Dual Pathways of Air Cavity Evolution during Droplet Impact on Superhydrophobic Nanoporous Surfaces

The impact of a liquid droplet on a solid surface generates a cylindrical air cavity along the droplet's central axis and entraps a thin air film underneath, with a liquid film potentially sandwiched in between. We observe that the air cavity produced by impacting a water droplet on superhydrophobic nanoporous surfaces evolves via two distinct pathways within a narrow Weber number range (We≈2-5): bulk-bubble entrapment or air-cushion development. Ultrafast synchrotron x-ray imaging reveals three air-cavity pinch-off mechanisms: (i) inertia-dominated axial implosion due to rapid droplet recoiling, (ii) capillary-wave-driven necking triggered by liquid film rupture, and (iii) hybrid pinch-off. We demonstrate that the stability of the sandwiched liquid film critically determines the pathway selection. Whereas intact liquid films enable isolation of the air cavity and air film, leading to bulk-bubble entrapment, submillisecond liquid film rupture (≲0.5  ms) redirects the air-cavity pinch-off dynamics, developing an air cushion underneath the droplet.