Reverse bouncing mechanisms: Droplet rebound on surfaces with spacing-gradient grooves

Understanding droplet rebound dynamics plays a crucial role in guiding technological applications such as inkjet printing, drug delivery, and microfluidics. This study investigates the complex directional rebound behaviors of droplets impacting on grooved surfaces with gradient spacing through a chemical potential lattice Boltzmann method. Through systematic observation of contact angle evolution, contact line pinning/depinning dynamics, force variations, internal momentum distribution, and velocity vector fields during rebound processes, a comparative analysis is performed for two distinct directional rebound modes, revealing the mechanisms underlying reverse bouncing. The results show that at lower Weber number (We = 42.4), droplets complete spreading and rebound along the wettability gradient after contraction. With the increase in Weber number, liquid penetration into grooves enhances, resulting in pinning at the right contact line and consequent asymmetrical droplet deformation. This pinning effect generates a hysteresis force that drives partial droplet contraction accompanied by rightward centroid migration, ultimately leading to rebound against the wettability gradient. Analysis of groove structure effects shows that an increase in the base value of the groove spacing d facilitates reverse droplet bouncing. And when H*(groove height-to-radius ratio) < 1, bottom contact promotes liquid column ascent, avoiding contact line pinning and enabling droplet rebound along the wetting gradient direction. This work provides insights for designing microstructure surfaces to control droplet motion.