SURFACE TENSION

Liquids have skin

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Molecules pull together

Water molecules attract each other, so the surface acts like a stretched film

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Strong for water

About 0.072 N/m, which is why insects can walk on a pond

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Bigger when small

The smaller the drop or channel, the more surface tension matters

At chip scale, surface tension can be stronger than gravity or a gentle pump.

CONTACT ANGLE

Does it wet or bead up?

Below 90° Hydrophilic: the liquid spreads and wets the surface

Above 90° Hydrophobic: the liquid beads up and avoids the surface

Glass Strongly hydrophilic

Untreated PDMS Hydrophobic, about 110°

The contact angle tells you whether a wall will pull liquid in or push it away.

CAPILLARY ACTION

Channels that fill themselves

A wetting wall curves the liquid's surface (the meniscus), and that curve creates a pressure that pulls liquid forward. A glucose strip uses this to sip a drop of blood in under a second.

With a hydrophilic channel, surface tension is the pump.

CAPILLARY CIRCUITS

Stop, go and wait, with no electricity

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Stop valve A sudden widening pins the meniscus, and the liquid stops

Trigger valve A second liquid arriving at the stop releases it

Capillary pump A field of tiny posts keeps pulling liquid at a steady rate

Timer Channel length and width set how long each step takes

Geometry alone can program a sequence of steps into a passive chip.

TREATING SURFACES

Changing the wall's chemistry

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Oxygen plasma

Makes PDMS hydrophilic, but it slowly recovers to hydrophobic over hours to days

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Anti-fouling coatings

PEG or albumin stop proteins and cells from sticking

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Hydrophobic coatings

Silanes or fluorinated layers help make water-in-oil droplets

Surface chemistry is a design choice, just like channel shape.

UNIT 4 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen how surface tension and wall chemistry let a chip fill, stop and steer liquid with no pump.