Abstract
The study of collective motion in bacterial suspensions has been of significant recent interest. To better understand the non-trivial spatio-temporal correlations emerging in the course of collective swimming in suspensions of motile bacteria, a simple model is employed: a bacterium is represented as a force dipole with size, through the use of a short-range repelling potential, and shape. The model emphasizes two fundamental mechanisms: dipolar hydrodynamic interactions and short-range bacterial collisions. Using direct particle simulations validated by a dedicated experiment, we show that changing the swimming speed or concentration alters the time scale of sustained collective motion, consistent with experiment. Also, the correlation length in the collective state is almost constant as concentration and swimming speed change even though increasing each greatly increases the input of energy to the system. We demonstrate that the particle shape is critical for the onset of collective effects. In addition, new experimental results are presented illustrating the onset of collective motion with an ultrasound technique. This work exemplifies the delicate balance between various physical mechanisms governing collective motion in bacterial suspensions and provides important insights into its mesoscopic nature.
Highlights
The phenomenon of collective motion has been an active area in the last few years
In this work we have introduced a simple model for studying correlation properties of mesoscopic collective motion in bacterial suspensions
Numerical simulations of this model demonstrate that both long-range hydrodynamic interactions and short-range collisions between bacteria are crucial to obtain agreement with experiment
Summary
The phenomenon of collective motion has been an active area in the last few years. Experiments have demonstrated that in the collective state a bacterial suspension can exhibit remarkable properties such as enhanced diffusivity, the formation of sustained whorls and jets, and the ability to extract useful work [1, 2, 3, 4, 5]. A variety of theoretical models have been considered recently, with fundamentally different mechanisms attributed to the onset of collective motion, from flagella reversal dynamics [4], pure hydrodynamic [7, 8, 9, 10, 11] to pure collisional [4, 12, 13, 14, 15, 16] interactions. While the existing models were able to reproduce some key experimental observations, such as the onset of collective motion above a certain critical concentration [6, 11, 17], but the agreement with experiment remains incomplete and mostly qualitative
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