Observation of the spreading and receding behavior of a shear-thinning liquid drop impacting on dry solid surfaces

In the present work, the impact dynamics of shear-thinning drops on dry solid surfaces was investigated and compared with that of Newtonian drops. The effects of liquid viscosity with shear-thinning characteristics, surface wettability and impact velocity on the spreading and receding behavior of the drops were investigated experimentally. Water, glycerin drops (Newtonian liquids), and xanthan drops (shear-thinning liquids) were impinged upon glass, stainless steel and parafilm-M substrates, which have hydrophilic, moderate and hydrophobic properties, respectively, at the impact velocities ranging from 0.8 to 4.0 m/s. The xanthan drops spread out more widely and receded more rapidly than the glycerin drops because the spreading and receding motion drastically reduced the viscosity and consequently produced a lower level of viscous dissipation. The impact velocity and the liquid viscosity with shear-thinning characteristics had a dominant effect on the spreading phase. In contrast, the surface wettability had only a minor effect on the spreading phase but a very significant effect on the receding phase. The effect of the impact velocity on the receding phase was limited to the low-viscosity drops (with a small degree of shear-thinning) on the parafilm-M (hydrophobic) substrate. That is, when the low-viscosity drops impacted on a hydrophobic substrate, the receding velocity increased greatly with higher impact velocity, resulting in partial or complete rebound behavior. On the other hand, drop receding was significantly suppressed on the glass (hydrophilic) substrate, and the receding velocity was almost insensitive to the impact velocity. Suppression of drop receding was more prominent with the liquid having a larger degree of shear-thinning characteristic (i.e., showing a larger reduction of viscosity with the increase of the shear rate).