Materials researchers at the Swiss Federal Institute of Technology in Zurich are developing a method to stabilise droplets in an emulsion by precisely coating them with particles. This approach will open up unimagined opportunities for creating new materials.
Mayonnaise is a prime example of an emulsion that consists of a water phase and an oil phase. Oil and vinegar are mixed to form countless droplets. Egg yolk is added as an emulsifier that coats the surface of the droplets, stabilising them. A fine, creamy mixture is formed if done correctly, but if the oil is added too quickly or at the wrong time, the mayonnaise congeals as the droplets are not stable enough: they dissolve, and the phases separate.
On-demand coverage of droplets
Materials researchers also have difficulty producing controlled interfaces of droplets in two-phase mixtures with stabilisers or emulsifiers. These interfaces are key to stabilising the droplets and ultimately the corresponding emulsion. Until now, researchers have failed to regulate either the extent of the particle coverage or the composition of the particles in the interfaces of such droplets.
Materials researchers from the Swiss Federal Institute of Technology in Zurich (ETH Zurich) and KU Leuven, a Belgian university, have developed a new method to specifically cover and design these droplet interfaces in emulsions using different particles. This method can be used to compute and set the quantity of particles required to achieve the correct degree of coverage. The researchers can also design particles to almost any shape and size.
A microfluidic arrangement makes it possible
The method is based on a microfluidic platform the size of a microscope slide that is used to produce tiny droplets. While these droplets form, a second phase with particles is fed in and attaches itself to the interface of the droplets. The researchers can control the quantity of particles by adjusting the velocity of the particle phase flowing through the developing droplets. Finally, this layer is surrounded by the phase in which the droplets come to rest. The finished droplets flow through a narrow and extended channel. As they travel through this channel, the phase with the particles that surround the droplets gradually dissolves into the surrounding solution. However, the particles have enough time to cover and stabilise the surface of the droplets.
This method was presented in the online journal "Nature Communications". To date, the process has been suitable merely for research since it only works on a minute scale. However, the ETH researchers are working on scaling up the method in order to process larger quantities. They are developing an apparatus that would be suitable for industrial testing methods based on the size and throughput.