Spreading Dynamics of Shear-Thinning Fluid Drop Impact on Hydrophobic Substrate

Droplet impact dynamics of the shear-thinning fluids on the hydrophobic surfaces have been experimentally studied and compared to the base fluid, water. In this work, hydrogels are prepared by lading water with Polyacrylamide polymer at three loading rates 1, 2, and 3 wt. %. The resultant hydrogel phase behaves as a shear thinning fluid with altered rheophysical properties. The presence of polymer additive modifies the entire spread dynamics by reducing the value of maximum spread due to the presence of finite yield stress and also by eliminating the bouncing effect of the base fluid. Despite the effect of inertial spreading of the droplet, dominant viscous forces act as dampers to both during the spreading and receding regimes. In particular, viscous forces prolong the receding period by an order of magnitude while completely suppressing secondary spreading. Finally, it is shown that shear-thinning fluids follow the well-known Tanner’s law (RR αα tt nn 7+3nn ; R: spread radius, t : time and n: shear thinning index), however, with a simple modification involving the substitution of shear thinning index obtained from rheology with the spreading exponent obtained from drop impact experiments.