Swarm behavior of self-propelled rods and swimming flagella

Abstract

Systems of self-propelled particles are known for their tendency to aggregate and to display swarm behavior. We investigate two model systems: self-propelled rods interacting via volume exclusion and sinusoidally beating flagella embedded in a fluid with hydrodynamic interactions. In the flagella system, beating frequencies are Gaussian distributed with a nonzero average. These systems are studied by Brownian-dynamics simulations and by mesoscale hydrodynamics simulations, respectively. The clustering behavior is analyzed as the particle density and the environmental or internal noise are varied. By distinguishing three types of cluster-size probability density functions, we obtain a phase diagram of different swarm behaviors. The properties of clusters such as their configuration, lifetime, and average size are analyzed. We find that the swarm behavior of the two systems, characterized by several effective power laws, is very similar. However, a more careful analysis reveals several differences. Clusters of self-propelled rods form due to partially blocked forward motion and are therefore typically wedge shaped. At higher rod density and low noise, a giant mobile cluster appears, in which most rods are mostly oriented toward the center. In contrast, flagella become hydrodynamically synchronized and attract each other; their clusters are therefore more elongated. Furthermore, the lifetime of flagella clusters decays more quickly with cluster size than of rod clusters.

Dynamics and efficiency of a self-propelled, diffusiophoretic swimmer

“…(34,35) with fixed concentrations far away of the swimmer are employed for numerical solution of Eqns. (20)(21)(22)(23)(24). The boundary conditions result in an infinite system of equations where the solute concentrations far away of the swimmer c {a,b} (∞) determine the reaction speed.…”

“…This procedure was described in detail by O'Brien and White 38 . For concrete numerical and analytical calculations we always rewrite the Stokes equation (22) by employing the stream function formalism 39 . The approximation Eq.…”

“…Furthermore, self-propelled small swimmers provide a unique chance to study artificial motion at the nanometer scale, e.g., to investigate anomalous diffusion [16][17][18] or non equilibrium thermodynamics 19,20 . Other interesting questions relating to mutual interactions of these swimmers and interactions with the environment are also beginning to be addressed [21][22][23][24] . In most of these experiments, a catalytic decomposition of hydrogen peroxide H 2 O 2 into oxygen O 2 (g) and water is employed for propulsion.…”

Transport Powered by Bacterial Turbulence

“…Introduction. Suspensions of bacteria or synthetic microswimmers show fascinating collective behavior emerging from their self-propulsion [1][2][3][4] which results in many novel active states such as swarming [5][6][7][8] and "active turbulence" [9][10][11][12][13][14]. In contrast to hydrodynamic turbulence, the apparent turbulent (or swirling) state occurs at exceedingly low Reynolds numbers but at relatively large bacterial concentrations.…”

Behaviour and Population Dynamics of Entomopathogenic Nematodes Following Application

“…They suggest that the initial aggregation is driven by random collisions between nematodes and continued collective motion is due to an attractive force arising from the surface tension of the water film (Gart et al). Other physical forces may also contribute to aggregation, and systems of self-propelled particles are known for their tendency to aggregate and display swarming behaviour (Yang, Marceau, & Gompper, 2010). For example, hydrodynamic interactions contribute to synchronisation and attraction of sperm (Yang, Elgeti, & Gompper, 2008).…”