Abstract: The understanding of droplet bouncing behavior, which attracts widespread attention, has played a significant role in guiding technologies such as inkjet printing, drug delivery, and microfluidics. The wetting gradient formed by grooved surfaces with varying spacing causes droplets to exhibit complex bouncing behaviors in different directions. Numerical simulations are employed to study this phenomenon. In this work, the Lattice Boltzmann method based on the chemical potential model is used to model and to compare the bouncing behaviors of droplets impacting surfaces with both uniform and gradient-spaced grooves. The droplet’s contact angle, contact line, forces, internal momentum, and velocity vectors during the bouncing process are analyzed to investigate the causes of reverse bouncing. The results indicate that the droplet undergoes a series of asymmetric deformations on the asymmetric groove. When the Weber number of the droplet is 42.4, the droplet completes spreading and tends to bounce along the wettability gradient toward the hydrophilic direction after contraction. However, when the Weber number increases to 117.8, the liquid penetrates deeper into the groove, and the asymmetry becomes more pronounced. This leads to the pinning of the right contact line, and the resulting hysteresis force causes part of the droplet on the interface to contract and shift its center of mass to the right. Consequently, the droplet rebounds against the surface wetting gradient. This study provides insights for the design of microstructured surfaces that manipulate droplet motion.

Key words: lattice Boltzmann method, numerical simulation, heterogeneous surfaces, wettability, droplet bouncing