A team of physicists has discovered unusual rotating crystals made of spinning particles that exhibit behaviors resembling living matter, such as twisting instead of stretching and self-reassembly after breaking. These materials, governed by transverse interactions, challenge conventional crystal growth rules. The findings, published in the Proceedings of the National Academy of Sciences, suggest potential applications in technology and biology.
Physicists from universities in Aachen, Düsseldorf, Mainz in Germany, and Wayne State University in Detroit, USA, have explored crystals composed of rotating particles. These solids display strange properties, including the ability to split into fragments, form unusual grain boundaries, and show controllable structural defects. The research, detailed in a study published in the Proceedings of the National Academy of Sciences, introduces a theoretical framework for predicting behaviors in systems with transverse interactions.
Transverse forces, which act perpendicular to the line connecting particle centers, cause objects to rotate around one another. Unlike central forces like gravity, these interactions lead to spontaneous rotation. Such forces appear in both synthetic materials, like certain magnetic solids, and biological systems. For instance, experiments at the Massachusetts Institute of Technology observed starfish embryos rotating around each other through coordinated swimming motions.
Professor Dr. Hartmut Löwen from Heinrich Heine University Düsseldorf noted: "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." One key property is odd elasticity, where pulling the material causes it to twist rather than stretch.
The crystals can disintegrate when rotating building blocks rub together intensely, breaking into smaller spinning crystallites. Remarkably, these fragments can later reassemble into a coherent structure. A multiscale theoretical model developed by the team, led by Professor Dr. Zhi-Feng Huang from Wayne State University and Professor Löwen, simulated these dynamics.
Contrary to typical crystal growth, large crystals under transverse interactions break into smaller units, while smaller ones grow to a critical size. Professor Huang explained: "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 Dr. Raphael Wittkowski from RWTH Aachen University and the DWI -- Leibniz Institute for Interactive Materials added: "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."
Co-author Dr. Michael te Vrugt from the University of Mainz stated: "Our far-reaching theory encompasses all systems evidencing such transverse interactions. Conceivable applications range from colloid research to biology." Professor Löwen highlighted potential uses: "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."