Stability of Drop and Bubble Collisions: Coalescence Maps

Predicting the outcome of droplet and bubble collisions is an invaluable tool for the optimisation of a wide range of industrial processes, including emulsion and foam stability in food, and also mineral and pharmaceutical processing. In particular, the stability of both drop and bubble collisions is of particular importance in lab-on-a-chip (LOC) devices, where the continual interaction of drop or bubble pairs is necessary for effective operation. Previously, our group has developed and applied experimental and theoretical methods to investigate the dynamic interactions between drop or bubble pairs using Atomic Force Microscopy (AFM). Here we apply our methods to quantitatively describe the outcome of both drop and bubble collisions under conditions of direct relevance to LOC devices.

Using an experimentally validated drop interaction model, which accounts for interface deformation, surface forces and thin film drainage, we are able to predict the outcome of droplet or bubble collisions over a wide range of important parameters directly relevant to LOC devices. The parameters of interest include drop or bubble size, approach velocity and interfacial tension (determined by the presence of surfactants within the system). Our results are presented in the form of ‘coalescence maps’, a practical and simple depiction of collision stability. Our characterisation of both drop and bubble collisions uncovers rich and complex behaviour, depending primarily upon the influence of different types of surface forces present.