Developing Simple and Reliable Tools for Chemical Analysis Using Droplet-Based Microfluidic Platforms

Two-phase flow is more complicated than single phase flow, which adds an additional layer of complexity to droplet microfluidic systems. This can cause problems with reliability, making use of droplet platforms for routine analysis particularly difficult. This work aims at developing new, reliable droplet microfluidic tools for chemical analysis by using simple design principles. The theory and demonstration of two simple droplet microfluidic viscometers with complementary operating modes is presented. The viscometers utilize either droplet frequency or the spacing of droplets in a droplet train to determine the viscosity of nanoliter-scale samples. Accuracy and precision were characterized and found to be equivalent to or better than many commercial viscometers. Depending on the specific design, viscosities spanning three orders of magnitude could be measured at shear rates spanning four orders of magnitude. Measurement of nonaqueous and aqueous samples was demonstrated, and a combination of surface fluorination and use of a fluorous surfactant was used to allow compatibility with protein solutions containing up to 26 mg/mL BSA. Viscosity could also be determined from a fluorescence trace of droplets, making simultaneous spectroscopic analysis and rheological characterization of samples possible. The simple designs allowed for reliable operation, and could allow for integration of viscometric measurement capabilities into existing droplet platforms with minimal changes to microfluidic architectures. A microfluidic injector capable of delivering 750 pL aqueous sample droplets to a capillary for separation by capillary electrophoresis is also presented. The design is extremely simple and consists of two straight channels interfaced out of plane in the shape of a cross. Injector precision and separation efficiency were characterized and found to be acceptable. To demonstrate use of droplets for storing temporal information, a droplet train containing a step change in riboflavin concentration was injected. Temporal resolutions down to 15 s were possible. Riboflavin assay in human urine was also performed to demonstrate a practical use for the injector. The simple nature of the design ensured reliable operation and may allow device fabrication by nontraditional means, which could make the design more available to research groups with limited access to expensive microfabrication equipment.