Visualization of the Cassie--Wenzel transition with X-ray microscopy*

Water droplets on hydrophobic surfaces with micropillar usually exhibit two wetting states: (i) the Cassie state when air is trapped between water and micropillars and (ii) the Wenzel state when air is completely replaced by water. A transition from the Cassie to the Wenzel states is essential in designing stable hydrophobic surfaces. Directly visualizing the Cassie-Wenzel (C-W) transition is difficult with conventional microscopies because of no transparency from micropillars. Here we suggest a powerful technique based on high-resolution high-penetration X-ray microscopy for clearly visualizing the C-W transition. Thanks to the X-ray penetrating into the opaque micropillars, we were able to directly explore the intermediate state during the C-W transition. We study on the transition dynamics regarding how air replacement by water was gradually propagated with position and time. We believe that the replacement dynamics would be explained as a kind of phase transition kinetics.