Figure 1. Particle motion with mere position-dependent propulsion velocity.

(a) Time evolution of the probability distribution P(x) of self-propelled particles with variable self-propulsion velocity for x≥−a as obtained from Brownian dynamics simulations (Methods). Although P(x) becomes asymmetric at long times, no systematic drift of the particles to regions of higher motility is observed. (b) Time evolution of (N⁺−N⁻)/(N⁺+N⁻), where N⁺ and N⁻ are the numbers of particles at x>0 and x<0, respectively. Obviously, a mere position-dependent motility does not induce a macroscopic particle current.