Air entrainment dynamics under bouncing Boger droplets*

Recent studies have revealed that air entrainment plays a crucial role in preventing contact between an impacting droplet and a smooth surface. In this study, we explore the air entrainment dynamics for different types of Boger droplets with similar zero-shear viscosities but varying elastic moduli. The submicron air layer is visualized during impact on an oil film (thickness ~ 3 μm) at We ≈ 9 with a high-speed total internal reflection microscopy (TIRM) technique. We observe that droplets with increasing elastic modulus enhance the thickness gradient of the air film. However, the droplet's top surface collapse is surprisingly uninhibited by an increase in the elastic modulus.