Dynamic Characteristics of Vibrating Droplets on Superhydrophobic Ratchet Surfaces

Droplets show apparent dynamic properties on a superhydrophobic vibrating ratchet surface. The work aims to explore the motion mechanism and influencing factors of droplets in this process. In this paper, a physical model of droplet motion was developed and the effect of various parameters on dynamic behaviors of droplets was explored. In the experimental section, firstly, a series of asymmetric ratchet surfaces with a certain inclination and height were fabricated with aluminum sheets. Next, the ratchet surfaces were uniformly sprayed and dried with a hydrophobic coating, Glaco Soft 99, and the procedure was repeated several times until the surfaces had a stable superhydrophobicity. At this point, it was not necessary to consider the adhesion between the droplet and the ratchet surface. Finally, the ratchet surface was fixed to the shaking table, a drop of deionized water was placed on the surface, a certain amount of vibration was loaded, and the droplet motion was captured at a rate of 1000 fps with a high-speed camera (Fastec Imaging Hispec 3). The dynamic behavior of the vibrating droplet was observed and studied. It was found that within a certain vibration range, in which the frequency action range was 10-100 Hz and the amplitude action range was 0-2 mm, the vibrating droplet on the superhydrophobic ratchet surface exhibited four different behaviors, i.e., stationary, directional creep, jumping, and rupture behaviors, as the vibration amplitude increased. The experimental results showed that the fastest motion of the vibrating droplet on the superhydrophobic ratchet surface can reach 8 cm/s, which was much faster than the results of similar studies. Considering the continuity of droplet motion, this work investigated the directional creeping behavior of the droplet. During the motion of the droplet, experimental pictures were taken every 10 ms, and the contact angle at both ends of each picture and the center of mass of the droplet were measured. A physical model of droplet motion was proposed by considering the driving force and resistance during the droplet motion using mechanical analysis. The accuracy of the model was verified by repeating the experiments and obtaining experimental values agreeing with the theoretical values. With the help of the model, the effects of vibration characteristics parameters, ratchet parameters and droplet volume on droplet motion characteristics were analyzed. For a certain size of droplet, there is an optimal vibration acceleration consisting of resonant frequency and optimized amplitude, which can make the droplet achieve the fastest motion velocity under this condition. Also, the ratchet parameters affects the droplet motion velocity. By adjusting the angle and height of the ratchet to appropriate values, the droplet motion can run faster. Finally, the droplet volume also affects the droplet motion velocity. As the droplet volume increased, the droplet motion velocity showed a trend of increasing first and then decreasing. Therefore, adjusting the droplet volume to a proper value will help the droplet move faster. This work provides a method and theoretical support for the subsequent manipulation of droplet motion. It has various applications in some fields such as enhanced heat transfer, liquid transportation, and aerospace. © 2023, Chongqing Wujiu Periodicals Press. All rights reserved.