CLEARANCE OF A THIXOTROPIC LIQUID FROM A CHANNEL - A MODEL OF COUGH (MUCUS, YIELD STRESS, AVALANCHE)

A newly discovered instability, resembling an avalanche, is proposed to explain how momentum and energy are transferred from exhaled air to the stationary blanket of mucus lining the walls of the respiratory tract during cough. This catastrophic event clears a significant fraction of mucus analog from a model airway (geometrically similar to a collapsed human trachea) in simulated steady-state cough experiments.;This instability can occur when air is forced through a channel lined with a liquid, rheologically similar to mucus. The clearance of liquid is always preceded by the appearance of small amplitude surface waves which coalesce into a well defined wavefront that propagates at velocities as high as 6 m/s in the direction of the airflow.;In the respiratory avalanche, both interfacial shear and static pressure forces contribute to the applied force on the liquid lining. These forces are taken into account in a physical model in which the law of conservation of momentum is applied to both the liquid and gas phases. A stability criterion is also derived which is verified by experiments. This phenomenon occurs when the sum of the stresses applied to the mucus analog exceeds its ultimate yield stress or the static frictional forces which hold it to the walls of the model airway.;Calculations of the stress developed at the interface between the mucus and the walls of the airway indicate that this instability should occur during cough but not during normal breathing.;It was also shown that introducing a lubricant between the thixotropic mucus analog and the solid surface to which it is applied reduces both the mean air velocity and pressure at which the instability occurs. It suggests that applying a lubricant at the interface between the airway wall and the mucus lining might facilitate cough. It also draws attention to the serous layer, a natural biological lubricant, which may also play an important role in clearance of mucus during cough.