Pseudochemotaxis in inhomogeneous active Brownian systems

Abstract

We study dynamical properties of confined, self-propelled Brownian particles in an inhomogeneous activity profile. Using Brownian dynamics simulations, we calculate the probability to reach a fixed target and the mean first passage time to the target of an active particle. We show that both these quantities are strongly influenced by the inhomogeneous activity. When the activity is distributed such that high-activity zone is located between the target and the starting location, the target finding probability is increased and the passage time is decreased in comparison to a uniformly active system. Moreover, for a continuously distributed profile, the activity gradient results in a drift of active particle up the gradient bearing resemblance to chemotaxis. Integrating out the orientational degrees of freedom, we derive an approximate Fokker-Planck equation and show that the theoretical predictions are in very good agreement with the Brownian dynamics simulations.

Anomalous fluxes in overdamped Brownian dynamics with Lorentz force

“…In fact, it has become a common practise to start with the overdamped equation of motion of the particle (Eq. (2)) as the model of the nonequilibrium system under study [4][5][6][7][8][9][10][11]. The overdamped equation of motion is generally obtained in a simple way: set the inertia term to zero in the velocity Langevin equation and rearrange to describe the dynamics of the slow position variable.…”

Composite Search of Active Particles in Three‐dimensional Space Based on Non‐directional Cues

“…Directional persistence is a key kinetic parameter of self‐propelled microswimmers, which is set by a competition between swimming speed and directional fluctuations; directional fluctuations can be of thermal origin or originate from active fluctuations of the propulsion mechanism itself. A microswimmer may thus control its directional persistence by different means: (i) transiently changing its speed, [31] for instance in response to the local concentration of a chemical [32] or external fields, [33] (ii) up‐ or down‐regulating any stochastic part of active propulsion, [34,35] or, possibly, (iii) transiently changing its effective size. Regarding option (iii), consider for example a hypothetical microswimmer with extended lever arms that can switch between an extended and a folded configuration: because the rotational diffusion coefficients of an immersed microswimmer scale as with its longest linear dimensions L to leading order, a two‐fold change of L would result already in an almost 8‐fold change of its rotational diffusion coefficient [36] .…”

“…Future work will address the effect of measurement noise in target zone detection. More complex strategies can be realized if agents respond in a gradual manner to the distance from the nearest target, e. g., in the form of distance‐dependent swimming speed, [31] distance‐dependent rotational diffusion coefficient, [5] or for dimers of elastically coupled active and passive particles in a spatial activity field [52]…”

Lorentz forces induce inhomogeneity and flux in active systems

“…A common starting point of the theoretical description of ABPs is the Langevin equation. Most models of ABP ignore inertial effects and are based on overdamped equations of motion [27,[37][38][39][40][41]. Due to the nonconservative nature of the Lorentz force, the overdamped equation of motion cannot be obtained by simply setting the mass of the particles to zero [4,42].…”