Microdroplets: Sorting

Sorting is not always needed when using these techniques for biological assays, but when it is, it is one of the most important steps. Sorting in microfluidics can be achieved in a variety of ways, including using electric and magnetic fields or by modulating the pressures in the channels. However, these sorting methods either place strict requirements on the properties of the drops or are limited in throughput. 

Newer sorting methods place fewer restrictions on the drops and are also capable of faster sorting. One such sorting method uses single-layer membrane valves. In this method, drops are flowed into a bifurcating channel that is slightly asymmetric; the upper path of the junction has a slightly lower hydrodynamic resistance than the lower path, so that, in the absence of other forces, all droplets flow up. However, by actuating the valve, the upper path is constricted, increasing its hydrodynamic resistance, and inverting the asymmetry; this causes the drops to flow into the lower channel. By timing the valve actuation with the droplet flow, individual drops can be deflected into the lower path, as shown in the above movie and in the wide-field movie of the same device below.

Another method to sort droplets utilizes dielectrophoresis, exploiting the fact that the conductive drops have a different dielectric constant from the non-conducting carrier oil. In an electric field, the drops polarize, creating an attractive interaction that pulls them towards the electrodes. This can be used to tug on select drops, deflecting them into a "keep" channel, as shown in this movie: