A group of scientists from Friedrich-Alexander-Universität Erlangen-Nürnberg (FAU), the University of Liège and the Helmholtz Institute Erlangen-Nürnberg for Renewable Energy have developed a microswimmer that appears to defy the laws of fluid dynamics: their design, including two beads that are linked by a direct spring, is moved by totally balanced oscillations. The Scallop theorem states that this can not be attained in fluid microsystems. The findings have actually now been published in the scholastic journal Physical Evaluation Letters.

Scallops can swim in water by quickly clapping their shells together. They are big enough to still be able to move forwards through the moment of inertia while the scallop is opening its shell for the next stroke. However, the Scallop theorem applies basically depending upon the density and viscosity of the fluid: A swimmer that makes symmetrical or mutual forward or backwards motions similar to the opening and closing of the scallop shell will likely stagnate an inch. ‘Swimming through water is as hard for tiny organisms as swimming through tar would be for people,’ states Dr. Maxime Hubert. ‘This is why single-cell organisms have comparatively complex ways of propulsion such as vibrating hairs or rotating flagella.’

Swimming at the mesoscale

Dr. Hubert is a postdoctoral researcher in Prof. Dr. Ana-Suncana Smith’s group at the Institute of Theoretical Physics at FAU. Together with scientists at the University of Liège and the Helmholtz Institute Erlangen-Nürnberg for Renewable Resource, the FAU group has actually developed a swimmer which does not appear to be limited by the Scallop theorem: The basic model consists of a direct spring that connects two beads of different sizes. Although the spring broadens and contracts symmetrically under time reversal, the microswimmer is still able to move through the fluid.

‘We originally checked this concept using computer system simulations,’ states Maxime Hubert. ‘We then built a functioning model’. In the useful experiment, the researchers positioned two steel beads determining just a couple of hundred micrometres in diameter on the surface of water consisted of in a Petri meal. The surface stress of the water represented the contraction of the spring and growth in the opposite direction was achieved with a magnetic field which triggered the microbeads to occasionally push back other.

Vision: Swimming robots for transporting drugs

The swimmer is able to move itself because the beads are of various sizes. Maxime Hubert says, ‘The smaller bead responds much faster to the spring force than the larger bead. This causes asymmetrical movement and the bigger bead is pulled along with the smaller sized bead. We are therefore utilizing the principle of inertia, with the difference that here we are worried about the interaction in between the bodies rather than the interaction in between the bodies and water.’

Although the system will not win any prizes for speed– it moves forwards about a thousandth of its body length during each oscillation cycle– the large simpleness of its construction and mechanism is an important development. ‘The principle that we have found might help us to build small swimming robots,’ says Maxime Hubert. ‘One day they might be used to transport drugs through the blood to an exact place.’

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