A two-layer Newtonian fluid model for muco-ciliary transport in the lung is developed where
the viscosity of the upper mucous layer is very much greater than the viscosity of the lower
periciliary layer. Theory is presented for both cases when the cilia penetrate, and do not
penetrate, the very viscous mucous layer. Calculations suggest that, in normal
circumstances, it is not essential for the cilia to penetrate the mucus to provide positive
transport. However, it does suggest that there is a weak optimal penetration depth of the cilia …