The diffusion properties of self-propelled particles which move at constant speed and, in addition, reverse their direction of motion repeatedly are investigated. The internal dynamics of particles triggering these reversal processes is modeled by a stochastic clock. The velocity correlation function as well as the mean squared displacement is investigated and, furthermore, a general expression for the diffusion coefficient for self-propelled particles with directional reversal is derived. Our analysis reveals the existence of an optimal, finite rotational noise amplitude which maximizes the diffusion coefficient. We comment on the relevance of these results with regard to biological systems and suggest further experiments in this context.
IntroductionActive matter systems-ensembles of self-driven particles-are a central subject of non-equilibrium statistical physics [1-4]:examples include micron-sized active colloids and rods driven by chemical reactions [5,6] or by the Quincke effect [7,8] as well as macroscopic collective motion patterns in bird flocks or sheep herds [9][10][11]. In particular, the study of bacterial systems as well as their theoretical analysis within simple self-propelled particle models has lead to interesting insights into the physics of active matter-consider, for example, the clustering of myxobacteria [12,13] or the dynamic vortex formation in dense suspensions of swimming bacteria [14][15][16][17].In order to understand the cooperative behavior of active particles as well as the associated pattern formation processes, reliable knowledge of the dynamics of individual entities is crucial. In this work, we therefore focus on the dynamics of individual active particles. We particularly consider particles that are able to reverse their direction of motion repeatedly. More precisely, particles follow an alternating motion pattern where rather persistent motion is interrupted by sudden reversals of the direction of motion.This type of motion has been reported in a variety of bacterial systems [18][19][20][21][22][23][24][25][26]. For instance, the soil bacterium Myxococcus xanthus constitutes a paradigmatic example of a bacterium exhibiting periodic reversals in the direction of motion:internal oscillations of the protein dynamics cause switches in cell polarity and, correspondingly, in the direction of motion [18,19,27,28]. Under certain conditions, the reversals of several, densely packed bacteria appear synchronously leading to remarkable accordion wave patterns [29,30]. Apart from myxobacteria, a variety of marine microorganisms exhibit run-and-reverse motion [20], such as Pseudoalteromonas haloplanktis and Shewanella putrefaciens [21]. Similar motion patterns were reported for Pseudomonas citronellolis [31], Paenibacillus dendritiformis [22] and Pseudomonas putida [23][24][25][26].More complex, three-step (run-reverse-flick) motion patterns, composed of rather straight runs, directional reversals and 90°turns, were found in the marine bacterium Vibrio alginolyticus [32,33]. In this con...