Experiment on the quasi-superhydrophobic levitation state of droplets with low surface tension

Abstract

<p indent=0mm>Droplet wettability and dynamic behavior of working fluids with low surface energy are widely used in various industrial and advanced technology fields, such as refrigerant spray cooling and microfluidic chip. Their wettability and dynamic behavior play a critical role in improving the efficiency of industrial processes. Some scholars have realized the wettability control of droplets of low surface energy fluids by changing the characteristics of micro-nano structures on solid surfaces. In addition, many external incentive ways, such as electrowetting, magnetic field, thermal stimulation, and supersonic vibration, have been proposed to control the droplet wetting state and motion behavior. However, such methods have limitations and no significant effect on low surface energy working fluids. Inspired by the above methods and the Leidenfrost phenomenon, this study made use of the action of the microporous airflow, which can regulate the wettability and motion behavior of low surface tension droplets without adding additional components to the droplets and without special requirements for the droplets themselves. A visual experimental system for wetting and dynamic behavior of droplets regulated by the microporous airflow was set up, and the experimental section of the regular array micropore was machined by laser. The momentum force generated by the microporous flow was used to balance droplet gravity, which overcomes the wetting behavior of the low surface tension droplets FC3283 and FC40 and the realizes the quasi-superhydrophobic levitation state of droplets with low surface tension. This study aimed to explore the regulatory mechanism of the microporous flow additional momentum force on the wettability and dynamic behavior of droplets with low surface energy. Experimental results showed that the motion behavior of droplets in the quasi-superhydrophobic levitation state could be divided into four stages (i.e., hovering, bobbing, bouncing, and detaching) with the decrease in droplet radius. The motion behavior of droplets was related to the dynamic balance of gravity, microporous flow momentum force, inertia force, and surface tension of droplets. In the bouncing stage, mutual conversion of energy was observed in the process of droplet falling and rising. A positive correlation was found between gravitational potential energy and surface energy. This kind of energy conversion made droplets oscillate periodically at a stable frequency. In addition, the deformation degree and amplitude of droplets with two kinds of low surface energy working fluids during the bouncing stage were compared, and the effect of viscosity on the droplet energy conversion process was analyzed. For wettability research, analogous to Young’s equation, combined with the force analysis of droplets at the inflection point during maximum deformation, the concept of pseudo contact angle was proposed to describe the wetting behavior of droplets. The pseudo contact angle of droplets in different oscillation stages was analyzed. In the hovering stage, the pseudo contact angle of droplets reached 153.6°. In the bouncing stage, as the droplet radius and falling velocity increased, the inertia of droplets increased and the surface tension decreased. Therefore, with the increase in Weber number, the pseudo contact angle of droplets decreased gradually. This method can supplement the external field force droplet manipulation method, and it provides a new method for potential applications in low surface tension droplet manipulation, lossless rapid directional transport, and microfluidic system design.