Bounce regime of droplet collisions: A molecular dynamics study

Yi Ran Zhang,Xi Zhuo Jiang,Kai Hong Luo

doi:10.1016/j.jocs.2016.03.011

Yi Ran Zhang, Xi Zhuo Jiang + Show 1 more

Open Access

https://doi.org/10.1016/j.jocs.2016.03.011

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Abstract

Droplet collisions have complex dynamics, which can lead to many different regimes of outcomes. The head-on collision and bounce back regime has been observed in previous experiments but numerical simulations using macro- or mesoscale approaches have difficulties reproducing the phenomena, because the interfacial regions are not well resolved. Previous molecular dynamics (MD) simulations have not reproduced the bounce regime either but have reported the coalescence and/or shattering regimes. To scrutinize the dynamics and mechanisms of binary collisions especially the interfacial regions, head-on collision processes of two identical nano-droplets with various impact velocities both in vacuum and in an ambient of nitrogen gas are investigated by MD simulations. With the right combination of the impact velocity and ambient pressure, the head-on collision and bounce back phenomenon is successfully reproduced. The bounce phenomena are mainly attributed to the “cushion effect” of the in-between nitrogen molecules and evaporated water molecules from the two nano-droplets. The analysis has verified and also extended the current gas film theory for the bounce regime through including the effects of evaporated water molecules (vapour). Some similarities and some dissimilarities between nanoscale and macro-/meso-/microscale droplet collisions have been observed. The study provides unprecedented insight into the interfacial regions between two colliding droplets.

Highlights

Droplet collisions are encountered in both natural and industrial processes, such as, in the formation of clouds and rain drops [6], in the operation of nuclear reactors and in the process of spraying [5]
Up to the present time, the published simulations have only reproduced coalescence and shattering regimes [8,11,12,16,22,26,31] but not the bounce regime. Such an absence is intriguing as it suggests the possibility that the bounce regime may not exist in nano-droplet collisions while it was observed in microand macro-droplet collisions
Results have shown that the binary droplet collisions in vacuum follow either coalescence regime or shattering regime regardless of how the impact velocities vary, which is consistent with previous studies [13,20]

Summary

Introduction

Droplet collisions are encountered in both natural and industrial processes, such as, in the formation of clouds and rain drops [6], in the operation of nuclear reactors and in the process of spraying [5]. Experimental studies have identified that under different atmospheric conditions most of head-on liquid-gas droplet collisions can be described using three distinct regimes: coalescence, bounce and reflexive separation [1,2,9,30]. Up to the present time, the published simulations have only reproduced coalescence and shattering regimes [8,11,12,16,22,26,31] but not the bounce regime Such an absence is intriguing as it suggests the possibility that the bounce regime may not exist in nano-droplet collisions while it was observed in microand macro-droplet collisions. 0.102 0.000 3.320 ent gas and vaporized water molecules on droplet behaviours during the bounce process

Molecular dynamics simulations

Reproduction of bounce regime by MD simulations

Explanation for bounce regime

Nano-scale collision regimes

Conclusions