Correlation lengths in turbulent bacterial suspensions
Résumé
Dense suspensions of swimming bacteria display remarkable properties such as the appearance of collective motion, i.e. local bacterial ordering associated with a characteristic correlation length-scale, reminiscent of turbulent flow behaviour. Here, we investigate this phenomenon as a function of system-size and bacterial concentrations using video microscopy over large fields of view (up to 3mm X 4mm). Using particle image velocimetry, we calculate the spatial correlation of the velocity vectors and extract a characteristic length scale L. At sufficiently, high bacterial concentrations, we found the length L is proportional to the smallest system size, i.e. here the height of the glass capillary. However, we found no saturation of L towards large system-sizes and L can reach ~500 µm, which is ~5x larger than the currently largest L reported for bacterial suspensions. This suggests the absence of an intrinsic length-scale of dense populations of such ‘pusher-like’ swimmers.