Diffraction patterns of a water-submerged superhydrophobic grating under pressure

We report on a study of superhydrophobic surfaces submerged in water in a fluidic chamber. A surface-treated transmission grating was used as a superhydrophobic layer that had a well-defined diffraction pattern when a laser beam passed through the water-submerged grating sample, indicating a Cassie-Baxter state with trapped air between the water and grating interfaces. By applying pressure to the water in the fluidic chamber, the diffraction pattern can be changed because of the volume reduction of trapped air or water penetration into the grating. Depending on the maximum value of applied pressure in the fluidic chamber, the diffraction pattern change can be either reversible or irreversible after the release of the pressure. We attribute the irreversible change under high applied pressure to the switching from a Cassie-Baxter state to a Wenzel state.