Enhancement of Coalescence-Induced Nanodroplet Jumping on Superhydrophobic Surfaces.

Coalescence-induced droplet self-jumping on superhydrophobic surfaces has received extensive attentions over the past decade because of its potential applications ranging from anti-icing materials to self-sustained condensers, in which a higher jumping velocity v is always expected and favorable. However, the previous studies have shown that there is a velocity limit with v ≤ 0.23 u for microscale droplets and v ≤ 0.127 u for nanoscale droplets, where u is referred to as the inertial-capillary velocity. Here, we show that the jumping velocity can be significantly increased by patterning a single groove, ridge, or more hydrophobic strip, whose size is comparable with the radius of coalescing droplets, on a superhydrophobic surface. We implement molecular dynamics simulations to investigate the coalescence of two equally sized nanodroplets (8.0 nm in radius) on these surfaces. We found that a maximum v = 0.23 u is achieved on the surface with a 1.6 nm high and 5.9 nm wide ridge, which is 1.81 times higher than the nanoscale velocity limit. We also demonstrate that the presence of groove, ridge, and strip alters coalescence dynamics of droplets, leading to a significantly shortened coalescence time which remarkably reduces viscous dissipation during coalescence; thus, we believe that the present approach is also effective for microscale droplet jumping.