Nonequilibrium diffusion of active particles bound to a semi-flexible polymer network: simulations and fractional Langevin equation

Abstract

In a viscoelastic environment, the diffusion of a particle becomes non-Markovian due to the memory effect. An open question is to quantitatively explain how self-propulsion particles with directional memory diffuse in such a medium. Based on simulations and analytic theory, we address this issue with active viscoelastic systems where an active particle is connected with multiple semi-flexible filaments. Our Langevin dynamics simulations show that the active cross-linker displays super- and sub-diffusive athermal motion with a time-dependent anomalous exponent $α$. In such viscoelastic feedback, the active particle always has superdiffusion with $α=3/2$ at times shorter than the self-propulsion time ($τ_A$). At times greater than $τ_A$, the subdiffusion emerges with $α$ bounded between $1/2$ and $3/4$. Remarkably, the active subdiffusion is reinforced as the active propulsion (Pe) is more vigorous. In the high-Pe limit, the athermal fluctuation in the stiff filament eventually leads to $α=1/2$, which can be misinterpreted with the thermal Rouse motion in a flexible chain. We demonstrate that the motion of active particles cross-linking a network of semi-flexible filaments can be governed by a fractional Langevin equation combined with fractional Gaussian noise and an Ornstein-Uhlenbeck noise. We analytically derive the velocity autocorrelation function and mean-squared displacement of the model, explaining their scaling relations as well as the prefactors. We find that there exist the threshold Pe ($\mathrm{Pe}^*$) and cross-over times ($τ^*$ and $τ^\dagger$) above which the active viscoelastic dynamics emerge on the timescales of $τ^* \lesssim t \lesssim τ^\dagger$. Our study may provide a theoretical insight into various nonequilibrium active dynamics in intracellular viscoelastic environments.