Absolute negative particle mobility

Noise effects in technological applications, far from being a nuisance, can be exploited with advantage — for example, unavoidable thermal fluctuations have found application in the transport and sorting of colloidal particles1,2,3 and biomolecules4,5,6. Here we use a microfluidic system to demonstrate a paradoxical migration mechanism in which particles always move in a direction opposite to the net acting force (‘absolute negative mobility’) as a result of an interplay between thermal noise, a periodic and symmetric microstructure, and a biased alternating-current electric field. This counterintuitive phenomenon could be used for bioanalytical purposes, for example in the separation and fractionation of colloids, biological molecules and cells.