Impact Force of Shear-thickening Drops

While the impact dynamics of Newtonian drops have been well-studied, the behavior of non-Newtonian drops, in particular those exhibiting strong shear thickening, remains much less understood. In this work, we combine high-speed imaging and force sensing to measure both the shape and impact force of cornstarch-water mixtures, a common model for shear-thickening fluids. We first show that, in the continuous shear-thickening regime, the impact force can be accurately predicted by models developed for viscous Newtonian drops. At high cornstarch mass fractions, where discontinuous shear thickening occurs, we draw an analogy to solid-sphere impacts on granular beds and demonstrate that the Darcy-Reynolds model captures the impact force. Strikingly, at high cornstarch mass fractions and high impact velocities, we identify a previously unreported impact regime in which the drop initially behaves like a liquid and then transitions to solid-like behavior at later times. We develop a hybrid model that combines features of liquid-drop and solid-sphere impacts, which quantitatively predicts the impact force across this new regime. Our study reveals the rich impact dynamics of shear-thickening fluids and illustrates how drop-impact experiments can extend the capabilities of conventional rheological measurements to understand the behavior of complex fluids at ultra-high shear rates.