Active-Passive Brownian Particle in Two Dimensions.

This paper presents a model for active particles in two dimensions with time-dependent self-propulsion speed undergoing both translational and rotational diffusion. Usually, for modeling the motion of active particles, the self-propulsion speed is assumed to be constant as in the famous model of active Brownian motion. This assumption is far from what may happen in reality. Here, we generalize active Brownian motion by considering stochastic self-propulsion speed $v(t)$. In particular, we assume that $v(t)$ is a two-state process with $v=0$ (passive state) and $v=s$ (active state). The transition between the two states is also modeled using the random telegraph process. It is expected that the presented two-state model where we call it active-passive Brownian particle has the characteristics of both pure active- and pure passive-Brownian particle. The analytical results for the first two moments of displacement and the effective diffusion coefficient confirm this expectation. We also show that a run-and-tumble particle (such as a motile bacterium) can be mapped to our model so that their diffusivities at large scales are equal.