
P
Physics of Fluids Group, Faculty of Science and Technology, Impact and Mesa;{+} Institutes, University of Twente, P.O. Box 217 7500AE Enschede, The Netherlands.
Lammertink
Rob G H
RG
Wessling
Matthias
M
Lohse
Detlef
D