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. 
W.J. Weigand, A. Messmore, D.D. Deheyn, A. Morales-Sanz, D.L. Blair,   J.S. Urbach, R.M. Robertson-Anderson
cond-mat.soft physics.bio-ph q-bio.TO