Review on coalescence

What is it about?

The term coalescence describes the confluence of two dispersed fluid particles after collision or the confluence of a drop with its corresponding interface. Coalescence is an important phenomenon in many everyday events, technical applications and products. Raindrops for example would not flow together to puddles and ponds but accumulate on the ground. In industrial processes (e.g., extraction columns, separators or stirred tanks) it is essential to control and influence the coalescence behaviour of dispersions.

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Why is it important?

Coalescence is the essential phenomenon in separation processes. Additionally, coalescence determines, together with the opposing mechanism of drop breakage, the drop size distribution within a dispersed system. But coalescence is not always desired: for example in cosmetic products like lotions and creams where water and oil are finely dispersed. Also in food and nutrition products such as milk where the fat is homogenised and in supplements where active ingredients are encapsulated in (double) emulsions coalescence has to be hindered. In these cases, the coalescence is significantly inhibited by additives which accumulate at the liquid/liquid interface (surfactants). Hence, the probability of two drops to coalescence might and often should vary significantly from application to application and, thus, its influencing parameters need to be understood. In the last decades, considerable research was done but the mechanisms and influencing parameters are still not understood in detail. This review summarizes the main steps during coalescence and its influencing factors. Furthermore, experimental techniques and simulation methods are reviewed and evaluated.