Back and forth motion can go places in noisy environments
-Ambarish Ghosh, Electrical Communication Engineering
We have developed a system of nanostructures that execute back and forth (reciprocal) motion in a fluidic environment under the action of an oscillating magnetic field. Although the structures are expected to have zero net displacement after one time period of oscillation, the presence of noise (thermal fluctuations) results in a net displacement which can be orders of magnitude higher than what would be expected from simple Brownian motion. In other words, the combined action of thermal noise and reciprocal motion can lead to a net displacement significantly larger than the action of fluctuations alone. This result was first obtained theoretically by Lauga, and was used to predict large motility in certain species of marine bacteria, undergoing reciprocal motion. The experimental realization, as shown in this paper was obtained with magnetized helices, which turned back and forth, and therefore showed to-and-fro motion in the presence of an oscillating magnetic field. Apart from the enhancement of diffusivity observed in this paper, there are several other interesting theoretical predictions on the behaviour of swimmers showing reciprocal motion; which may now be realized in this novel experimental system.
Observation of Enhanced Diffusivity in Magnetically Powered Reciprocal Swimmers, Pranay Mandal and Ambarish Ghosh, Phys. Rev. Lett. 111, 248101 (2013)