Size dependent diffusion and dispersion of particles in mucin

ABSTRACT Mucus, composed significantly of glycosylated mucins, is a soft rheologically complex vis-coelastic material lining respiratory, reproductive, and gastrointestinal tracts in mammals. Mucus may present as a gel or as a fluid, and serves as a barrier to the transport of microbes including, harmful particles, and inhaled atmospheric pollutants. Studies on mucin gels have provided insight into swelling kinetics, and the diffusion and permeability of molecular constituents such as water. The transport and dispersion of micron and sub-micron tracer particles in mucin gels and solutions differs from the motion of molecular water since the much larger tracers may interact with microstructure and larger features of the mucin network. Here, using brightfield and florescence microscopy, high speed particle-tracking, and passive microrheology, we study the thermally driven stochastic movement of 0.5 − 5.0 µ m tracer particles in 10% mucin solutions at neutral pH, and in 10% mucin mixed with unmodified limestone rock dust, modified limestone, and crystalline silica. Particle trajectories, mean square displacements, and the displacement probability distributions, are used to assess tracer diffusion and transport. Complex moduli are concomitantly extracted using microrheology techniques. We find that under the conditions analyzed in our experiments the mucin solution present as a highly viscous, weakly viscoelastic fluid rather than as a viscoelastic gel. For small to moderately sized tracers with diameter than 2 µ m, effective diffusion coefficients follow predictions of classical Stokes-Einstein relationship. Diffusivity in rock-dust laden mucin is surprisingly larger than in bare mucin. Probability distributions of squared particle displacements indicate that heterogeneity, transient trapping, and electrostatic interactions impact tracer transport, especially for larger tracers. Our results prompt further exploration of physiochemical and rheological mechanisms mediating particle transport in soft, viscoelastic biopolymer networks and materials.