Coalescence Mechanism Induced by Different Wetting States of Ti and Al Droplets on Rough Surfaces

There is currently increasing interest in droplet transportation and coalescence on rough surfaces. However, the relationship among wettability, coalescence mode, and substrate characteristics (roughness and nanopillar height) remains unclear. In this work, two coalescence modes, climbing coalescence and contacting coalescence, are first observed in the dynamic behaviors of Ti and Al droplets on rough substrates. Due to the nonsynchronized wetting state transition of the droplets, the coalescence mode with increasing substrate characteristics differs, transitioning from contacting coalescence to climbing coalescence and then returning to the contacting mode. In general, the mode of coalescence correlates closely with the respective wetting states. Typically, Ti and Al droplets coalesce in the contacting mode when they have the same wetting state, but if they have different wetting states, they coalesce in the climbing mode. Our results emphasize the complicated relationship between the surface structure and the wettability of droplets, which could provide insights into self-assembly, three-dimensional printing, and microfluidic devices.