Dynamics of Shear-thickening Drop Impact*

We investigate the impact dynamics of non-Newtonian drops, focusing specifically on cornstarch-water mixtures, a premier model for shear-thickening fluids. By combining high-speed photography, force sensing, and high-speed stress microscopy, we simultaneously measure the shape, impact force, and stress distributions of impacting drops as a function of the mass fraction of cornstarch. Our force measurement shows a substantial increase in the maximum impact force beyond a threshold cornstarch mass fraction that correlates with the discontinuous shear thickening. Our observation further shows that the time it takes to reach maximum impact force depends non-monotonically on the mass fraction of cornstarch. Particularly, the time lengthens at low mass fractions in the viscous fluid regime and shortens at high mass fractions in the elastic solid regime. We analyze stress distributions beneath impacting drops and conduct numerical simulations to elucidate this unusual trend. Altogether, our study offers valuable insights into the impact dynamics of shear-thickening fluids and provides a method to assess the rheology of cornstarch-water mixtures under high shear rates of drop impact.