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.