Regimes of Head-On Collisions of Equal-Sized Binary Droplets.

Yi Ran Zhang,Kai H. Luo

doi:10.1021/acs.langmuir.8b04277

Abstract

Through molecular dynamics simulations, head-on collision processes of two identical droplets with a diameter of 10.9 nm are elaborately scrutinized over a wide range of impact Weber numbers (from 6.7 to 1307) both in vacuum and in an ambient of nitrogen gas. As the impact Weber number exceeds a certain critical value, a hole or multiple holes in apparently random locations are observed in the disklike structure formed by two colliding droplets. We name this a new “hole regime” of droplet collisions, which has not yet been reported in previous studies. As the impact Weber number increases, the number of holes increases. The hole or holes may disappear unless a second critical impact Weber number is exceeded, when the merged droplet is likely to experience dramatic shattering. It is also found that the existence of ambient gas provides a “cushion effect” which resists droplet deformation, thus delaying or even preventing the appearance of hole formation and shattering regimes. Moreover, increasing ambient pressure suppresses hole formation. A model based on energy balance is proposed to predict droplet behaviors, which provides a more accurate estimate of the maximum spreading factor compared to previous models. Finally, we further extend the current nanoscale droplet collision regime map and analyze the similarities and dissimilarities between nano- and macroscale droplet collision. Our study extends the current understanding on nanodroplet collisions.

Highlights

Droplet collisions are encountered in both natural and industrial processes,[1−4] for example, the formation of rain drops,[5] the operation of nuclear reactors,[6] and the process of spraying.[7]
Other investigations using experimental and numerical methods[9−11] such as the level set, volume of fluid, and lattice Boltzmann method (LBM) provide a rich picture of droplet collision outcomes, but the details of the collision dynamics are difficult to obtain through experiments and continuum numerical methods, especially when the approaching droplets are within a distance comparable to the molecular mean-free path
Our study successfully observed a new hole regime in nanoscale droplet collision at high impact number and proposed a model based on energy balance to estimate the spreading factor of the merged droplet

Summary

INTRODUCTION

Droplet collisions are encountered in both natural and industrial processes,[1−4] for example, the formation of rain drops,[5] the operation of nuclear reactors,[6] and the process of spraying.[7]. We have employed molecular dynamics (MD) simulations to investigate head-on collisions of nanodroplets, which successfully reproduced the head-on collision and bounce-back regime for the first time by any numerical simulation Such phenomena have only been observed in head-on collisions of microdroplets in experiments.[12] Previous numerical studies, including those by discrete, mesoscopic LBMs,[13−18] failed to predict this regime because the interfacial region of thickness comparable to the molecular mean-free path was not resolved. Our study successfully observed a new hole regime in nanoscale droplet collision at high impact number and proposed a model based on energy balance to estimate the spreading factor of the merged droplet.

MD SIMULATIONS

RESULTS AND DISCUSSION

CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES