Understanding how rapidly two or more chemical compounds react is fundamentally important in chemistry, biology, and materials science. Experimental techniques for measuring the rates of reactions under different conditions are valuable, especially if they allow one to do experiments that can not be performed by hand in a test tube -- that is, if they allow measurement of very rapid reactions (e.g. 1 ms, or 1/1000th of a second) using minute volumes of solutions (e.g. 30 nanoliters, or approximately 1/1000th of a single droplet).
Rustem Ismagilov and his collaborators at the Chicago Materials Research Center have developed a simple pressure-driven microfluidic device which, when coupled to standard instrumentation, addresses many of the recurring experimental challenges associated with these systems. This particular device, like other microfluidic devices, allows control of networks of fluid streams, where each stream can be analyzed, separated, and/or combined according to design.
Ismagilov's system, however, overcomes a number of the limitations of other microfluidic systems. It rapidly and thoroughly mixes samples using a process called chaotic advection, ensuring a nearly-homogeneous distribution of species in solution. It also transports solutions to the reaction vessel without dispersion (i.e. the reactants are localized within a given volume), which permits the accurate determination of the final reactant and product concentrations in the reaction mixture. This device also consumes samples at ~10,000 times lower rate than devices that rely on other methods of mixing, such as turbulence, a key consideration when using difficult-to-synthesize reactants.
Mixing, in the Ismagilov device, is achieved by incorporating a winding channel in the microfluidic system, where the flow moving through this section experiences different relative velocities along the side walls, therefore mixes. This process replicates the unsteady, chaotic fluid motion induced in flow cavities (apparatus used to study fluid motion and properties) by moving walls, only in this case, the fluid moves relative to the walls, with mixing occuring on the order of milliseconds. Effective mixing is especially critical when studying diffusion-controlled reactions (like some electron transfer reactions) where the final products obtained are often highly dependent upon the rate and method of mixing. Often pockets of pure reactant are found in poorly mixed solutions, leading to unwanted side reactions, before the reaction of interest can take place.
Dispersion is eliminated by localizing the reagents within aqueous plugs (droplets large enough to completely fill the flow channel). Streams of reagents, separated by an inert central stream, flow into a microchannel and are then injected into a flow of water-immiscible oil, spontaneously forming plugs. These plugs remain intact as they travel throughout the microfluidic system; the rapid mixing, described above, takes place entirely within each plug. Also, by incorporating different channel widths in the device, streams of plugs can be easily merged or divided.
Because this microfluidic system allows one to control complex reaction networks, and incorporates rapid mixing without dispersion, it may eventually serve a broad community of chemists, biochemists, and biophysicists as an inexpensive complement to stopped-flow instruments. It may also impact traditional areas of microfluidics where miniaturization and speed are becoming important—e.g. in high-throughput screening, combinatorial synthesis, analysis, and diagnostics.