A world first at the microscopic scale: metamaterials that can shrink and expand on their own
PHYSICS image: C. Huygelen
Soft structures that can take on different shapes without any external drive. Leiden physicists Daniela Kraft and Julio Melio created them in their lab. They present their groundbreaking research on microscale metamaterials in Nature - a breakthrough that opens the door to smart, reconfigurable materials and microscopic robots.
‘Metamaterials have completely changed the way we think about materials,’ explains Professor of Experimental Physics Daniela Kraft. ‘In these systems, movements are no longer set by the material itself, but by the structure – the way particles are connected. We set out to create such functional structures at the microscopic scale. And we succeeded.’
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Watch the video on the original website orBuilding the structures block by block
Together with Martin van Hecke, a professor at Leiden University and AMOLF, Kraft and Melio designed a metamaterial made from colloidal microparticles: tiny silica spheres. PhD candidate Melio assembled these particles into carefully engineered building blocks. Everything was made on a tiny scale – about ten times thinner than a human hair.
‘Diamond shape arrangements turned out to be the key’, Melio explains. We fixed the particles inside each diamond firmly together to make them mechanically stable. By then connecting the diamonds to each other at a single point, they can rotate relative to one another. Starting from just a few units, we gradually built more complex architectures, eventually realizing the so-called Kagome lattice (see video).’
Motion powered by thermal energy
Under an optical microscope, the structures reveal a remarkable feature: they fold and unfold spontaneously. ‘At this scale, particles are constantly in motion due to thermal energy’, explains Melio. ‘That intrinsic motion drives the transformation of our structures, entirely without external input. Remarkably, this motion is not random. When one set of quadrilaterals rotates in one direction, the neighboring set rotates in the opposite direction. As a result, the structure contracts and then expands again.
Controlling the motion
The team subsequently introduced external control over the movements of the microscopic metamaterial externally. By using magnetic microparticles, they made the metamaterial responsive to an external magnetic field. Switching the field on or off, caused the structure to shrink or expand on command. An important step towards real-world applications of the material.
Theoretical physicist Silke Henkes helped develop a framework to describe how thermal motion interacts with the metamaterial’s architecture: ‘It was an absolute pleasure to collaborate over the past four years. Together, we developed a theoretical framework to understand how thermal fluctuations drive our microscopic metamaterials – and our experiments matched the model perfectly.’
From discovery to innovation
This is just the beginning, according to Van Hecke: ‘It’s wonderful that we have now managed to create metamaterials at such a small scale. This opens the door to translating many other metamaterial concepts to the microscopic world.’
Kraft sees broad potential ahead. This design could form the basis for smart materials or microrobots that autonomously respond to their surroundings – the field in which her research group has built an international reputation.
Nature
Pivoting colloidal assemblies exhibit mechanical metamaterial behavior
Julio Melio1, Martin van Hecke1,2, Silke E. Henkes3 and Daniela J. Kraft1
1 Huygens-Kamerlingh Onnes Laboratory, Leiden University, P.O. Box 9504, 2300 RA Leiden, The Netherlands
2 AMOLF, 1098 XG Amsterdam, The Netherlands
3 Lorentz Institute, Leiden University, P.O. Box 9506, 2300 RA Leiden, The Netherlands