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2. Encapsulation

from Agresti et. al PNAS 2010.

When working with cells, beads, or other "discrete" reagents, encapsulation into drops with high efficiency is imperative, because empty droplets waste reagent and screening time. However, efficient encapsulation of discrete reagents can be challenging, due to the irregular flow properties of the reagents. For example, cells growing in a media solution are dispersed randomly, so that when introduced into a microfluidic device, they enter at random intervals. When encapsulating them into drops, this results in Poisson loading of the cells, causing some drops to contain cells, but others to be empty.

One way to increase encapsulation efficiency is to organize the reagents before they enter the drop maker. This can be achieved using inertial ordering, in which lift forces generate interactions that organize the reagents, as shown in this movie,

From Ed et. al Lab on a Chip 8, 1262-1264 (2008)

For deformable reagents, like soft microgel particles or cells, another way to achieve efficient encapsulation is to use close-packing forces; if most of the surrounding solution is removed the particles pack together so that when introduced into a microfluidic device they flow more regularly. This allows them to be encapsulated much more efficiently, as shown in this movie,

from Abate et. al App. Phys. Lett. 2009