Evidence of Droplet Coalescence in Extensional Flow Using Microfluidic Devices

Coalescence under flow is one of the major causes for emulsion or foam destabilization in food processes and consequently, understanding the hydrodynamic part of the coalescence mechanism(s) is key in order to prevent it. In our experimental investigations, we use microfluidic methods to vary the parameters governing size and capillary number of head-on collisions. Drops were exposed to an initially predominantly compressional and subsequently extensional flow field by means of a flow-through device with an expansion followed by a contraction. Our results show that even when the transient compressional flow is insufficient to allow the film to drain to critical thickness for rupture, the drops will often coalesce in the following 'extensional' phase. A food emulsifier was observed to apparently enhance coalescence in extension for small concentration, but provide stabilization for higher concentration, by a specific mechanism.