Active Brownian dynamics in a configurable viscoelastic medium

Active particles, both natural and artificial, in complex fluids exhibit rich dynamics, including anomalous diffusion and non-Boltzmann behavior due to the interplay between fluid response, thermal fluctuations, and self-propulsion. Studying these microswimmers in viscoelastic media is challenging because their propulsion mechanisms and viscoelastic responses locally alter material properties. Here, we present a novel experimental framework utilizing configurable viscoelastic fluids to study the dynamics of artificial microswimmers with precise control over all viscoelastic parameters. Our approach employs a diffusing optical trap to create tunable viscoelastic environments for Pt-coated Janus colloids. By programming a piezo-mirror to scan the trap along computer-generated Brownian trajectories, we generate Maxwell-Voigt responses that mimic diverse soft materials. Through independent control of viscoelastic timescales and moduli using laser power and trap diffusion profiles, we observe three dynamical regimes: short-time Brownian diffusion, intermediate anomalous diffusion, and long-time post-relaxation. Numerical simulations of harmonically bound active Brownian particles with long-time diffusion validate our experimental findings and corroborate our conclusions.