Swarming dynamics in bacterial colonies

Abstract

We determine and relate the characteristic velocity, length, and time scales for bacterial motion in swarming colonies of Paenibacillus dendritiformis growing on semi-solid agar substrates. The bacteria swim within a thin fluid layer, and they form long-lived jets and vortices. These coherent structures lead to anisotropy in velocity spatial correlations and to a two-step relaxation in velocity temporal correlations. The mean squared displacement of passive tracers exhibits a short-time regime with nearly ballistic transport and a diffusive long-time regime. We find that various definitions of the correlation length all lead to length scales that are, surprisingly, essentially independent of the mean bacterial speed, while the correlation time is linearly proportional to the ratio of the correlation length to the mean speed.

Pattern formation in active particle systems due to competing alignment interactions

“…Thus, complex spatio-temporal patterns may emerge in interacting active particle systems. There are numerous experimental realizations of active matter, for example driven filaments on the molecular scale [6], colloidal systems [7,8], bacterial colonies [9][10][11][12][13][14][15][16][17] as well as macroscopic systems like flocking birds [18] and schooling fish [19].…”

“…On the one hand, the quantitative analysis of fascinating phenomena like clustering [11] and vortex formation [20] provide an a e-mail: grossmann@physik.hu-berlin.de b e-mail: pawelr@princeton.edu excellent opportunity to test theoretical predictions, and, on the other hand, experiments often reveal unforeseen novel phases of active matter. Recently, a number of experiments reported the emergence of dynamic vortex patterns in dense suspensions of Bacillus subtilis [12][13][14][15], Paenibacillus dendritiformis [16] as well as Escherichia coli [17]. In particular, it was observed that bacteria locally align their body axis.…”

Motion of two micro-wedges in a turbulent bacterial bath

“…Examples range from bacterial suspensions [8,9], spermatozoa [10,11], human crowds [12] to suspensions composed out of artificial self-propelled particles [13,14,15,16,17,18]. Such systems have frequently been studied in the last year in bulk focusing on clustering [19,20,21,22,23,24], swarming [25,26,27,28] and complex swirling or turbulence [29,30,31,32,33,34,35,36]. A static confinement has been shown to be able to stabilize these structures [37], accumulate and guide active particles [38,39,40,41,42,43].…”

Nonlinear spontaneous symmetry breaking in active polar films

“…We can thus for small enough ϵ approximate the solution by truncating the series at k = N , to obtain a system of 2N ODEs for θ 1 (t), ϕ 1 (t), ..., ϕ N (t), θ N (t). To capture the asymmetry in the film, we need to have at least N = 2, for which we start by assuming that λ 1 = 1, λ 2 = 4: this allows us to keep the contribution of θ 1 , ϕ 1 as the dominant one up to the order ϵ 4 , at which point θ 2 , ϕ 2 appear; the assumption we check to be consistent a posteriori.…”