Effect of Vibration on Droplet Impact Dynamics on Superhydrophobic Surfaces under Moderate Low Pressure

The rebound behavior of droplets impacting superhydrophobic surfaces is ubiquitous in natural and industrial processes and is closely associated with a wide range of practical applications. In special application scenarios such as aircraft and high-altitude equipment, surfaces are often subjected to both vibration and low pressure. However, existing studies are mostly based on atmospheric pressure or stationary surfaces. It remains unclear how moderate low pressure and external vibration jointly affect droplet spreading. In this work, high-speed photography was employed to first investigate the impact dynamics of droplets on a stationary superhydrophobic surface under low pressures (97, 74.6, and 60 kPa). The influence of pressure on the spreading process was examined. Subsequently, at 60 kPa, the effects of vibration phase, frequency, and amplitude on droplet impact were studied. The results show that the maximum spreading coefficient increases gradually with decreasing pressure. This can be attributed to the reduction of gas-phase dissipation during droplet spreading under low pressure. Meanwhile, vibration introduces an additional phase-dependent energy input mechanism. The maximum spreading coefficient exhibits a pronounced periodic variation with the initial phase angle. Moreover, as the vibration frequency and amplitude increase, the fluctuation of the maximum spreading coefficient with phase becomes more significant. Based on energy conservation analysis, an empirical mathematical model was developed for the maximum spreading diameter of a droplet impacting a vibrating superhydrophobic surface under low pressure. The model elucidates the regulatory mechanisms by which pressure and vibration modulate energy input and dissipation during impact. This study contributes to understanding the interfacial dynamics of droplet impact under coupled low-pressure and vibration conditions, and provides useful insights into droplet impact behavior under relevant environmental conditions.