= Capillary tube
A <capillary tube> is a tube with a small internal bore. Its large wall-area-to-fluid-volume ratio makes interfaces important: <surface tension> and the <contact angle> at the wall can produce <capillary action>, while <viscosity> strongly limits flow through a narrow passage.
For an ideal circular tube of radius $a$ in contact with a liquid reservoir, a static meniscus model gives $h=2\gamma\cos\theta/(\rho ga)$, where $\gamma$ is <surface tension>, $\theta$ the <contact angle>, $\rho$ the liquid <mass density> and $g$ <gravitational acceleration>. This follows by balancing the meniscus pressure jump with <hydrostatic pressure>. The sign permits a depression for a nonwetting liquid. Comparing tubes of different radii tests the predicted inverse-radius scaling within the model's assumptions.
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