The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

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

by Amy Stetten

Abstract

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 ow at the fluid interface. In this thesis, we show that aqueous 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 ow. However, aerosolization of these dispersions causes the vesicles to 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 are completely miscible, they can maintain a detectable “effective 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 uid 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 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 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.