Transport Processes Driven by Electrochemically-Generated Gradients in Concentration of Electro-Active Surfactants

University Of Wisconsin-Madison, Madison WI

Investigators

Abstract

Nicholas L. Abbott U of Wisconsin - Madison "Transport Processes Driven by Electrochemically-Generated Gradients in Concentration of Electro-Active Surfactants." This project involves a combined experimental and theoretical study of transport processes in the bulk of solutions that are driven by externally controlled gradients in concentration of surfactant. The experimental approach revolves around the use of redox-active surfactants and electrochemical methods to generate gradients in surfactant concentration within microfluidic channels. The amphiphilic properties of the redox-active nano-scale surfactants can be substantially and reversibly changed by electrochemically-controlled oxidation and reduction of the surfactant. By lining the microfluidic channels with two electrodes - one at which the surfactant is "turned on" and the other at which the surfactant is "turned off", the research will demonstrate general and facile methods that generate transient and steady-state gradients in surfactant concentration across the bulk of a solution contained within a channel. Spatial concentration gradients of micelles are also investigated. Using micrometer-sized droplets of oil as a model system and concentrations of surfactant below the critical micelle concentration, the influence of the gradients in concentration of the redox-active surfactants on the transport of the droplets is investigated. Marangoni-stresses at the surfaces of the droplets induced by the externally controlled gradient in surfactant concentration will drive the motion of the droplets across the gradient. The collective motion of ensembles of droplets driven by externally controlled gradients in surfactant concentration are also investigated. A complementary analytical and numerical study (finite elements and boundary element methods) of the motion of droplets in the presence of gradients in concentration of surfactant is performed. Because the kinetics of adsorption and desorption of the surfactants is expected to have a strong influence on the migration of the droplet and because they are a required input for the simulation, parameters characterizing the kinetics of adsorption of the redox-active surfactants at oil-aqueous interfaces are experimentally determined. In addition to aiding interpretation of the experiments, the simulations will provide a venue to explore key parameters that underlie the phenomenon (such as the role of the kinetics of adsorption) and guide the design of subsequent experiments. Broader Impacts: First, this research contributes to society's knowledge of interfacial phenomena and nanotechnology which underlies a range of commodity products (e.g., foods, pharmaceuticals) and processes (e.g., emulsion polymerization, oil processing). Second, the development of methods that permit control of transport processes in electrochemically-controlled gradients in concentration of surfactant have the potential to form the basis of technologies for performing separations of molecular species (such as drugs), for directing the meso-scale assembly of particles, and for sorting of emulsion droplets or driving emulsion droplets into arrays (meso-scale materials synthesis). While the research proposed is fundamental in nature, the principles that will emerge from it have technological promise. For example, the potential for specialized micro-drug delivery systems is also possible. Third, the blend of discovery-oriented experimentation and detailed fundamental analysis provides an unusual environment for the education of graduate students in both experimental and theoretical methods in interfacial science. Fourth, by incorporating undergraduate researchers into the project (as the PI and co-PI have done in the past, including female and minority undergraduate participants), the proposed research will stimulate undergraduates to consider the opportunities that follow from higher education in science and engineering.