FUNDAMENTALS
Capillary Action in Microfluidics
Capillary action — the same force that pulls water up a thin straw — can drive fluid through a microfluidic chip with no pump at all. It powers lateral-flow tests and many point-of-care devices. Here is the physics and how to design with it.
What is capillary action?
Capillary action is the movement of liquid through a narrow space driven by surface tension and the balance between adhesive forces (liquid to the wall) and cohesive forces (liquid to itself). In a wettable (hydrophilic) channel, the liquid is pulled forward and fills the channel on its own.
Capillary pressure
The driving capillary pressure is set by the surface tension, the contact angle (wettability) and the channel size, as described by the Young–Laplace relation. Smaller channels and more wettable walls give a stronger pull. You can estimate it with our capillary pressure calculator.
Why it is useful
Capillary filling needs no syringe pumps, tubing or power — which is exactly what makes cheap, portable point-of-care devices and paper microfluidics possible.
Designing capillary-driven chips
- Control wettability through surface treatment.
- Use capillary stops and valves (geometry or hydrophobic patches) to time and sequence flow.
- Account for evaporation and for surfaces ageing over time.
Limits
Capillary flow is less precise and controllable than pressure-driven flow (see flow control), and it is sensitive to surface contamination and ageing.
Frequently asked questions
What is capillary action in microfluidics?
The pump-free movement of liquid through a narrow channel driven by surface tension and wettability, pulling fluid into hydrophilic channels.
How does capillary action move fluid without a pump?
Surface tension acting at the wetting liquid front generates a capillary pressure that draws the liquid through the channel, so no external pump is required.
What affects capillary pressure?
Surface tension, the contact angle (how wettable the walls are) and the channel dimensions — smaller, more wettable channels pull harder.
What are capillary action's limitations?
Less precise control than pressure-driven flow, and sensitivity to evaporation and to surface contamination or ageing.
Pump-free by design
Designing a capillary-driven chip?
Surface and geometry are everything. Upload your design and we will advise on wettability and manufacture, or book a call.
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