Orientation · Topic 2 of 3
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Why the microscale behaves differently
As channels shrink, viscosity and surface tension take over while inertia and gravity fade.
What you’ll learn
- How the balance of forces shifts as size shrinks
- Why microscale flow is smooth (laminar) rather than turbulent
- Why surface tension and diffusion come to dominate
The concept
When a channel shrinks, not all forces shrink at the same rate. Inertial and gravitational effects scale with volume (∝ L³), while viscous and surface-tension effects scale with area or length (∝ L² or L). Make L small and the volume-dependent effects — inertia, gravity, buoyancy — fade relative to viscosity and surface tension.
The practical consequence is that microscale flow is dominated by viscosity. Flow is laminar (smooth and layered) rather than turbulent, so streams travel side by side and blend only by diffusion. Surface tension becomes strong enough to hold droplets together and to pull liquid into channels by capillary action.
These are not obstacles to fight but tools to design with: predictable laminar streams, capillary-driven filling, and stable droplet compartments are all direct consequences of small size.
Why it matters
Laminar flow is deterministic, so you can design exactly where each fluid goes.
It also explains why mixing is hard (there is no turbulence to help) and why droplets and capillary filling work so reliably.
The equation
The Reynolds number captures the balance — inertial forces over viscous forces.
Variables
| Symbol | Variable | Unit |
|---|---|---|
| Re | Reynolds number | — |
| ρ | Fluid density | kg/m³ |
| v | Mean velocity | m/s |
| Dₕ | Hydraulic diameter | m |
| μ | Dynamic viscosity | Pa·s |
Worked example
Compare water flowing at 1 cm/s through a 100 µm channel with the same water in a river:
Re_chip = (1000)(0.01)(100×10⁻⁶) / (1×10⁻³) ≈ 1
The microchannel sits at Re ≈ 1 (firmly laminar), while a river can exceed Re ≈ 10⁶ (fully turbulent) — the same fluid, utterly different behaviour.
Try it yourself
Put these numbers into the reynolds number calculator and see the result for your own channel.
Open the Reynolds number calculator →Common mistakes
Watch out for:
- Trying to mix two microscale streams by “stirring” — turbulence is not available; mixing is diffusion-limited.
- Ignoring surface tension and capillary effects that are negligible at large scale but dominant here.
Keep going
Related concepts
Related tools
Further reading
- Theoretical Microfluidics — Henrik BruusCovers the scaling of forces in detail. Verify the current edition.
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Why the microscale behaves differently
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Major applications
Orientation
Where microfluidics is used — diagnostics, single-cell biology, organ-on-chip, flow chemistry, and cooling.
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