A drop of a thick cornstarch-water mixture falls onto a table. Normally, such a mixture solidifies instantly upon a sharp impact: you can even run across a pool filled with it. But in this experiment, things went differently. The high-speed camera captured that in the first instant after contact, the drop spreads out like an ordinary liquid. Only later, when the impact speed drops, does it suddenly seize up and behave like a solid.
This order (first liquid, then solid) came as a surprise. Usually, for such mixtures, it's the opposite: the harder the impact, the faster they solidify. Here, at the highest speeds, the mixture momentarily 'forgets' its unusual nature and flows like water with starch. Only when the motion slows down does it remember that it can solidify, and the drop clumps together.
This result helps us better understand the behavior of complex fluids that change their properties depending on how quickly they are compressed. Such mixtures are used in body armor, brake fluids, and even cooking. Now scientists know that under very sharp loads, these materials can behave differently than old models predicted. And that matters for those designing protective gear or industrial processes where such fluids experience rapid impacts.
as written by the authors arXiv:2601.12642
Combining high-speed photography with direct force measurements, we investigate the impact dynamics of drops of cornstarch-water mixtures -- a premier example of shear-thickening fluids -- across a wide range of impact conditions. Our study identifies three distinct impact regimes. In addition to the liquid-like and solid-like behaviors generally expected for the impact-induced response of shear-thickening fluids, we uncover a counterintuitive regime in which high-concentration cornstarch-water mixtures display a liquid-like response at the onset of impact when shear rates are high and only transition to a solid-like behavior at later times as shear rates reduce. By integrating the classic drop-impact theory with the Reynolds-Darcy mechanism for dilatancy, we develop a unified model that quantitatively describes the impact dynamics of shear-thickening drops across all regimes. Our work reveals the unexpected response of shear-thickening fluids to ultra-fast deformation and advances fundamental understanding of drop impact for complex fluids.
If you slowly lower your finger into a cornstarch-water mixture, it will sink in easily, like into thick syrup. But if you punch the surface with your fist, the mixture instantly turns solid and won't let your hand through.