Drop detachment modes in microfluidics devices

Johns Hopkins University, Baltimore MD

Investigators

Abstract

National Science Foundation - Division of Chemical &Transport Systems Particulate & Multiphase Processes Program (1415) Proposal Number: 0651035 Principal Investigators: Stebe, Kathleen Affiliation: Johns Hopkins University Proposal Title: Drop detachment modes in microfluidics devices Intellectual Merit Drops surrounded by a continuous phase are formed in microfluidics devices to sequester proteins, lipids, cellular fragments, reagents for materials manufacture, etc. The individual drops allow cross contamination to be minimized, and provide small "reactors" in which assays can be performed or products can be made. The drops are often formed in the presence of surfactants, either to prevent the adsorption of fragile reagents (e.g. proteins) and to reduce the work required to produce the drop-continuous phase interface. In the presence of surfactants, a wide range of drops can be formed in a flow focusing device by simply tuning the relative flow rates of the drop and continuous fluids. Surfactants likely play a strong role in determining the regime of drop detachment. However, this has not been established in a quantitative fashion with well characterized surfactants. We intend to study drop formation and detachment in a flow focusing device for three regimes of surfactant mass transfer; adsorption-desorption controlled surfactants, diffusion controlled surfactants and mixed kinetic-diffusion controlled surfactants in flow fields relevant to drop detachment in microfluidics. The question naturally arises- why study all three limits? Furthermore, how do you know which limit applies, and how does it depend on the surfactant related thermodynamic or kinetic parameters? Finally, what is the appropriate model to adopt for the mass flux of surfactant in a microfluidics device? A major focus of this work is to address these questions in numerics and experiment. We intend to establish the relative importance of bulk diffusion flux and adsorption/desorption kinetic fluxes as a function of drop length scale. We hypothesize that kinetics should dominate on length scales relevant to microfluidics. If this can be established, this would greatly simplify the analysis of drop dynamics in the length scale of ten microns and below. We propose to perform experiments using well characterized surfactants in terms of the surfactant thermodynamics and transport kinetic constants to compare to our numerical predictions. (The surfactants will be characterized by pendant drop analysis, in which the PI has extensive experience. Broader Impacts Multiphase lab-on-a-chip devices nearly always contain deliberately added surfactant to reduce the work required to produce drops. They typically use drops to sequester reagents (e.g. proteins, peptides and fragments) that are surface active. At present, there are no selection rules to guide the relationship between surfactant formulations, injection modes and detachment conditions. Meanwhile, experimental evidence for the rich behavior of surfactant-laden drops in terms of phase diagrams of break up modes reveal complex behavior with a number of transitions. By developing improved control over these systems, we support expanded use of multiphase flow microfluidics devices for diagnostics, screening and manufacturing. The PI regularly welcomes undergraduate and high school students into her laboratories, and has directed more than 24 such students over the past 12 years. Of these students 9 were women, and 3 were African American. These students have been drawn from various channels, including interested undergraduates enrolled at JHU, and the JHUMRSEC-REU and high school outreach programs. The PI is regularly invited to speak at outreach events, such as the Whiting School Engineering Open House, or presentations for the Center for Talented Youth to draw students into the science and engineering fields.