Coalescence kinetics of high internal phase emulsions observed by a microfluidic technique

High internal phase emulsions (HIPEs) come with challenges regarding droplet stability given the increased level of stress exerted on the thin films separating droplets owing to small inter-droplet distances and large number of contacts. Proteins are frequently used as emulsifiers for HIPE food products, and they seem to work from a pragmatic point of view because besides offering repulsive barriers against coalescence they form a viscoelastic network that limits film drainage. However, the mechanistic understanding of how dynamic conditions influence the stability of protein stabilized emulsions is still incomplete because most studies in the literature employ classical light scattering measurement methods to assess emulsion stability which require the dilution of the samples given their turbid nature. The prior thus renders results that are not ideally suited to extrapolate the destabilization kinetics to the properties of the original droplet structure. In this study, we quantify coalescence using a ‘purposedly’ designed microfluidic chip for the formulation of high internal phase emulsions (volume fractions range 0.64–0.94). On average, >50,000 hexadecane droplets stabilized with β-lactoglobulin are evaluated in each experiment. Below protein monolayer coverage, coalescence increases with the flow velocity of the dispersed phase and the volume fraction (φd) as a result of droplet-droplet compression. We find that the formation of tightly packed droplet structure generates film stretching, and allows for increased interaction over time contributing to coalescence. Emulsion stability was improved with increasing continuous phase viscosity until hexagonal closed packing is reached (φd > 0.91) after which more coalescence occurs, since the increased compression counteracts the effects of viscous dissipation in the separating films which hinder drainage. Our observations provide new insights into the evolution of droplet interactions as a function of volume fraction, and the coalescence mechanisms responsible for the destabilization of protein-stabilized HIPEs.