Foams, gels, thin films

A cappuccino tastes best if it's made with soft, creamy milk froth. It's difficult to achieve similar success with oat or almond milk. Size is one reason for this: tiny protein particles stabilise the foam and provide fine air bubbles. Cappuccino froth illustrates how something that is only a few nanometres in size (millionths of a millimetre) can change the macrocosm that is perceived by an individual. For Professor Regine von Klitzing, researching these bridges between large and small is a tremendous challenge. And although her team at the Institute for Condensed Matter Physics at the TU Darmstadt is not dealing with milk froth, the material class to which the tasty topping belongs is called “soft material”. By this, physicists mean a kind of mixed aggregate state between “solid” and “liquid”. This includes gels such as blancmange, elastic plastics such as rubber and, as already mentioned, foams.

It isn't only the beguiling taste of some representatives that makes soft material so interesting. Technology should also benefit, for instance with intelligent surface coatings that act as sensors or that change their shape and become active themselves. Von Klitzing's team wants to thoroughly research the complex world of soft material, including creating brand new materials with specific functions and properties. Von Klitzing has decades of experience in this field. Her team combines expertise with comprehensive experimental facilities. “We cover the entire range from the synthesis of new materials to their characterisation,” the physicist sums up one strength.

Switching gels by signals

The researchers see a potential for intelligent surfaces in certain nanogels. They consist of a spherical net of chain-like molecules called polymers that dissolve in water. When warm, the polymers do not dissolve so well in water. The tiny spheres push the wetness out. The consequence: they shrink. And because this occurs at a specific temperature depending on the particular material, the nanogel can be “switched” between large and small. Gels also exist that respond to other signals. “Some can be switched with the pH values, others by moisture or magnetic fields,” explains von Klitzing. The team tests gels made from various polymers and adds nanoparticles to them. For instance, gel particles to which gold nanoparticles have been added can be “switched over” with a focused light beam.

One other focus of von Klitzing's research are very thin liquid films that are enclosed between solid surfaces or air. “We are researching the interactions in these films with certain additives such as molecules or particles, which is one example of how our research has an interdisciplinary base on the borderline between physics and chemistry,” she says. One aim is to understand what stabilises thin films. This knowledge is important for the cosmetics, pharmaceutical and food industries, all of which often use foams, emulsions and suspensions. Emulsions are a mixture of two non-miscible liquids and consisting of fine droplets of one of the two components in the other, for instance fat droplets in water. With suspensions, solid particles float in a liquid. Foams are created by has bubbles in a liquid. In all three cases, thin liquid films separate the particles, bubbles or droplets.

The particles, droplets or bubbles tend to combine in order to minimise the energy of the system. In certain emulsions or foams, this is prevented by what is known as the Pickering effect: if particles settle on the surface of the bubbles or droplets, they cannot combine to form a bubble or droplet. “We use environment-sensitive particular such as nanogels to switch the stability of the foams,” says von Klitzing.

The team uses neutron scattering to study the microscopic properties of these “switchable” foams. A new neutron source in Lund in Sweden, the European Spallation Source (ESS), provides more detailed insights than any other existing ones. Von Klitzing's co-worker Matthias Kühnhammer is involved in the development of a new kind of flexible sample environment for the ESS. It will make it possible to investigate the samples far more quickly with the very highly intensive neutron ray there. “Our holder should therefore allow the quickest possible sample change,” says Kühnhammer. To this end, the device contains a number of cylinders that can be pushed around like a magazine in order to measure the next sample. The Darmstadt researchers are collaborating on the project that is funded by the Federal Research Ministry for Education and Research (BMBF) with the University of Bielefeld and TU Munich.

Stabilizing foams

The researchers do not only use particles to stabilise foams. For instance, they apply surfactants to the boundary surfaces between liquids, longish molecules with their ends in one or other of the liquids. Added polymers bring the surfactants closer together, which increases their effects and therefore stabilises the liquid film. Another stabiliser is based on the physical law of maximum entropy. This requires systems to strive for a secondary state; in a sense, to seek freedom of movement. Thread-shaped polymers that are attached to the surfaces of particles would be inhibited if they came too close to each other, so they keep their distance in order to maximise entropy. The film between them remains stable.

In order to understand these and other mechanisms, the researchers use various measuring methods, such as atomic force microscopy or neutron scattering, to investigate their foams and gels. They have their mechanical properties, electrical charge or response to pressure in their sight.

And who knows: maybe in ten years' time the barista will be asking how stable and foam you want your cappuccino froth to be on a scale of one to ten – and set the coffee machine accordingly.

The “Flexiprob” project in which Regine von Klitzing's team investigates foams and nanogels is part of the BMBF funding programme “Researching condensed materials with large facilities 2016-2019”. The scientists at Darmstadt are developing a flexible samples environment in co-operation with the University of Bielefeld (Prof. Thomas Hellweg) and the TU Munich (Prof. Peter Müller-Buschbaum).