Active Diffusion of Self-Propelled Particles in Flexible Polymer Networks

Biopolymer networks having a meshwork topology, e.g., extracellular matrices and mucus gels, are ubiquitous. Understanding the diffusion mechanism of self-propelled agents, including Janus colloidal particles, through such biopolymer networks is thus of paramount importance. Here, for the first time, we computationally explore this issue in depth by explicitly modeling three-dimensional biopolymer networks and performing Langevin dynamics simulations of the active diffusion of the self-propelled tracers therein. We demonstrate that the diffusion dynamics of the active tracers feature rich, distinct physics depending on the mesh-to-particle size and Peclet number (Pe). When the particle is smaller than the mesh size ratio, it moves as if in free space with decreased mobility depending on the polymer-occupation density and Pe. However, when the particle size is increased to be comparable to the mesh size, the active particles explore the polymer network via the trapping-and-hopping mechanism. If the particle is larger than the mesh, it captures the collective viscoelastic dynamics from the polymer network at short times and the simple diffusion of the total system at large times. We study the trapped time distribution, flight-length distribution, mean-squared displacement, and long-time diffusivity on varying the Pe number and the tracer size. Finally, we discuss the scaling behavior of the long-time diffusivity with Pe, where we find a Pe range that yields a nontrivial power law. The latter turns out to originate from a large fluctuation of the trapped, activated tracers in conjugation with responsive polymer networks.