Physics-Biology interface seminar: Carine Douarche


14:00 - 15:00

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Collective motion in bacterial suspensions

Carine Douarche (Université Paris-Sud)

Warning: special seminar time.

Suspending highly motile Escherichia coli in a liquid lowers the viscosity of the solution at low shear rate. At higher cell concentrations, a regime of near zero viscosity can be reached. In this work, we investigate the system-size dependence of the rheological response of an E. coli suspension as a function of shear rate and bacteria concentration using a low-shear Couette rheometer. Additionally, we image the suspensions in a cone-plate rheo-imaging setup allowing direct visualisation of the collective organisation under shear. We find the flow becomes banded and viscosity decreases to near zero at a bacterial concentration close to where collective motion appears in absence of flow.

Indeed, dense suspensions of swimming bacteria display remarkable collective motion, i.e. local bacterial ordering associated with a characteristic correlation length, reminiscent of turbulent flow behaviour. Using video microscopy over large fields of view (up to 3 mm x 4 mm), and particle image velocimetry, we calculate the spatial correlation of the velocity vectors and extract a characteristic length scale. At sufficiently high bacterial concentrations, we find this length to be proportional to the smallest system size. However, the absence of saturation towards large system-sizes suggests there is no intrinsic length-scale in these dense populations of such ‘pusher-like’ swimmers.

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