Non-Newtonian Droplet Spreading Dynamics: on the Effects of Shear Thinning and Viscoelasticity

The spreading dynamics of a droplet impacting a substrate may influence applications such as inkjet printing and firefighting. In this study, we investigate how the fluid's rheology affects the droplet impact on different substrates. Single droplets were released to fall onto the surface. We have tested three types of non-Newtonian fluids: shear-thinning, viscoelastic, and fluids exhibiting both characteristics. We analyzed spreading dynamics across Reynolds, Weber, and Weissenberg numbers, and revealed a nonmonotonic relation between inertial, capillary, viscous, and elastic forces. Notably, we found that viscoelastic effects emerge when the fluid relaxation time is comparable to the droplet spreading time, resulting in reduced spreading due to elastic energy storage. Fluids with significantly longer relaxation times exhibit minimal viscoelastic influence, as the spreading occurs too quickly for elasticity to respond. We also analyze spreading velocities and substrate-dependent variations to highlight dynamics influenced by non-Newtonian properties. Overall, our results may promote physics-informed design strategies to enhance droplet retention in various conditions, a critical step for improving firefighting efficiency.