Editing Digital microfluidics (section)

== Droplet formation ==
There are two ways to make new droplets with a digital microfluidic device. Either an existing droplet can be split in two, or a new droplet can be made from a reservoir of material.<ref name="Cho_2003">{{Cite journal | vauthors = Cho SK, Moon H, Kim CJ |date= February 2003|title=Creating, transporting, cutting, and merging liquid droplets by electrowetting-based actuation for digital microfluidic circuits|journal=Journal of Microelectromechanical Systems|volume=12|issue=1|pages=70–80|doi=10.1109/JMEMS.2002.807467 |url=http://www-mtl.mit.edu/researchgroups/mems-salon/kevin_cho_2003.pdf}}</ref> Both processes are only known to work in closed devices,<ref name="Berthier_2008" /><ref>{{Cite journal| vauthors = Chang JH, Kim DS, Pak JJ |date=2011-05-02|title=Simplified Ground-type Single-plate Electrowetting Device for Droplet Transport|journal=Journal of Electrical Engineering & Technology<!-- This is the Korean journal, not the predatory Indian one--> |volume=6|issue=3|pages=402–407|doi=10.5370/JEET.2011.6.3.402|issn=1975-0102|doi-access=free}}</ref> though this often is not a problem as the top plates of DMF devices are typically removable,<ref name="Kirby_2013" /> so an open device can be made temporarily closed should droplet formation be necessary.
[[File:Droplet_Splitting_Figure.png |alt=A droplet being split in a digital microfluidic device. Initially, the droplet's has a shape like a spherical section. The charged electrodes on either side pull the droplet in opposite directions, causing a bulb of liquid on either end with a thinner neck in the middle, not unlike a dumbbell. As the ends are pulled, the neck becomes thinner and when the two sides of the neck meet, the neck collapses, forming two discrete droplets, one on each of the charged electrodes.|thumb|576x576px|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 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,<ref name="Fair_2007" /> this droplet will move towards both active electrodes. Liquid moves to either side, which causes the middle of the droplet to neck.<ref name="Cho_2003" /> For a droplet of the same size as the electrodes, splitting will occur approximately when <math>R_{neck}/R_{end}=-1</math>, as the neck will be at its thinnest.<ref name="Cho_2003" /> <math>R_{neck}</math> is the [[radius of curvature]] of the [[Meniscus (liquid)|menisci]] at the neck, which is negative for a concave curve, and <math>R_{end}</math> 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.<ref name="Cho_2003" /><ref name="Teh_2008">{{cite journal | vauthors = Teh SY, Lin R, Hung LH, Lee AP | title = Droplet microfluidics | journal = Lab on a Chip | volume = 8 | issue = 2 | pages = 198–220 | date = February 2008 | pmid = 18231657 | doi = 10.1039/B715524G }}</ref>

