(6dd) Droplet Microfluidics in Physical and Biological Systems

The exploitation of microdroplets produced within microfluidic environments has recently emerged as a new and exciting technological platform for applications within the chemical and biological sciences. Interest in microfluidic systems has been stimulated by a range of fundamental features that accompany system miniaturization. Such features include the ability to process and handle small volumes of fluid, improved analytical performance and low unit cost. Moreover, microfluidic systems that generate and utilize a stream of sub-microlitre droplets dispersed within an immiscible continuous phase have the added advantage of allowing microscopic investigation of physical properties of complex fluids such as emulsions, suspensions and foams. I will describe here three different studies. All of which use the microdroplet platform to establish fundamental understanding of different physical systems as well as to develop devices for biomedical applications. First I will explore the transition from reversible to chaotic behavior in the oscillatory shear flow of water-in-oil emulsions. The emulsion is being flown through a microchannel and is forced to rearrange due to a central constriction. The surprising results show that while at the Stokes flow limit, the emulsion exhibit behaviors that vary from complete reversible to complete irreversible depending on the volume fraction, velocity and viscosity ratio. I will present here the first direct visualization of the phenomenon. This study is an important step in understanding the microscopic rearrangements of droplets and particles near jamming. The second study deals with the break-up of droplets in a concentrated emulsion during its flow as a 2D monolayer in a microchannel consisting of a narrow constriction. The probability of droplet break-up increases with flow rate and the rate of deformation of the drops. The probabilistic nature of the break-up process arises from the stochastic variations in the packing configuration of the drops as they enter the constriction.  The results of this study shed new light on the stability of concentrated emulsions and the resulting applications in droplet microfluidics and two-phase flow in porous media. Finally, I will describe the development of a new method for rapid detection of tuberculosis using droplet-based microfluidics. Ultimately, this tool will be able to detect tubercle bacilli in a sensitive, rapid, specific and quantitative manner at low cost. This tool will be particularly important in resource-limited settings where TB is the most prevalent.