Centrifugation is a widespread laboratory technique used to separate mixtures into fractions characterized by a specific size, weight or density. We demonstrate that centrifugation can be also used to separate swimming cells having different motility. To do this we study self-propelled bacteria under the influence of an external centrifugal field. Using dynamic image correlation spectroscopy we measure the spatially resolved motility of bacteria after centrifugation. A significant gradient in swimming-speeds is observed for increasing centrifugal speeds. Our results can be reproduced by a model that treats bacteria as "hot" colloidal particles having a diffusion coefficient that depends on the swimming speed.
@article{arxiv.1310.2753,
title = {Motility fractionation of bacteria by centrifugation},
author = {Claudio Maggi and Alessia Lepore and Jacopo Solari and Alessandro Rizzo and Roberto Di Leonardo},
journal= {arXiv preprint arXiv:1310.2753},
year = {2014}
}