Air entrapment mechanism of a drop impacting a microstructured surface

Understanding how water droplets interact with microstructured surfaces is crucial for many technologies, yet direct visualization of wetting and air entrapment mechanism during drop impact remains challenging. Here, we use Brewster angle to study the evolution of air bubbles and wetting states of an impinging drop on a silicon micropillar array. By tuning the incident angle of a p-polarized 532 nm laser to the Brewster angle of select interfaces, we suppress unwanted reflections and achieve high-contrast interface-specific imaging. This approach allows us to directly observe the formation and evolution of central and peripheral trapped air bubbles, the morphology of the wetting front, and the thickness profile of the expanding liquid lamella. We found that short pillars (2 μm) consistently trap a central air bubble while tall pillars (20 μm) lead to suspended bubbles on pillar tops and the formation of peripheral air rings. The arrangement and shape of pillars affect the morphology of the trapped air and the anisotropy of the wetted area. Using interferometric fringe analysis, we quantitatively mapped the local thickness of the expanding liquid lamella, with results matching theoretical predictions from Fresnel reflectance. This study proves that Brewster angle imaging is a powerful and non-invasive technique for probing hidden interfacial processes during rapid wetting transitions. Our results provide new knowledge on the fluid-structure interaction of a drop impacting a microstructured surface