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Active microrheology of Chaetopterus mucus determines three intrinsic lengthscales that govern material properties

We characterize the scale-dependent rheological properties of mucus from the Chaetopterus marine worm and determine the intrinsic lengthscales controlling distinct rheological and structural regimes. Mucus produced by this ubiquitous filter feeder serves a host of roles including filtration, protection and trapping nutrients. The ease of clean mucus extraction coupled with similarities to human mucus rheology also make Chaetopterus mucus a potential model system for elucidating human mucus mechanics. We use optically trapped microsphere probes of 2-10 microns, to induce oscillatory strains and measure mucus stress response. We show that viscoelastic properties are highly dependent on the strain scale (l) with three distinct regimes emerging: microscale: l_1<4 microns, mesoscale: l_2~4-10 microns, and macroscale: l_3>10 microns. While mucus response is similar to water for l_1 indicating that probes rarely contact the mucus mesh, for l_2 the response is distinctly more viscous and independent of probe size, demonstrating that the mucus behaves as a continuum. However, this principally viscous mesoscale response is distinct from the largely elastic macroscopic mucus response. Only for l_3 does the response mimic macroscopic elasticity, with rigid constraints strongly resisting microsphere displacement. Our results demonstrate that a uniform mesh model for mucus with a single lengthscale modulating the crossover from water-like to elastic is too simplistic. Rather, the mucus responds as a hierarchical network with a loose microscopic mesh controlling mechanics for l_2, coupled with a mesoscale rigid scaffold responsible for the macroscopic gel-like mechanics beyond l_3. Our results shed important new light onto the design of drug delivery platforms, preventing pathogen penetration, and improving filtration, coating and clearance capabilities of mucus.