Impact force of liquid drops

By synchronizing high-speed photography with fast force sensing, we simultaneously measure the temporal evolution of the shape and impact force of impacting drops on solid surfaces over a wide range of Reynolds numbers (Re). We show that at high Re when inertia dominates the impact processes, the early time evolution of impact force follows a square-root scaling, quantitatively agreeing with a recent self-similar theory. The observation demonstrates the existence of upward propagating self-similar pressure fields during the initial impact of liquid drops at high Re. At intermediate Re when viscous forces set in, we analyze the early time scaling of the impact force of viscous drops using a perturbation method, which shows a similar square-root temporal scaling with the coefficient inversely proportional to the square root of Re. The analysis quantitatively matches our experiments and predicts the trends of the maximum impact force and the associated peak time with decreasing Re. Finally, we also discuss the influence of viscoelasticity on the temporal signature of impact forces.