The conventional method<ref name="Pollack_2000">{{Cite journal| vauthors = Pollack MG, Fair RB, Shenderov AD|date=2000-09-11|title=Electrowetting-based actuation of liquid droplets for microfluidic applications|journal=Applied Physics Letters|volume=77|issue=11|pages=1725–1726|doi=10.1063/1.1308534|bibcode=2000ApPhL..77.1725P|issn=0003-6951}}</ref><ref name="Cho_2003" /> of splitting an existing droplet by simply turning the splitting electrodes on and off produces new droplets of relatively equal volume. However, the new droplets formed by the conventional method show considerable difference in volume.<ref>{{Cite journal| vauthors = Nikapitiya NY, Nahar MM, Moon H |date=2017-06-16|title=Accurate, consistent, and fast droplet splitting and dispensing in electrowetting on dielectric digital microfluidics|journal=Micro and Nano Systems Letters|volume=5|issue=1|page=24|doi=10.1186/s40486-017-0058-6|bibcode=2017MNSL....5...24N|issn=2213-9621|doi-access=free}}</ref><ref name="Banerjee_2012">{{cite journal | vauthors = Banerjee A, Liu Y, Heikenfeld J, Papautsky I | title = Deterministic splitting of fluid volumes in electrowetting microfluidics | journal = Lab on a Chip | volume = 12 | issue = 24 | pages = 5138–5141 | date = December 2012 | pmid = 23042521 | doi = 10.1039/c2lc40723j }}</ref> This difference is caused by local perturbations due to the rapid mass transport.<ref name="Banerjee_2012" /> Even though the difference is negligible in some applications, it can still pose a problem in applications that are highly sensitive to variations in volume,<ref name="Liu_2014">{{Cite journal| vauthors = Liu Y, Banerjee A, Papautsky I |date=2014-01-10|title=Precise droplet volume measurement and electrode-based volume metering in digital microfluidics|journal=Microfluidics and Nanofluidics|volume=17|issue=2|pages=295–303|doi=10.1007/s10404-013-1318-2|s2cid=16884950|issn=1613-4982}}</ref><ref>{{Cite journal| vauthors = Vergauwe N, Witters D, Atalay YT, Verbruggen B, Vermeir S, Ceyssens F, Puers R, Lammertyn J | display-authors = 6 |date=2011-01-26|title=Controlling droplet size variability of a digital lab-on-a-chip for improved bio-assay performance|journal=Microfluidics and Nanofluidics|volume=11|issue=1|pages=25–34|doi=10.1007/s10404-011-0769-6|s2cid=93039641|issn=1613-4982}}</ref> such as immunoassays<ref>{{cite journal | vauthors = Shamsi MH, Choi K, Ng AH, Wheeler AR | title = A digital microfluidic electrochemical immunoassay | journal = Lab on a Chip | volume = 14 | issue = 3 | pages = 547–554 | date = February 2014 | pmid = 24292705 | doi = 10.1039/c3lc51063h }}</ref> and DNA amplification.<ref>{{cite journal | vauthors = Chang YH, Lee GB, Huang FC, Chen YY, Lin JL | title = Integrated polymerase chain reaction chips utilizing digital microfluidics | journal = Biomedical Microdevices | volume = 8 | issue = 3 | pages = 215–225 | date = September 2006 | pmid = 16718406 | doi = 10.1007/s10544-006-8171-y | s2cid = 21275449 }}</ref> To overcome the limitation of the conventional method, an existing droplet can be split by gradually changing the potential of the electrodes at the splitting region instead of simply switching them on and off.<ref name="Banerjee_2012" /> Using this method, a noticeable improvement in droplet volume variation, from around 10% variation in volume to less than 1% variation in volume, has been reported.<ref name="Banerjee_2012" />

=== 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.<ref name="Cho_2003" /><ref>{{Cite book| vauthors = Fan SK, Hashi C, Kim CJ |title=The Sixteenth Annual International Conference on Micro Electro Mechanical Systems, 2003. MEMS-03 Kyoto. IEEE |chapter=Manipulation of multiple droplets on N×M grid by cross-reference EWOD driving scheme and pressure-contact packaging |date=2003|pages=694–697|doi=10.1109/MEMSYS.2003.1189844|isbn=0-7803-7744-3 |s2cid=108612930}}</ref> 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.<ref name="Elvira_2012">{{cite journal | vauthors = Elvira KS, Leatherbarrow R, Edel J, Demello A | title = Droplet dispensing in digital microfluidic devices: Assessment of long-term reproducibility | journal = Biomicrofluidics | volume = 6 | issue = 2 | pages = 22003–2200310 | date = June 2012 | pmid = 22655007 | pmc = 3360711 | doi = 10.1063/1.3693592 }}</ref> Because this distance varies, the volumes of dispensed droplets will also vary within the same device.<ref name="Elvira_2012" />

Due to these inconsistencies, alternative techniques for dispensing droplets have been used and proposed, including drawing liquid out of reservoirs in geometries that force a thinner neck,<ref name="Cho_2003" /><ref name="Nikapitiya_2014">{{Cite book | vauthors = Nikapitiya NJ, You SM, Moon H |title=2014 IEEE 27th International Conference on Micro Electro Mechanical Systems (MEMS) |chapter=Droplet dispensing and splitting by electrowetting on dielectric digital microfluidics |date=2014|pages=955–958|doi=10.1109/MEMSYS.2014.6765801 |isbn=978-1-4799-3509-3|s2cid=45003766}}</ref> using a continuous and replenishable electrowetting channel,<ref name="Liu_2014"/> and moving reservoirs into corners so as to cut the reservoir down the middle.<ref name="Teh_2008" /><ref name="Nikapitiya_2014" /> Multiple iterations of the latter can produce droplets of more manageable sizes.