Coalescence-Induced Jumping of Two Unequal-Sized Nanodroplets.

Coalescence-induced self-propelled jumping of droplets on superhydrophobic surfaces has potential applications for condensation heat transfer enhancement, anti-icing, self-cleaning, antidew, and so forth. However, most of the previous studies focused on two identical droplets which are not commonly encountered in the nature. In this work, coalescence-induced jumping phenomena of two unequal-sized droplets on superhydrophobic surfaces were investigated theoretically and numerically. First, by introducing modified inertial-capillary velocity (uic*) and Ohnesorge number (Oh*) with consideration of radius ratio (r*) of two coalescing droplets, we proposed a generalized inertial-capillary scaling law for the jumping velocity of coalesced droplets, which is expected to be applicable for both two identical droplets and two unequal-sized droplets coalescing on superhydrophobic surfaces. Subsequently, we employed molecular dynamics simulations to investigate the coalescence-induced jumping process of two unequal-sized nanodroplets. Our simulations showed that the dimensionless jumping velocity (vj/uic*) well follows the generalized inertial-capillary scaling law with vj/uic* ≈ 0.127 in a specific Oh* range; however, it rapidly reduces and finally vanishes when the radius ratio of large droplet to small droplet is larger than a certain threshold value. Our simulations also revealed that nonjumping of two unequal-sized droplets with a very large radius ratio is due to that the larger droplet swallows the small one, so that the liquid bridge has no chance to impact the solid surface, and hence the "liquid bridge impacting substrate" mechanism fails in this circumstance.