Experimental Study of Gas-Liquid Compound Droplet Impact on Superhydrophobic Surface.

The impact dynamics of gas-liquid compound droplets (GLCD) on surfaces play a crucial role in optimizing processes such as atomization, coating, and microencapsulation. The dynamic behaviors of GLCD impacting superhydrophobic surfaces under varying impact heights, viscosities, and gas-liquid volume ratios (Φ) were investigated via high-speed photography. Three collision categories are defined according to GLCD morphology evolution under different parameters, i.e., oscillation-breakup, rebound collision, and impact-breakup. As the impact height increases, the occurrence of droplet breakup shifts progressively from the retraction stage to the spreading stage. Notably, the maximum rebound coefficient of the compound droplet initially increases and then decreases with increasing impact height, and finally increases again due to the bubble breakup, which differs from the behavior of the homogeneous droplets. Furthermore, the maximum spreading and rebound heights of the GLCD are suppressed by increasing the liquid-phase viscosity. For Φ = 0.48 GLCD, increasing the liquid-phase viscosity from 4.7 to 5.5 mPa·s raises the critical breakup impact height from 90 mm to 120 mm. Meanwhile, the average dimensionless breakup time also increases from 0.54 to 1.72. As the Φ increases, the liquid film of the GLCD gradually thins, resulting in a decrease in the maximum spreading coefficient and enhanced probability of droplet breakup. Additionally, increasing the Φ from 0.48 to 0.56 leads to an increase in the average dimensionless breakup time by 0.88. This study provides a fundamental understanding of the dynamic behavior of compound droplets and offer valuable insights for related engineering applications.