Experimental study on the dynamics of droplet impacting on a hydrophobic surface

An experimental visualization is undertaken to investigate the impact dynamic behaviors of water, absolute ethanol, and low surface energy droplets with different viscosities impacting on hydrophobic surfaces. The behaviors of droplets, including spreading, rebounding and oscillation retraction are observed and quantitatively characterized by transient spreading factor and maximum spreading diameter. Effects of droplet impact velocity, surface wettability, and droplet viscosity on the impact dynamics are explored and analyzed. As the droplet impact velocity increases, the droplet kinetic energy increases, resulting in that the spreading factor and spreading velocity increase simultaneously and the maximum spreading diameter of droplets increases with a gradual slower speed. Hydrophobic surfaces are not easily wetted by water droplets due to their low surface energy, leading to the partial rebound of water droplets when they impact on the hydrophobic surfaces. However, this phenomenon does not occur when low surface energy droplets such as absolute ethanol and simethicone impact on hydrophobic surfaces at the same velocity. The increasing droplet viscosity enhances the viscous dissipation, slowing down the impact process and inhibiting the droplet spreading, oscillation and retraction behaviors. Based on the energy conservation method, a universal model for the maximum spreading factor of low surface energy droplets with different viscosities impacting hydrophobic surface was established. According the experimental results, a new spreading time model t m =2 D 0 / U 0 was proposed to enhance applicability of the model for low surface energy droplets with high viscosity, reducing the calculation error to less than 10%.