Microbubble entrainment on thin liquid films under drop impacts.

This study reveals how drops impacting thin liquid films leave behind radial microbubble trains - here defined as large-area microbubbles (LAMs) - over a region comparable to the maximal surface coverage of the spreading phase. Using a thin, minimally compliant viscous oil film, the trapped bubbles are immobilized and quantified via high-speed imaging techniques across varying drop velocities and surface inclinations. The setup enables the characterization of microbubble entrainment (e.g., bubbles per unit area) as a function of the drop inertia, visco-capillary dynamics, and fluid instabilities. The formation of LAMs is driven by a thin intervening air gap and contact line instability, analogous to classic coating instabilities. Microbubble entrainment is absent if intermolecular forces fail to initiate wetting across the lubricating air layer (e.g., drop bouncing). Once the contact line forms, wetting instabilities induce air tubes to become unstable via the Rayleigh instability, leading to radial and azimuthal microbubble coverage under the thin air gap.