Determination of the interaction mechanism of 10 µm oil-in-water emulsion droplets using optical tweezers

An Chen, Yu Jing, Fu-Ning Sang, Shao-Wei Li, Jian-Hong Xu

A comprehensive understanding of the stability of an emulsion relies on a quantitative understanding of the dynamic interaction forces between the dispersed emulsion droplets. Microscale droplets have been widely used in various applications, for e.g., in food, cosmetics, controlled drug release, and biology. The dynamic interaction forces between two deformable emulsion droplets in the size range of 20–200 µm have been well-analysed using bubble/drop probe atomic force microscopy, whereas those of the emulsion droplets below 10 µm are rarely studied. In this work, we quantify the interaction forces between a pair of 1,6-hexanediol diacrylate (HDDA) droplets with a 10 µm diameter in an aqueous phase using optical tweezers (NanoTracker™ 2). By capturing two oil droplets in two separate optical traps and bringing them into close proximity, their interaction forces can be measured as a function of the separation distance. The force profiles are analysed using a theoretical model based on the augmented Young–Laplace equation that includes the effects of disjoining pressure. The interaction forces between two 10 µm HDDA oil droplets with SDS molecules adsorbed at their interfaces could be well-described using the classical Derjaguin–Landau−Verwey–Overbeek theory. This work demonstrates a useful methodology to quantify the interaction forces between microscale emulsion droplets with pN force precision, providing a deeper understanding of the stabilization mechanism of O/W emulsions and can be extended to many other emulsion systems.

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