Detecting the sub-states of grating superhydrophobic surfaces with naked eyes

We propose a convenient method to detect the detailed wetting states of grating-type superhydrophobic (SHPo) surfaces under water. Although the silvery plastron makes the nonwetted state easily discernable from the wetted, which appears black, various sub-states of the nonwetted state have so far been indistinguishable without special techniques such as confocal microscopy. For certain applications, however, different sub-states within the nonwetted state may have dramatically different effects. Applied for drag reduction experiments, here we develop an observation strategy that reveals the sub-states of plastron on grating SHPo surface to naked eyes for the first time. The results are confirmed by visualizing the liquid-air interfaces on the submerged surface with optical microscope.