What is it about?

The outcome of a head-on collision of two immiscible liquid droplets is studied. When they collide, the droplets can either bounce off each other, coalesce, or separate with the formation of smaller droplets. We are especially interested in the separation. Three different distributions of the liquids are possible here: either two encapsulated droplets form (single reflex separation), or a single encapsulated droplet plus a droplet made solely of the encapsulating liquid emerge, the latter being found either on the impact side (reflexive separation) or opposite to it (crossing separation). The aim of the study is to understand and predict the liquid distribution after separation. The analysis of a large amount of experimental and simulation data has been performed to consider various conditions, including partial and total wetting and a wide range of Weber and Reynolds numbers. This has helped us to understand the factors that influence the final liquid distribution. We have derived two dimensionless parameters that predict the mechanism in very good agreement with experimental and numerical results.

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Why is it important?

Collisions of immiscible droplets are not yet as well studied as collisions of droplets of the same liquids, although they are relevant in many engineering and life-science applications. In this study, we focus on head-on collisions of immiscible droplets that have separation as a collision outcome. We explain the underlying mechanisms. The derivation of two dimensionless parameters allows the prediction of the liquid distribution after separation.


Thanks to the wide range of data we obtained, we were able to gain a comprehensive picture of the physical processes. Experimental investigations and numerical simulations, as well as the subsequent analytical derivation of the two dimensionless parameters, complemented each other excellently. Immiscible fluids are an fascinating field of research and I look forward to further collaborations.

Kathrin Schulte
University of Stuttgart

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This page is a summary of: Liquid distribution after head-on separation of two colliding immiscible liquid droplets, Physics of Fluids, October 2023, American Institute of Physics, DOI: 10.1063/5.0168080.
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