剪切变稀流体液滴撞击疏水表面回弹现象及最大铺展的研究

非牛顿流体液滴撞击固体表面的行为广泛存在于多种工农业生产中, 然而目前相关研究主要关注牛顿流体, 非牛顿流变特性对液滴撞击动力学的影响机制还有待探索. 本文研究了纯剪切变稀流体(质量分数≤ 0.03%的黄原胶水溶液)液滴撞击疏水表面后的最大铺展及回弹行为. 通过高速摄像技术捕获液滴撞击疏水表面的运动过程及形态变化, 研究了液滴的铺展回缩过程. 实验结果表明, 在相同We下, 剪切变稀特性对液滴撞击疏水表面后的铺展阶段影响很小, 但对回缩阶段影响很大. 黄原胶浓度增加使得液滴依次表现出部分回弹、完全回弹和表面沉积三种不同的回弹行为. 利用能量守恒定律推导出了液滴能在疏水表面上回弹的临界无量纲高度ξ_c理论值. 发现牛顿流体与非牛顿流体液滴最大无量纲高度ξ_max均符合标度律ξ_max ~ αWe斜率随黄原胶浓度增大而减小. 基于有效雷诺数Re_eff, 提出了一种有效黏度μ_eff表达式, 并据此建立了剪切变稀流体的最大无量纲直径β_max预测模型. 该模型在较广We区间与实验测量值取得了良好一致. 

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.