Experiment and Simulation of the Singular Jet of Viscous Droplets Impacting on a Superhydrophobic Surface

The impact of liquid droplets on surfaces has fascinated scientists for over a century, motivated by a variety of applications such as additive manufacturing, spray cooling, and more recently, epidemic prevention and control. Previous research has shown that when water droplets gently impact a superhydrophobic surface, they can shoot out a small diameter but extremely high velocity jet, called a singular jet. Pure water is one of the most widely used working fluids in industry. However, the evolution of their internal flow field structure, velocity vector, and pressure distribution have yet to be fully studied. The impact and singular jet behaviors of droplets on superhydrophobic surfaces are investigated using a high-speed imaging system. We prepared Newtonian working fluids with viscosities of 0.9-27.7 mPa.s by proportioning the glycerol / water solution. Using the nanosilica deposition technique, superhydrophobic surfaces were prepared with a static contact angle of approximately 158 degrees. To simulate the impact process of droplets on superhydrophobic surfaces, a numerical model was constructed based on finite element scheme coupled with a level-set method. The simulation and experimental results showed good agreement. The impact conditions for the occurrence of the singular jet behavior of viscous fluid droplets are summarized. The experimental results showed that when pure water droplets hit superhydrophobic surfaces in the lower We number range, entrained bubbles can be observed, which also directly lead to the occurrence of a singular jet. However, when the viscosity of the droplet is greater than 14.2 mPa.s, even if the impact velocity is enhanced (We > 100), the singular jet behavior no longer appears. From the We -Re phase diagram, it can be observed that the singular jet behavior mainly occurs in the region with Re = 700-1 000. The range of We numbers for the singular jet is wider. However, no singular jet phenomenon occurred in the regions where Re < 300 or Re > 1100. The experimental results also showed that the viscous force reduced the maximum jet velocity of the droplet, which caused the jet velocity to become flat. The numerical simulation results indicate that the singular jet is related to the cavity formation during the retraction stage of the impact droplets. Moreover, a larger pressure concentration area was found at the center of the droplet when the singular jet occurred. The interfacial morphology between the gas and liquid at the bottom of the cavity inside the droplet was significantly affected by changes in the viscosity. The gas-liquid interface at the bottom of the cavity could change from an upward convex into a downward concave shape with the increase in viscosity. Therefore, an upward jet cannot be formed. The simulation results showed that the surface tension is in the opposite direction, while the curved gas-liquid level at the bottom of the cavity is reversed. In combination with the experimental and numerical simulation methods, the generation and regulation mechanisms for the singular jet of viscous fluid droplets were determined. Regulating the viscosity of the working fluid significantly influences the singular jet behavior when the droplets impact superhydrophobic surfaces. This study provides a theoretical basis for the regulation of droplet dynamics.