An experimental study of binary collisions of miscible droplets with non-identical viscosities

Karrar H Al-Dirawi,Andrew E Bayly

doi:10.1007/s00348-019-2874-3

Abstract

The dynamics of binary collisions of equi-diameter droplets with non-identical viscosities have been investigated experimentally and compared to previously generated data from identical droplet collisions (Al-Dirawi and Bayly in Phys Fluids 31(2):027105, 2019). Three hydroxypropyl methylcellulose (HPMC) aqueous solutions, 2%, 4%, and 8% HPMC, were used to generate the droplets of different viscosities, 2.8, 8.2, and 28.4 mPa s, respectively. High-speed imaging techniques were applied to observe and capture the collision outcomes. Collision outcomes were characterised and regime maps were generated. The non-identical viscosity droplet collisions produced regime maps with well-defined boundaries which are comparable in shape to the conventional regime maps of identical droplet collisions. The boundaries of the bouncing and reflexive separation regimes of the non-identical collisions show intermediate position between the identical cases of the low and the high viscosity droplets. However, the boundary of the stretching separation regimes of the non-identical collisions showed good agreement with the boundary of the identical case of the lower viscosity droplet. Moreover, the ability of models developed for predicting the regimes boundaries of collisions of identical viscosity droplets was assessed for the non-identical collisions. They proved capable in the non-identical cases, and the changes in adjustable parameters were consistent with the underlying physical basis of the models.Graphic abstract

Highlights

1.1 Binary droplet collisions theoryDroplet collisions are important to many areas of physics and technology, such as atmospheric studies, combustion science and spray drying (Ashgriz and Poo 1990; Qian and Law 1997; Francia et al 2016; Post and Abraham 2002)
A new collision regime has been identified in case of non-identical viscosities at high viscosity ratio (i.e., 2% vs. 8% hydroxypropyl methylcellulose (HPMC)), where a relatively significant asymmetry in dynamics due to the differences in viscosity is observed
Viscosity is an important factor in determining the collision behaviour of droplets

Summary

Introduction

Droplet collisions are important to many areas of physics and technology, such as atmospheric studies, combustion science and spray drying (Ashgriz and Poo 1990; Qian and Law 1997; Francia et al 2016; Post and Abraham 2002). In many of these areas, the droplet collisions occur between droplets of different properties. Understanding, and predicting the outcome, of droplet collisions of non-identical droplets is, of great interest in this and other areas

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Conclusion