Wetting and Spreading Behaviors of Impacting Metal Droplet Regulated by 2D Ultrasonic Field.

The wetting and spreading behaviors of metal droplets on solid substrates are critical aspects of additive manufacturing. However, the inherent characteristics of metal droplets, including high surface tension, elevated viscosity, and extreme temperatures, pose significant challenges for wetting and spreading on nonwetting substrates. Herein, this work proposes a strategy that employs a two-dimensional (2D) orthogonal ultrasonic field to construct a vibration deposition substrate with radial vibration amplitude gradient, thereby enhancing the wettability and adhesive strength of impacting metal droplets ejected by a piezoelectric micro-jet device. First, a 2D ultrasonic vibration device is designed based on the combination of longitudinal vibration modes. Additionally, oblique and circular vibration trajectories are synthesized. The vibration amplitude distributions and trajectories of the deposition substrate are verified utilizing the finite element method. Subsequently, the experimental results demonstrate that the contact angle is decreased by 24.7%, the spreading diameter is increased by 10.3%, and the adhesive strength is enhanced by 5.4 times compared to deposition on a static substrate. The 2D ultrasonic field facilitates the transition of metal droplets from a non-wetting state to a wetting state on the nonwetting substrate, which highlights the versatility and adaptability of ultrasonic strategy for expanding the applications of metal droplets.