Drop impact experiments of non-Newtonian liquids on micro-structured surfaces

The spreading dynamics of Newtonian liquids have been extensively studied in hydrophilic and hydrophobic surfaces and their behaviors have been extensively explored over the last few years. However, the drop impact of non-Newtonian liquids still needs further study. Luu and Forterre ( J. Fluid Mech. , 2009, 632 , 301) successfully found scaling laws for yield-stress fluids on hydrophilic surfaces. They also uncovered interesting and yet unexplained regimes when the impact was made on a superhydrophobic surface. In this work, we perform drop impact experiments on micro-patterned surfaces with two types of non-Newtonian liquids: one showing shear-thickening behavior and another one showing shear-thinning behavior. Our results show that a typical shear-thickening liquid such as cornstarch - at least at the relatively low concentration of 30% w/w - spreads according to the scaling laws of Newtonian liquids, whereas visco-elastic liquids like Carbopol behave as predicted by Luu and Forterre for impacts on hydrophilic surfaces, but show different scaling laws when they impact on superhydrophobic surfaces. The spreading dynamics of different non-Newtonian liquids are explored in this work at high Weber numbers in hydrophilic and hydrophobic surfaces with unexpected results.