Falling Hard and Bouncing Back: Impact Force of Shear-thickening Drops

While the impact dynamics of drops of Newtonian fluids have been extensively studied, our understanding of non-Newtonian drop impact is still rudimentary. Here, by synchronizing high-speed imaging and force sensing, we simultaneously measure the shape and the impact force of an impacting drop of cornstarch-water mixtures, a commonly used model for shear-thickening fluids. A sharp transition from viscous to elastic impact dynamics is identified with an increasing mass fraction of cornstarch. Particularly, we find that the maximum impact force increases drastically above a threshold cornstarch mass fraction, directly correlated with the shear-thickening rheology of cornstarch-water mixtures. More interestingly, the time of the maximum force increases at low mass fractions in the viscous regime and decreases at high mass fractions in the elastic regime. We illustrate the origin of this unexpected non-monotonic trend by measuring the stress distributions underneath impacting drops and via numerical simulations. Our study reveals the behaviors of shear-thickening fluids in the fast process of drop impact and sheds light on the unique dynamics of non-Newtonian drop impact.