Impact Force and Stress Distribution of Drop Impact

Abstract

Drop impact is ubiquitous and relevant to many important natural phenomena and industrial applications. Although the kinematics of drop impact has been extensively studied through simulations and high-speed imaging, the understanding of drop impact is still far from fully understood. The studies of dynamics such as the impact force and the stress distribution of drop impact are still relatively scarce. The impact force and the stress distribution lead to the most important consequence throughout the industrial processes and are crucial factors to erosion on substrates or waterjet cutting. Here, we systematically investigate the impact force and the stress distribution of drop impact through experimental studies. To measure the impact force of drop impact, we synchronize the high-speed camera and the piezoelectric force sensor to obtain the temporal evolution of the impact force and the morphology of drop impact over several orders of magnitudes of Re. We verify the force-time scaling proposed by the self-similar theory at the high $Re$ regime. In the finite Re regime, we consider the effects from the viscosity of liquids and analyze the scaling by using a perturbation method, which matches our experimental results very well. The influence of viscoelasticity is also discussed. To obtain the temporal evolution of the stress distribution, which has not been measured experimentally, we develop a novel technique "high-speed stress microscopy." We confirm the propagation of the self-similar non-central maximum pressure and shear stress predicted by theories, and the shear force is also quantified. Moreover, we discover the impact-induced surface shock waves, which are crucial to the origin of erosion induced by drop impact. Furthermore, we measure the shear stress distribution of drop impact on micropatterned surfaces with high-speed stress microscopy. We investigate the influence of micropillars on substrates to the displacement distribution, the shear stress distribution, and the shear force. We hypothesize that the change of shear stress distribution may result from the formation of vortices. Finally, the results show that on the micropatterned surface, the maximum shear stress is suppressed, which is helpful for mitigating erosion to substrates. Our studies provide the experimental results for understanding the dynamics of drop impact. In addition to the pioneering works of measuring the stress distribution, high-speed stress microscopy can be applied to complicated conditions such as non-Newtonian drop impact and varying the ambient pressure. Besides, it opens the door for experimental exploration of the detailed information inside an impacting drop, including the patterns of the flow and the boundary layer.