STUDY ON REBOUND BEHAVIOUR AND MAXIMUM SPREADING OF SHEAR-THINNING FLUID DROPLET IMPACTING ON A HYDROPHOBIC SURFACE

Controlling the non-Newtonian fluid droplet impact process is of profound importance not only in academic interesting but also in the practice applications. However, the existing research about droplet impacting on solid surface mainly focuses on the Newtonian fluid, and the mechanism of non-Newtonian properties on droplet impact dynamics remains to be explored. In this study, the maximum spreading and rebound behaviour of shear-thinning fluids (xanthan gum aqueous solution with mass fraction ≤ 0.03%) droplets impacting on hydrophobic surface have been investigated experimentally. The morphological changes of droplets impact onto hydrophobic surface were captured by means of high-speed imaging technology, the spreading and recoiling process were studied. The experimental results show that under the same We, xanthan gum concentration showed little effect on the maximum spreading of droplets. However, the droplets differed greatly with different concentration in the recoiling stage, and with the increase of xanthan gum concentration three kinds of rebound behaviours, namely partial rebound, full rebound and deposition, were exhibited. The theoretical value of critical dimensionless recoiling height ξ_c for droplet rebound on the hydrophobic surface was obtained by using the energy conservation law, and the maximum dimensionless recoiling height ξ_max of droplets was found to be consistent with the scalar law ξ_max ~ αWe, with the slope decreasing with increasing xanthan gum concentration. Based on the effective Reynolds number Re_eff, an effective viscosity μ_eff expression was proposed, and the maximum dimensionless diameter β_max prediction model of shear-thinning fluid droplets was established. The predicted value of β_max obtained by the model achieved good agreement with the experimental measured value over a wide range of We.