Capillary and Transport Phenomena of Liquid Drops Spreading on Miscible Liquid Subphases

It has long been known that drops of soluble surfactant solution induce Marangoni flows at air-liquid interfaces. These surfactant drops create a surface tension gradient, which causes an outward convective flow at the <br>fluid interface. In this thesis, we show that aqueous<br>phospholipid dispersions may be used for this same purpose. In aqueous dispersions, phospholipids aggregate into vesicles that are not surface-active, so these dispersions do not initiate Marangoni flow. However, aerosolization of these dispersions causes the vesicles to<br>shear open, allowing access to the surface-active lipid monomer within. Deposition of lipid via aerosolization leads to surface tensions as low as 1 mN/m on water and can induce spreading on entangled polymer subphases even in the presence of pre-deposited phospholipid layers. Most methods for introducing a surfactant solution to a liquid subphase involve dropwise deposition, either by pipette or by aerosol. In order to better understand the behavior of these miscible drop/subphase systems, we study drops of solvent as they slowly diffuse into their polymer solution subphase. Previous work has shown that, even when two fluids<br>are completely miscible, they can maintain a detectable \eeffective interface" for long times. Effective interfacial tension has been probed using a number of methods, but that work has not extended to the three-phase system of a fluid drop on top of a miscible pool. By observing drop shapes, we show that these drops obey immiscible wetting conditions, that their shapes obey the augmented Young-Laplace equation, and that the low density difference of these<br>systems provides a unique ability to probe very small pressures across the drops. The ability to use phospholipids naturally found in the lung as spreading agents on low<br>surface tension surfaces of macromolecular solutions, in tandem with an understanding of the effective interfaces controlling drop shape, yields a deeper understanding of how we might improve surfactant-driven delivery of therapeutic agents along the complex airway surfaces in the lung.