Dynamic interfacial tension measurements during the first milliseconds of emulsion formation

Many food products are emulsion-based systems, and generally they are made using high shear devices that induce fast droplet break-up (i.e., in the millisecond range) in the presence of emulsifiers. Emulsifiers have two roles; decreasing the oil-water interfacial tension, and further stabilizing the droplets. It is expected that emulsifier adsorption occurs at similar time scales as droplet breakup during emulsification; it is therefore crucial to understand the early steps of emulsion droplet formation and emulsifier adsorption. However, conventional tensiometry techniques are not able to measure changes in interfacial tension related to emulsifier adsorption at such small timescales. A device that can enable the measurement of interfacial tension the millisecond range is the microfluidic Y-junction, which is the focus of the present work. Here, the droplet size depends on the shear of the continuous phase, and on the interfacial tension. A reduction in the interfacial tension as a result of emulsifier adsorption therefore leads to the formation of smaller droplets. Based on the droplet size and flow rates, the interfacial tension at the moment of droplet breakup, also called dynamic interfacial tension, can be measured in the millisecond range. A model was constructed that relates the droplet size to the shear exerted by the continuous phase and the interfacial tension. With that model the interfacial tension at the moment of droplet formation (typically between 0.5-10 milliseconds) could be calculated. The surfactant coverage of the oil droplet depends on the process conditions, leading to different values of the dynamic interfacial tension at the moment of droplet break-up, and we were able to link this to droplet formation time and emulsifier concentration. The proposed method allows exploration of surfactant and protein behavior at the oil-water interface at very short time-scales, which is not possible through any other technique. It is expected that this information will help the food industry to optimize their ingredient formulation and processing conditions, using adsorption behavior as a starting point of their product and process designs. In that respect, the suggested technique could revolutionize the way emulsions are made.