Thin film dynamics with complex interfaces [electronic resource] : a study of tear film lipids and lung surfactants
Keywords: Chemical Engineering.

In this dissertation, we examine the drainage and dewetting dynamics of thin films from curved substrates. We emphasize an important aspect of this problem that has not received previous attention: the interaction of rheologically complex insoluble monolayers at the surface of the draining liquid. A major part of this thesis work is centered around the development of a new experimental device called the Interfacial Dewetting and Drainage Optical Platform (i-DDrOP), which enables the dynamic measurement of thin aqueous films that possess viscoelastic interfacial properties. We utilize the i-DDrOP to focus on two biological systems of interest: the tear film and the lung surfactant film. Tear film dynamics: The tear film that protects the ocular surface is a multiplexed structure composed of an aqueous phase capped with an insoluble meibomian lipid layer (meibum) spread from glands lining our eyelids. This meibum layer has been shown to be highly viscoelastic using interfacial rheology measurements. Past work has focused on the role of this layer in reducing evaporation, although conflicting evidence on its ability to reduce evaporative loss has been published. We present here the beneficial effects that are derived through the interfacial viscoelasticity of the meibomian lipid film. By measuring the drainage and dewetting dynamics of model tear films with the i-DDrOP, we offer evidence that these layers strongly stabilize the films due to their ability to support surface stresses. This alternative view of the role of meibum can help explain the origin of meibomian gland dysfunction, or dry eye disease, where improper compositions of this lipid mixture do not offer the proper mechanical resistance to breakage and dewetting of the tear film. Work is also done on examining the interaction between tear film lipids and silicone hydrogel (SiHy) contact lenses. The interplay between lenses and the tear film is central to the comfort experienced by the user. During the duration of a blink-cycle, the tear film can potentially break up and dewet on the anterior surface of the contact lens, leading to lipid depositions that modify the wettability of the lens surface. This deposition is an ongoing clinical problem with acute ophthalmological repercussions including symptoms of dry eyes and reduced vision for the contact lens wearer. Using the i-DDrOP, we investigate the dewetting and subsequent deposition of tear film lipids on SiHy surfaces. Our findings reveal valuable information about the wettability characteristics of these surfaces that has important applications for the eye care industry. Lung surfactants: The surfactant lining the walls of the alveoli increases pulmonary compliance and prevents collapse of the lung at the end of expiration. In premature born infants suffering from surfactant deficiency, clinically-approved lung surfactant replacements such as Survanta, Curosurf and Infasurf, are injected to facilitate breathing and prevent respiratory failure. However, clinical data shows varied degrees of efficacy and mortality for these commercial systems. Additionally, surface rheology measurements of these three systems reveal surprisingly different interfacial properties. Using the i-DDrOP, we test the ability of these commercial surfactant films to stabilize against drainage, and suggest different interfacial stress contributions, including dilatational and Marangoni stresses, for these surfactant systems. Understanding these fundamental different modes of stabilization can benefit the design of more effective replacement therapeutics for treating respiratory disorders.