Abstract
Pattern formation in 3D random media has been a topic of interest in soft matter and biological systems. However, the onset of long-range microscopic ordering has not been explored in randomly moving self-propelled particles due to a lack of model systems as well as local probe techniques. In this article, we report on a novel experiment, using motile Escherichia coli bacteria as a model system, to study the onset of dynamic correlation and collective movement in three-dimension. We use fluctuation of an optically trapped micron-size bead as a detector of correlated bacterial motion, and further study this behavior by analyzing the motility of fluorescent bacteria in a confocal volume. We find evidence of dynamic correlation at very low volume fractions (0.01). We show that the magnitude of this correlation strongly depends on the interbacterial distances and their coupling modes. This opens up possibilities to probe long-range pattern formation in actively propelled cells or organisms coupled through hydrodynamics and/or chemical signaling.