The Interfacial Physics Group

Carnegie Mellon University, Department of Physics

Spreading and Wetting Characteristics of a Drop of Pure Surfactant and Surfactant Solution on Liquid Subphases

by Ramankar Sharma

Abstract

Gradients in the surface excess concentration of surfactants adsorbed at a liquid/vapor interface create surface tension gradients that drive Marangoni flows. This phenomenon may be used to enhance the uniformity of aerosol drug delivery in the lungs of patients with obstructive pulmonary diseases, such as cystic fibrosis. We are investigating the potential use of surfactant-generated Marangoni flows to develop self-dispersing aerosols with improved uniformity of drug delivery in the treatment of partially obstructed lungs. For liquid aerosol systems, an active agent is formulated with an aqueous surfactant solution to enhance the spreading of aerosol droplets after deposition on the airway surface liquid (ASL).

Experiments with deposition of surfactant-laden aqueous drops on model subphases show significantly enhanced drop spreading relative to surfactant-free controls. We find that the surfactant escapes the drop’s contact line and spreads across the liquid subphase surface faster than the contact line. Our work with deposition of surfactant-laden aerosol droplets shows that the resulting Marangoni flows not only cause greater spreading of individual aerosol droplets on the liquid subphase surface but, more importantly, drive a large scale convective expansion of the entire field of deposited droplets across the subphase. The distances spanned by the aerosolized surfactant droplet field expansion are comparable to the lengths of airways deep in the lung where sub-monolayer droplet deposition is expected during aerosol inhalation.

Surfactant-induced Marangoni flows can also be used to enhance dispersal of dry powder medications in lung airways. We find that a physical mixture of solid surfactant and a model drug powder also disperses much further than the surfactant-free powder on the same ASL mimic liquid subphases. We characterized the forces acting on a single moving particle at the interface under the influence of Marangoni flows.

These spreading results suggest that the addition of surfactants to aerosol formulations could enhance aerosol drug delivery by increasing the extent of post-deposition spreading in lungs.