TWO LIQUIDS

Oil and water don't mix

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Two phases

Water droplets travel inside a stream of oil

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Surfactant

A soap-like additive keeps droplets from merging

⚖️

Capillary number

Ca = μ v ÷ γ compares the oil's drag with surface tension

When viscous drag beats surface tension, the oil pinches off a droplet.

DROPLET MAKERS

A T-junction in action

T-junction Water enters from the side and the oil snaps off plugs

Flow focusing Oil squeezes the water stream from both sides through a narrow neck

Size control The flow ratio and channel width set the droplet size

Each droplet is the same size, so each is an identical tiny reactor.

TINY TEST TUBES

What you can do with a droplet

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⚡

Thousands per second

Droplets can be made at kilohertz rates

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Merge and split

Combine a cell droplet with a reagent droplet, or split one in two

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Sort

An electric pulse pulls glowing droplets into a collection channel

A droplet is a test tube about a billion times smaller than a real one.

USES

Droplets changed biology

Droplet digital PCR splits a sample into about 20,000 droplets and counts how many light up. Droplet single-cell sequencing, introduced around 2015, pairs each cell with a barcoded bead so thousands of cells can be sequenced at once.

Counting droplets turns a fuzzy measurement into a precise digital one.

DIGITAL MICROFLUIDICS

Moving drops with electricity

🔲

In electrowetting-on-dielectric (EWOD), a grid of electrodes sits under a hydrophobic coating. Switching an electrode on makes the surface above it more wettable, so a drop slides onto it. Software can move, merge and split drops on the grid with no channels or pumps.

Electrowetting turns a chip into a programmable drop-moving board.

UNIT 12 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen how chips make uniform droplets, what those droplets are used for, and how electrodes can move drops with no channels.