Cluster crystals consist of a core of organic polymers surrounded by DNA molecules (right). Pressed together (left), they exhibit properties of both crystals and liquids. Image: Natasa Adzic, University of Vienna.
Cluster crystals consist of a core of organic polymers surrounded by DNA molecules (right). Pressed together (left), they exhibit properties of both crystals and liquids. Image: Natasa Adzic, University of Vienna.

More than 20 years ago, researchers predicted that, with sufficiently high density, certain particles would form a new state of matter with properties of both a crystalline solid and a flowing liquid. Scientists from the University of Vienna in Austria, and the Forschungszentrum Jülich and the University of Siegen, both in Germany, have now succeeded in creating this state in the laboratory. Their experimental concept, reported in a paper in Nature Communications, opens up the possibility for further development and could pave the way for additional discoveries in the world of complex states of matter.

Through their research efforts, the scientists were finally able to disprove an intuitive assumption that in order for two particles of matter to merge and form larger units (i.e. aggregates or clusters), they must be attracted to each other. As early as the turn of the century, a team of soft matter physicists headed by Christos Likos, a theoretical physicist at the University of Vienna, predicted on the basis of theoretical considerations that this does not necessarily have to be the case. They suggested that purely repulsive particles could also form clusters, provided the particles are fully overlapping and their repulsion fulfils certain mathematical criteria.

Since then, further theoretical and computational work has demonstrated that, if compressed under external pressure, such clusters would develop a crystalline order similar to conventional materials such as copper and aluminium. Put simply, a crystalline order means a periodic lattice structure in which all the particles have fixed positions.

In contrast to metals, however, the particles in these cluster crystals are highly mobile and continuously jump from one lattice site to the next, giving them properties that are similar to liquids. Each particle will at some point be found at each lattice site. But producing particles with the necessary characteristics to form such cluster crystals has proved difficult.

Now, Emmanuel Stiakakis at Forschungszentrum Jülich and his colleagues, in close collaboration with theoreticians from Vienna and polymer chemists from Siegen, have managed to produce particles with the required characteristics, in the form of hybrid particles with a pompom-like structure. The cores of these particles are made of organic polymers, to which DNA molecules are attached, sticking out in all directions like the threads of a pompom.

This structure allows the molecules to be pushed close together and thus to be sufficiently compressed. At the same time, the combination of the electrostatic repulsion of naturally charged DNA molecules and a weak interaction between the polymers at the center of the particles ensures the necessary overall interactions.

"DNA is particularly well suited for our intentions, as it can be assembled relatively easily in the desired shape and size due to the Watson–Crick base pairing mechanism. In combination with polymer cores, the shape and repulsion of the hybrid particles can be fine-tuned and different variations can be produced relatively quickly," explains Stiakakis, who conducts research at Forschungszentrum Jülich’s Institute of Biological Information Processing. He has long been using DNA molecules to investigate aspects of self-assembling soft matter.

"After extensive efforts and by applying numerous experimental methods, including biochemical synthesis and characterization as well as X-ray scattering and light scattering, we have now been able to bring a more than 20-year search for cluster crystals to a successful conclusion," says Likos. He now anticipates the discovery of further complex states of matter that will be formed by the new macromolecular aggregates.

This story is adapted from material from the University of Vienna, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.