We encounter emulsions in everyday life, for example in the form of food, cleaning products or cosmetics. They are also used as media for catalytically activated reactions to make it easier to separate the products later. Emulsions consist of two immiscible liquids, such as an oil phase and a water phase. In order to be able to mix the actually immiscible liquids, so-called emulsifiers are added to form emulsions that are as stable as possible. As a rule, drops form in the second liquid. Depending on the application, surfactants, polymers (including proteins) or silica particles can be used as emulsifiers.

It is particularly attractive to be able to switch the emulsions via external stimuli (temperature, pH value or light). For this purpose, soft gel-like particles can be used as emulsifiers. Such microgels react to the external stimulus with a volume phase transition, which can then lead to the sudden destabilisation of the emulsion, for example. However, the microgels are often highly swellable in polar solvents (such as water), so that an emulsion with oil droplets in the water phase, a so-called oil-in-water emulsion, is formed.

In order for catalytic processes to run continuously, for example, a water-in-oil emulsion is often necessary. The research team led by physics professor Regine v. Klitzing and doctoral student Sebastian Stock (Soft Matter at Interfaces, TU Darmstadt) was able to show that mixing hydrophilic, soft stimuli-responsive PNIPAM microgel particles with hydrophobic, hard silica nanoparticles enables a water-in-oil emulsion even for very non-polar oils. Due to the model-like nature of these particles, the researchers were able to draw conclusions about their simultaneous interfacial adsorption behaviour for the first time. Understanding the interaction of multiple stabilisers makes it possible to adjust the type of emulsion while at the same time adjusting stimuli-sensitive properties. The strategy presented is of importance for industries such as pharmaceuticals and chemicals, where the production technologies and application of emulsions play a major role.