SMALL SCALE

Channels the width of a hair

📐

Size

Channels are usually tens to hundreds of micrometers wide

💧

Volume

A whole chip may hold only a few microliters

🐢

Flow rate

Pumps run at microliters per minute, far slower than a tap

At this size, fluids behave differently from what you see in a cup.

LAMINAR FLOW

Smooth layers, no swirling

In a microchannel, fluid slides in smooth parallel layers. There are no eddies, so two streams can run side by side for a long distance without mixing.

Mixing happens only by slow diffusion across the layers.

THE NUMBER

The Reynolds number

Re = ρ v D_h ÷ μ

ρ is the fluid's density, v its speed, D_h the channel's hydraulic diameter, and μ its viscosity. Flow in pipes becomes turbulent above Re of about 2,000. For culture medium moving at 1 mm/s through a 100 µm channel, Re is about 0.1.

Chips live far below the turbulent range, so flow stays smooth.

WHAT IT MEANS

Design consequences

Predictable Laminar flow is steady and easy to calculate, so shear can be set precisely

Side by side Two fluids can share a chip, which is how blood and brain compartments are built

Slow mixing A drug added to one stream spreads only gradually, so dosing needs planning

Smooth flow is a gift: it lets engineers design the forces cells feel.

PRACTICAL TROUBLE

Bubbles and clogs

At small scales surface tension is strong. An air bubble can lodge in a channel, block the flow, and damage the cell layer. Chip users degas their medium, wet the channels carefully, and prime tubing before connecting it.

Most chip failures are mundane: air, leaks, and dirt.

UNIT 10 STUDY COMPLETE

💧

Ready for the Fab Challenge?

You've seen why fluid in a chip flows in smooth layers and what that means for design.