Capillary Action

Definition

How water climbs up narrow tubes against gravity — the physics behind paper towels soaking spills and plants drinking.

$h = \frac{2\gamma\cos\theta}{\rho g r}$

Unit: m

Technical definition

Liquid rise in narrow tubes from the balance of adhesive (liquid-wall) versus cohesive (liquid-liquid) forces plus surface tension. Jurin's law: h = 2γcosθ/(ρgr) — rise height is inversely proportional to tube radius.

Example

Water rises ~3 cm in a 1 mm glass tube. In plant xylem (microscopic), capillary action plus transpiration pull lifts water to the tallest treetops.

Common confusion

Capillary action alone cannot explain tall trees — it only lifts water centimeters in realistic tubes. Trees use transpiration pull (evaporation-driven tension) as the main engine, with capillarity assisting.