Effect of nano-pillared surfaces with an arrangement density gradient on droplet coalescence dynamics.

The ability to predict and control the coalescence of droplets is of great importance for both industrial and technological applications, including 3D printing, micro-cladding, and self-assembly. Here, a textured surface decorated with nano-pillared arrays was designed and its arrangement density (f) was found to significantly affect the coalescence dynamics of droplets through changing their wettability. A large arrangement density f of the nano-pillared arrays would induce a Cassie wetting state for droplets, which supports the coalescence process. But when decreasing f to a value that produces a Wenzel wetting state, the coalescence is heavily impeded by the nano-pillars. However, a very small arrangement density f is also favorable for coalescence because the pinning effect resulting from the nano-pillars becomes ignored. More importantly, special substrates were well designed by nano-pillars with a density gradient in order to control the coalescence dynamics for some potential applications. This work helps to shed light on the coalescence dynamics of droplets on a microtextured surface modified with different arranged nano-pillars and thereby provides guidance on how to control their behaviors.