Physics Foundations · Topic 1 of 6
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Laminar flow
Why microscale flow moves in smooth, parallel layers — and what that means for mixing.
What you’ll learn
- What laminar flow is and how it differs from turbulent flow
- Why it dominates at the microscale
- What it implies for mixing and channel design
The concept
In laminar flow, fluid moves in smooth layers (“laminae”) that slide past one another without cross-currents. Adjacent streams stay separate and follow predictable paths set entirely by the channel geometry.
Turbulence — the chaotic eddies that mix fluid rapidly — needs inertia to overcome viscosity. At the microscale viscosity wins overwhelmingly, so flow stays laminar across nearly all practical conditions.
Two streams meeting in a microchannel therefore flow side by side and blend only where molecules diffuse across the interface. This is exactly why designers reach for long serpentine channels or herringbone structures when they need fast mixing.
Why it matters
Deterministic flow lets you position fluids precisely — laminar co-flow and gradient generators depend on it.
It also means mixing must be engineered deliberately, never assumed.
The equation
Below Re ≈ 2000 (in a pipe), flow is laminar.
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
Water at 1 cm/s in a 100 µm channel:
Re = (1000)(0.01)(100×10⁻⁶) / (1×10⁻³) ≈ 1
Re ≈ 1 is far below the ~2000 threshold, so the flow is laminar.
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:
- Expecting turbulent mixing inside a microchannel — it almost never happens.
- Treating the pipe transition value (~2000) as a hard rule for every geometry; it is a guideline.
Keep going
Related concepts
Related tools
Further reading
- Theoretical Microfluidics — Henrik BruusVerify the current edition.
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Laminar flow
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Reynolds number
Physics Foundations
The single dimensionless number that predicts whether flow is laminar or turbulent.
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Reynolds number
Physics Foundations