Persistent motion of a Brownian particle subject to repulsive feedback with time delay

Abstract: Based on analytical and numerical calculations we study the dynamics of an overdamped colloidal particle moving in two dimensions under time-delayed, non-linear feedback control. Specifically, the particle is subject to a force derived from a repulsive Gaussian potential depending on the difference between its instantaneous position, $\mathbf{r}(t)$, and its earlier position $\mathbf{r}(t-\tau)$, where $\tau$ is the delay time. Considering first the deterministic case, we provide analytical results for both, the case of small displacements and the dynamics at long times. In particular, at appropriate values of the feedback parameters, the particle approaches a steady state with a constant, non-zero velocity whose direction is constant as well. In the presence of noise, the direction of motion becomes randomized at long times, but the (numerically obtained) velocity autocorrelation still reveals some persistence of motion. Moreover, the mean-squared displacement (MSD) reveals a mixed regime at intermediate times with contributions of both, ballistic motion and diffusive translational motion, allowing us to extract an estimate for the effective propulsion velocity in presence of noise. We then analyze the data in terms of exact, known results for the MSD of active Brownian particles. The comparison indeed indicates a strong similarity between the dynamics of the particle under repulsive delayed feedback and active motion. This relation carries over to the behavior of the long-time diffusion coefficient $D_\mathrm{eff}$ which, similarly to active motion, is strongly enhanced compared to the free case. Finally we show that, for small delays, $D_\mathrm{eff}$ can be estimated analytically.