Erosion by dripping drops: the stress distribution and surface shock wave of drop impact*

Drop impact causes severe surface erosion, dictating many important natural, environmental and engineering processes and calling for tremendous prevention and preservation efforts. Despite extensive studies on the kinematics of impacting drops, it is still far from clear why drop impact is so effective in erosion. Here, we develop a method of high-speed stress microscopy, which maps the shear stress and pressure distributions of drop impact—the key dynamic factors responsible for erosion—with unprecedented spatiotemporal resolutions. Our experiments reveal the fast propagation of self-similar noncentral stress maxima underneath impacting drops and quantify the shear force on impacted substrates. Moreover, we examine the deformation of impacted substrates and uncover impact-induced surface shock waves. Our study opens the door for quantitative measurements of the impact stress of liquid drops and sheds light on the origin of the superior ability of drop impact in erosion.