Drop impact on lubricated surfaces
Authors:   , Zhang Lige , Sun Ying 
Published: February 2023

Drop impact on surfaces is ubiquitous in natural and industrial processes. When the impacting drop approaches the surface, an interstitial air film of thickness ~ O(10⁰ - 10²) nm evolves between the drop and surface, and the stability of the air film determines the impact outcome. A stable air film leads to drop bounce-off, whereas an unstable air film leads to drop-surface contact and subsequent drop spreading and bubble entrapment. Therefore, understanding the air film dynamics during pre- and post-impact is imperative to applications such as droplet deposition manufacturing, spray coating, and liquid repellent surfaces. For drop impact onto a dry substrate, nanometer sized local asperities will rupture the air film and entrap air bubbles at random locations. A lubricated substrate, on the other hand, is atomically smooth and enables controlled drop-surface contact by isolating the local asperities. In this experimental study, we used total internal reflection microscopy (TIRM), which enables the measurement of air film of thickness ~ O(10⁰ - 10²) nm to investigate air film dynamics before and after contact for a liquid drop impacting a lubricated surface. At atmospheric conditions, the drop can bounce or initiate contact at different air film locations on a lubricated substrate, depending on the Weber number, We = [rho]_[l]U₀²R/[gamma], where [rho]_[l] is the liquid density, U₀ the impact velocity, R the drop radius and [gamma] the surface tension. When low viscosity drop such as water, impacts at We < 3, the air film underneath the impacting drop is stable, leading to droplet bounce-off. For the drop impact at We ~ 4, the undulation takes place at the dimple region of the air film due to the capillary wave, resulting in air film instability and dimple mode contact at the drop's central axis. The contact transition from dimple mode to film mode, where the latter corresponds to contact initiation at the thin film right outside the dimple, occurs at impacts We > 6. When the drop viscosity [mu]_[l] increases, the air film instability at impact We ~ 4 is suppressed by the viscous damping of capillary wave. When the ambient pressure is reduced, the air film drains faster than atmospheric pressure, shifting the stable air film to the unstable regime. A novel kink mode of contact on the lubricated substrate, where contact occurs at the drop-air interface with the highest local curvature, near the maximum radial extent of the drop is observed for low impact We < 3 and reduced pressure. The post-contact droplet spreading entraps two types of bubbles: (a) surface and (b) bulk, where the former occurs when the bubbles remain at the droplet-surface interface and the latter occurs within the droplet due to impact-induced air cavity. The dimple mode of droplet-surface contact shows an absence of central surface bubble due to the contact point at the drop center and subsequent axisymmetric spreading. The surface radial bubbles can be suppressed by tuning the impact velocity, liquid surface tension, and viscosity. Early-stage post-contact spreading dynamics are then studied for the dimple mode, where a visco-capillary scaling for the spreading radius r_[wet] versus time t of r_[wet]~t is observed.