X-ray microscopic observations of Cassie-Baxter wetting behaviors*

Cassie-Baxter wetting is prevailing in nature, for instance on lotus leaves and the body of the water strider, as well as on hydrophobic micropatterned surfaces, enabling to superhydrophobicity. Cassie-Baxter wetting is widely known to exhibit a flat bottom of a droplet on the top of the micropatterned surface. Occasionally a droplet has been found to be partially impaled into micropatterns and its bottom looks curved as observed with interference techniques, confocal microscopy, X-ray microscopy, and simulations. Here we clearly observe Cassie-Baxter wetting behaviors with high-resolution high-speed X-ray microscopy. We demonstrate that occurrence of flat or curved bottoms of a droplet is dependent on droplet volume, initial wetting, and micropattern geometry. Consequently, new interpretation for Cassie-Baxter wetting is suggested with respect to minimization of surface free energy.