This proposal addresses the formation, propagation, rupture and prevention of liquid boluses in tubes with applications to human airways and multi-phase industrial flows. Two projects are outlined involving interfacial fluid mechanics related to the pulmonary system: Project I - perturbed capillary-elastic instabilities which are fundamental to the stability of small airways as they close and re-open; Project II - liquid plug propagation in a flexible tube, a fundamental mechanism of airway reopening and also of liquid bolus delivery of surfactants and medications into the lung; Both projects include theoretical studies and bench-top experiments which are designed to investigate the underlying fluid and interfacial dynamics with an eye toward applications to biomedicine and technology.
In Project I, the stability of the thin liquid lining which coats the airways is examined. Instabilities in this lining may cause closing off of the small airways by capillary-elastic interactions which form a liquid bolus or plug. The closure and subsequent reopening of these airways contributes to the shape of the pressure-volume curve in cycled lungs and measuring the occurrence of airway closure is one component of a standard pulmonary function test. Certain disease states, e.g. hyaline membrane disease, in newborns who lack sufficient surfactant lead to pathological airway closure, thus impairing gas exchange. The proposed research in Project I addresses the nature of airway closure and subsequent bolus formation as it is affected by cyclic perturbations from respiratory motion, including oscillation of the radial wall boundary and oscillation of fluid shear stress (from tidal air flow) at the air-liquid interface. We have preliminary results which indicate, for example, that oscillatory shear on the air-liquid interface can prevent the capillary instability from leading to closure. Examining the parameter ranges over which this preventive perturbation may hold will be one of the important specific goals for research.
A current technique for the treatment of hyaline membrane disease (surfactant deficiency in neonates) is to deliver surfactant into the lung by instilling it directly into the neonatal trachea. Other clinical situations which involve instillation of liquids into the lung are liquid ventilation, delivery of genetic material for gene-therapy treatment and drug delivery during cardiopulmonary resuscitation. The instilled surfactants and liquids spread by surface and gravity forces and by convection with the airflow if they form a bolus or plug in the airway. The proposed research in Project II addresses the fluid mechanics and transport of a liquid bolus in a flexible tube to advance our knowledge and understanding of how the plug delivers its liquid and potential solutes to the airways and alveoli. Experiments on bolus flow through a tube (airway) bifurcation is introduced with symmetric and non-symmetric conditions, the latter stemming from either gravity effects or the presence of a bolus in one of the bifurcation branches prior to delivery of a new bolus upstream.
In addition to the pulmonary application discussed above, this research addresses issues which are basic to other engineering fields. As a technologically related counterpart to airway closure, the stability of similar free surfaces is important in annular extrusion methods for manufacturing small hollow fibers. These may be materials processed for a number of applications. Also, there are methods of co-extrusion manufacturing (e.g. optical fibers) in which there is an inner plug core of liquid and an outer annulus of a different liquid extruded simultaneously, one around the other. Understanding the coupled behavior of our biomedical problem will shed light on these materials processing examples, since the approaches will have many similarities and the stability behavior is likely to overlap in certain parameter ranges where, for example, one surface is more unstable than the other. In addition, the formation of liquid plugs in two-phase flows through porous media follow similar behavior to airway closure phenomena and arise in important applications of oil recovery.
