Rapid dynamics at the initial moments of liquid droplet impact

School of Engineering and Applied Sciences

Harvard University

Before a falling drop can contact a solid surface, it must displace the air beneath it. In the classical paradigm of droplet impact, the liquid droplet initially contacts the surface at a point centered on the impact axis, and spreads outward to wet the surface; however, this picture completely ignores the surrounding air. Recent calculations and experiments show that as the falling drop pierces through the air, the air fails to drain, and compresses. Inviscid calculations show that as the air compresses, the pressure in the gas layer deforms the surface of the drop, and as the drop continues to deform, the liquid does not initiate contact with the surface, but skates over a nanometer-thin film of air at a strikingly high velocity. These dynamics take place at fleeting timescales and diminutive lengthscales, and are obscured by the bulk of the drop, making experimental observation difficult. We address this challenge using a novel implementation of TIR microscopy to directly image the dynamics of the liquid-air interface. We show that the liquid air interface skates over a nanometer thin film of air, as predicted by theoretical calculations, but also that the leading edge of the liquid air interface suddenly transitions and lifts off away from the surface at a timescale that depends on liquid viscosity. The observed lift-off transition has implications for viscous splashing, where sheet ejection is observed to be delayed for higher viscosity liquids.