FUNDAMENTALS
Key Dimensionless Numbers in Microfluidics
Microfluidics is governed by a handful of dimensionless numbers that tell you which physical effects dominate. Understanding them is the fastest route to good chip design. Here are the key ones.
Why dimensionless numbers?
Each dimensionless number is a ratio of two competing forces or effects. Its value tells you which one wins — without needing the full equations — so you can predict how a device will behave before building it.
Reynolds number (Re)
Inertial versus viscous forces. In microfluidics Re is very low, so flow is laminar, not turbulent. Estimate it with our Reynolds number calculator.
Péclet number (Pe)
Advection (being carried by flow) versus diffusion (spreading by molecular motion). A high Pe means a stream travels far before it mixes — which is why micromixers are needed. It sets mixing lengths and gradient stability.
Capillary and Weber numbers
- Capillary number (Ca) — viscous forces versus surface tension; governs droplet formation and break-up.
- Weber number (We) — inertia versus surface tension; relevant to droplet stability at higher speeds.
- Knudsen, Dean and others appear in gas, curved-channel (inertial) and specialised flows.
Using them in design
Together these numbers predict whether flow is laminar, how far mixing takes, and how droplets form — so you can size channels and set flow rates up front. Our free lab tools compute several of them.
Frequently asked questions
What are the most important dimensionless numbers in microfluidics?
The Reynolds number (inertia vs viscosity), the Péclet number (advection vs diffusion), and the capillary and Weber numbers (viscous and inertial forces vs surface tension).
What is the Péclet number?
The ratio of advection to diffusion. A high Péclet number means fluid is carried far before it mixes, which is why microfluidic devices need dedicated micromixers.
What is the capillary number used for?
It compares viscous forces to surface tension and governs how droplets form and break up in droplet microfluidics.
Design by the numbers
Want the physics on your side?
Use our free calculators to size your device, then upload your design for a manufacturing quote.
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