Invisibility Cloak for Magnetic Obstacles

Cloaking refers to a physical method that wraps an object in a kind of invisibility cloak, which makes it unrecognizable. So far, cloaking has only been studied with waves – such as light or sound waves. It involves guiding waves around an object or obstacle, much like water in a river flowing around a rock. As a result, the waves reach their destination as if the obstacle were not there – so it becomes "invisible". Anna Rossi, Thomas Märker, Nico Stuhlmüller, Daniel de las Heras and Thomas Fischer from the Chair of Experimental Physics and Theoretical Physics at the University of Bayreuth have now developed a method to implement cloaking for particle motion.

The research team lets small particles, so-called colloids, flow in a magnetic field of a checkerboard pattern. The colloids are paramagnetic, so they only behave magnetically when they are near a magnet or an external magnetic field. Calculated changes of the magnetic field create areas on the chessboard that remain untouched by particle transport and thus become "invisible". The cloaked areas alter the motion only while the colloids are circumventing the obstacle, but not after they have passed the cloak. The particles arrive at their destination at the same time as particles on the route without obstacles. "We have also shown experimentally that if the obstacle shape is chosen correctly, the size of the obstacle does not matter – it can be any size, and the particles still arrive at their destination in time," says Ms. Rossi.

The results were obtained in cooperation with the University of Kassel and the Polish Academy of Sciences.