Motion and rupture properties of droplets on inclined surfaces under ultrasonic excitation

In this paper, an active anti-frosting method based on ultrasonic driving droplets is proposed. The dynamic behaviors of droplets, including asymmetric spreading, directional motion and rupture, are systematically examined on inclined surfaces under 35 kHz ultrasonic excitation, with controlled variations in droplet volume, vibration velocity, and surface inclination angle. The results reveal that droplets exhibit pronounced asymmetric deformation and directional motion toward the pressure antinode, eventually stopping due to the resistance effect of the reversed acoustic radiation force. A dual-peak evolution in contact angle hysteresis is observed, where the first peak transitions from negative to positive with increasing surface inclination, highlighting the competing influences of gravity and ultrasonic wave on morphological control. Furthermore, the acoustic radiation force is quantitatively characterized through both Newton's second law and the acoustic radiation pressure integration model. Theoretical and experimental results exhibit good agreement in the overall trend, while their deviation attributed to droplet asymmetry, is found to correlate linearly with a proposed composite dimensionless number. Additionally, the formation of satellite droplets is captured and classified into four stages: stretching, necking, rupture, and retraction. Satellite droplets typically emerge within a narrow dimensionless time of 0.6∼0.8 and account for <0.5 % of the main droplet volume. A zonal rupture map is established based on the proposed dimensionless number, which enables effective classification of satellite-free droplet, critical satellite droplet, and satellite droplet regions. This work can provide data basis and technical support for optimizing active anti-frosting technologies by controllable liquid transport.