Exotic roto-crystals
It sounds bizarre, but they exist: crystals made of rotating objects. Physicists from Aachen, Düsseldorf, Mainz and Wayne State (Detroit, USA) have jointly studied these exotic objects and their properties. They easily break into individual fragments, have odd grain boundaries and evidence defects that can be controlled in targeted fashion. In an article published in the Proceedings of the National Academy of Sciences (PNAS), the researchers outline how several new properties of such ‘transverse interaction’ systems can be predicted by applying a comprehensive theory.
Transverse forces can occur in synthetic systems, such as in certain magnetic solids. They exist in systems of living organisms too, however. In an experiment observing a host of starfish embryos conducted at American university MIT, it was found that through their swimming movements the embryos influence each other in a manner leading them to rotate around one another. What biological function this may have is not yet understood. The common thread in these systems is that they involve rotating objects.
Professor Hartmut Löwen of the Institute of Theoretical Physics II at Heinrich Heine University Düsseldorf (HHU) explains: “A system of many rotating constituent elements exhibits a qualitatively new behavior that is non-intuitive: At high concentrations, these objects form a solid body of rotors which possess ‘odd material’ properties.”
The property of ‘odd elasticity’, for example, is affected: When a conventional material is pulled, it deforms in the pull direction, but an odd elastic material will not deform, but rather twist.
Such an odd twisted solid can spontaneously disintegrate into many rotating crystallites if its building blocks rub against each other so strongly that they form fragments. Remarkably, in addition to breaking up into pieces, the crystal can also reassemble itself.
A team of physicists led by first author Professor Zhi-Feng Huang of Department of Physics and Astronomy at Wayne State University and corresponding author Professor Löwen has developed a cross-scale theory for such odd crystals. Applying this theory, model calculations were run which have yielded conclusions regarding potential new applications for such strange solids.
It was observed that large transversely interacting crystals intrinsically decay into small rotating crystal units. Smaller crystals, however, will grow until they reach a critical size. This behavior runs counter to normal crystal growth, i.e. that crystals grow increasingly larger under favourable thermodynamic conditions.
Professor Huang comments: “We have discovered a fundamental property of nature underlying this process which determines the relation between the size of the critical fragments and their rotation speed.”
Study co-author Professor Raphael Wittkowski of the DWI – Leibniz Institute for Interactive Materials and of RWTH Aachen University, further elucidates: “We furthermore demonstrated how defects in the crystals exhibit dynamics of their own. The formation of such defects can be influenced from outside, which allows properties of the crystals to be specifically controlled with a view to usage applications.”
“Our far-reaching theory encompasses all systems evidencing such transverse interactions. Conceivable applications range from colloid research to biology,” declares co-author Dr. Michael te Vrugt, Assistant Professor at the University of Mainz.
Professor Löwen added: “The model calculations indicate concrete application potential. The novel elastic properties of these new crystals could be exploited to invent new technical switching elements, for example.”
Digression: central and transverse forces
In physics, fundamental interactions such as gravity between two masses and the Coulomb force between two charged bodies, are called central forces because they act along a line connecting the centres of the two bodies. Central forces cause bodies to move towards or away from each other.
Transversal interactions have been recently discovered, however, which are an exotic phenomenon in which the mutually exerted force acts perpendicular to the axis of their centres. This spontaneously causes the two bodies to start rotating around each other.
Original publication
Z.-F. Huang, M. te Vrugt, R. Wittkowski, H. Löwen, Anomalous grain dynamics and grain locomotion of odd crystals, PNAS 122 (42) e2511350122 (2025).
DOI: 10.1073/pnas.2511350122
Text: Arne Claussen [Heinrich-Heine-University Düsseldorf]