Pair velocity correlations among swimming Escherichia coli bacteria are determined by force-quadrupole hydrodynamic interactions

This Letter examines the pair velocity correlations among Escherichia coli (E. coli) bacteria swimming freely in a microfluidic channel. A large number of bacterial tracks are obtained using a particle tracking algorithm, and the longitudinal and transverse pair velocity correlation functions are evaluated. A theoretical analysis traces the origin of correlated motion between bacterial pairs to the translation of a bacterium driven by the force-quadrupole velocity disturbance caused by the swimming of neighboring bacteria. Both theory and experiments indicate that the longitudinal and transverse correlation functions are positive and negative, respectively. (c) 2007 American Institute of Physics.