User:Patrick Braid/sandbox

Droplet Formation

There are two ways to make new droplets with a digital microfluidic device. Either an existing droplet can be split apart, or a new droplet can be made from a reservoir of material. Both processes can generally only be done in a closed device, though this often is not a problem as the top plates of DMF devices are typically removable, so an open device can be made temporarily closed should droplet formation be necessary.

A side-on and top-down view of a droplet being split in a DMF device, where the progression of time is shown left to right.

From a an Existing Droplet

A droplet can be split by charging two electrodes on opposite sides of a droplet on an uncharged electrode. In the same way a droplet on an uncharged electrode will move towards an adjacent, charged electrode, this droplet will move towards both active electrodes. Liquid moves to either side, which causes the middle of the droplet to neck For a droplet of the same size as the electrodes, splitting will occur approximately when $R_{neck}/R_{end}=-1$, as the neck will be at its thinnest. $R_{neck}$ is the radius of curvature of the menisci at the neck, which is negative for a concave curve, and $R_{end}$ is the radius of curvature of the menisci at the elongated ends of the droplet. This process is simple and consistently results in two droplets of equal volume.

From a Reservoir

Creating a new droplet from a reservoir of liquid can be done in a similar fashion to splitting a droplet. In this case, the reservoir remains stationary while a sequence of electrodes are used to draw liquid out of the reservoir. This drawn liquid and the reservoir form a neck of liquid, akin to the neck of a splitting droplet but longer, and the collapsing of this neck forms a dispensed droplet from the drawn liquid. In contrast to splitting, though, dispensing droplets in this manner is inconsistent in scale and results. There is no reliable distance liquid will need to be pulled from the reservoir for the neck to collapse, if it even collapses at all. Because this distance varies, the volumes of dispensed droplets will also vary within the same device. Due to these inconsistencies, alternative techniques for dispensing droplets have been proposed, including drawing liquid out of reservoirs in geometries that force a thinner neck and moving reservoirs into corners so as to cut the reservoir down the middle. Multiple iterations of the latter can produce droplets of more manageable sizes.