SMOOTH LAYERS

Two streams that refuse to mix

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Laminar

Fluid slides in smooth, parallel layers, like cards in a deck

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Turbulent

Eddies and swirls churn the fluid, as in a river or a stirred cup

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On a chip

Flow is almost always laminar, so streams run side by side

In a microchannel, two liquids can flow next to each other and barely mix.

THE NUMBER

The Reynolds number

Re = ρ v D_h ÷ μ Inertia (keeps fluid moving) divided by viscous friction (slows it down)

ρ and μ Density and viscosity of the liquid; water is about 1000 kg/m³ and 1 mPa·s

v and D_h Speed and channel size (the hydraulic diameter)

Above about 2,000 Pipe flow turns turbulent

On a chip Re is usually below 1: deep in laminar territory

Small size and slow speed make Re tiny, so friction wins and flow stays smooth.

THE FLOW PROFILE

Fast in the middle, still at the walls

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This curved profile is called Poiseuille flow. It means particles near the centre race ahead while those near the walls lag, which spreads a sample out as it travels.

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No slip

Liquid touching a wall doesn't move at all

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A parabola

Speed rises smoothly to a maximum in the centre of the channel

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Mean vs. maximum

In a round tube the centre flows twice as fast as the average

The walls hold the liquid back, and that drag is what shear stress on cells comes from.

PUT IT TO WORK

Using laminar flow on purpose

Hydrodynamic focusing Side streams squeeze a sample into a thin ribbon, so cells pass a laser one at a time

H-filter Small molecules diffuse across into a side stream while big particles stay put

Patterning cells Different stream contents touch different cells in the same channel

Smooth, predictable flow is a design tool, not just a fact of life.

A STRANGE WORLD

Life at low Reynolds number

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At Re far below 1 there is no coasting: stop pushing and the flow stops instantly. Flow is also reversible, so dye stirred one way can be unstirred by turning the handle back. Bacteria live here: they swim with corkscrew flagella because a simple back-and-forth stroke would get them nowhere.

In microfluidics, inertia is almost gone and every motion is ruled by friction.

UNIT 2 STUDY COMPLETE

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Ready for the Fab Challenge?

You've seen why microchannel flow is smooth, how to calculate the Reynolds number, and how engineers put laminar flow to work.