Active microrheology determines scale-dependent material properties of Chaetopterus mucus

We characterize the lengthscale-dependent rheological properties of mucus from the ubiquitous Chaetopterus marine worm. We use optically trapped probes (2-10 mu m) to induce microscopic strains and measure the stress response as a function of oscillation amplitude. Our results show that viscoelastic properties are highly dependent on strain scale (l), indicating three distinct lengthscale-dependent regimes at l(1)<= 4 mu m, l(2)approximate to 4-10 mu m, and l(3)>= 10 mu m. While mucus response is similar to water for l(1), suggesting that probes rarely contact the mucus mesh, the response for l(2) is distinctly more viscous and independent of probe size, indicative of continuum mechanics. Only for l(3) does the response match the macroscopic elasticity, likely due to additional stiffer constraints that strongly resist probe displacement. Our results suggest that, rather than a single lengthscale governing crossover from viscous to elastic, mucus responds as a hierarchical network with a loose biopolymer mesh coupled to a larger scaffold responsible for macroscopic gel-like mechanics.