Microdroplets: Parallel Dropmaking
Even though microfluidic devices form drops quickly, they do not form large volumes of drops, because each droplet is very small. For example, a typical drop maker forms about 1 mL of reagent per hour of operation. This is sufficient for most biological applications, but it is not for industrial applications, in which the devices are used to synthesize chemical or reagents, since these applications typically require a minimum production rate of 1-100 L per hour. In these instances, the best way to increase production is to increase the number of drop makers. However, to do this successfully requires that the channels feeding the array and collecting the drops be properly designed, so as to distribute the fluids evenly and at equal pressures. This can be achieved using feeding channels called distribution plates.
Another application of drop formation is synthesizing emulsion libraries, which are collections of drops that are the same size but encapsulate distinct reagents. They are often needed in biological assays, particularly when a large library of compounds must be tested against another compounds, or series of compounds. However, making libraries using a single drop maker is a tedious and labor-intensive process, since it requires each compound to be emulsified separately, and the products of all to be pooled into the library. Parallel emulsification makes this much faster and easier by creating the emulsions of the different compounds at the same time, as shown in the following movie:
In this movie, a parallel drop formation device is used to emulsify 8 distinct solutions at the same time. The device consists of 8 T-junction drop makers, each having its own inner phase inlets, allowing for the introduction of a distinct solution. The products from each drop maker are pooled in a common outlet, from which they are transported to the collection vial. In this example, the device is operated using a technique called syringe vacuum microfluidics, in which a vacuum applied to the exit of the device is used in conjunction with hydrodynamic resistor channels to pump the fluids through the devices. This affords the important advantage that only a single pumping system is needed to operate all the devices.