Abstract
Bacteria exhibit collective motion when the freely-swimming cells are cultivated and become tightly packed. This collective motion has some spatio-temporal patterns that are not found for the freely-swimming cells. In this study, we conducted the observation and numerical simulation to find some characteristics in the collective motion. In the microscopic observation, the overall flow velocity of the bacterial swarming is analyzed by the particle image velocimetry. The correlation between two velocity vectors is calculated. With the increase in the number density of cells, the correlation length becomes larger, up to 10 micrometers. In the numerical simulation, the effect of alignment of cells due to their interaction is considered, where the close cells have only repulsion like a collision of the rigid rodlike bodies. With increase in number density of the model cells, the correlation length of the velocity correlation becomes larger, which agrees with the observed result.
