Entrapment of microbubbles during drop impact onto a nano-rough solid surface

Drop impacts on solids are ubiquitous in natural and industrial processes. Air that is entrapped during the impact process can negatively impact the quality or effectiveness of coatings or processes. As a drop approaches a solid surface, the pressure in the intervening air layer becomes sufficient to deform the bottom of the drop entrapping a central disc of air upon impact, which has been well studied for smooth surfaces. We investigate the formation of a thick band of microbubbles that is formed surrounding the central air disc during drop impact onto microscope slides coated with a nano-particle based superhydrophobic coating (R_q ≈ 70-140 nm) using ultra-high-speed interferometry at rates up to 5 million fps. The effects of the size and structure of the nano-roughness on the central air disc and the mechanisms for the formation of the microbubbles will be presented. See Langley et al., Soft Matter, 2018, DOI: 10.1039/c8sm01070f