Coalescence and Rebound Dynamics in Two Droplets Train Impacting on a Heterogeneous Wettability Surface.

Droplets that can be steered and rebound off surfaces are fundamentally interesting and important due to their promising potential in numerous applications, such as anti-icing and -fogging, spray coating, and self-cleaning. Heterogeneous wettability surfaces have been shown to be an effective means of droplet manipulation. This paper combines numerical simulation with theoretical analysis to investigate the dynamics of two droplets training impacting on and bouncing off a heterogeneous surface (superhydrophobic substrate decorated with a hydrophilic strip). First, the time evolutions of the droplet morphology and velocity vectors are examined to explore the particular dynamic behaviors. At different ratios of the impact velocity, three distinct rebound patterns are observed, and a regime diagram is established. After that, the effects of the impact conditions and surface wettability on the rebound performance of the coalesced droplet are studied systematically. Special attention is paid to the variations of the rebound height and the lateral transportation distance with the Weber number of two droplets and the distance between the impacting point and the hydrophilic stripe. Moreover, a theoretical analysis of two droplets' impact is performed based on the energy conservation. The obtained scaling laws match well with the numerical data in the trend. Our research may strengthen the understanding of the interactions between droplets, which is valuable for the manipulation of multiple droplets in related fields.