  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.
