# A foam model highlights the differences of the macro- and microrheology of respiratory horse mucus

## TL;DR

This study compares the micro- and macrorheology of horse mucus and a hydrogel model, revealing a foam-like microstructure in mucus that influences particle transport and differs significantly from the gel's behavior.

## Contribution

It introduces a foam model for horse mucus microstructure, explaining the differences between micro- and macrorheology and its impact on particle transport.

## Key findings

- Mucus exhibits nonlinear viscoelastic behavior at smaller deformations than HEC.
- Mucus has a foam-like microstructure supported by cryo-SEM images.
- The foam microstructure influences particulate transport in mucus.

## Abstract

Native horse mucus is characterized with micro- and macrorheology and compared to hydroxyethylcellulose (HEC) gel as a model. Both systems show comparable viscoelastic properties on the microscale and for the HEC the macrorheology is in good agreement with the microrheology. For the mucus, the viscoelastic moduli on the macroscale are several orders of magnitude larger than on the microscale. Large amplitude oscillatory shear experiments show that the mucus responds nonlinearly at much smaller deformations than HEC. This behavior fosters the assumption that the mucus has a foam like structure on the microscale compared to the typical mesh like structure of the HEC, a model that is supported by cryogenic-scanning-electron-microscopy (CSEM) images. These images allow also to determine the relative amount of volume that is occupied by the pores and the scaffold. Consequently, we can estimate the elastic modulus of the scaffold. We conclude that this particular foam like microstructure should be considered as a key factor for the transport of particulate matter which plays a central role in mucus function with respect to particle penetration. The mesh properties composed of very different components are responsible for macroscopic and microscopic behavior being part of particles fate after landing.