Since the former is proportional to the drop radius while the latter is proportional to the cube of the drop radius, a sufficiently small drop or bubble can defy gravity and remain attached to a solid surface. This angle reflects the balance of interfacial tensions between the solid and the fluids at the point where the fluid interface meets the solid surface. On the other hand, when the contact angle exceeds , the liquid moves away from the gas and is said to be nonwetting. Liquid rises in a capillary to an equilibrium position determined by the balance of the surface tension force and the gravity force acting on the column of elevated fluid. Another example of the effect of surface tension acting in a static fluid is the rise of a liquid in a small diameter tube, or capillary tube, extending vertically above the surface of a liquid exposed to air, as shown in figure 2.12 .
Capillary action can be important when liquids infiltrate into porous materials.
For example, the upward flow of liquid by capillary action is essential to the maintenance of a candle flame. Radiant heat from the candle flame melts the wax at the base of the candle wick. The melted wax is drawn upward in the wick by capillary action, evaporating as it rises because of heat transfer from the flame surrounding the wick. The vapor then diffuses into the flame, burning by mixing with the surrounding air and feeding back some heat to maintain the process of melting and evaporation.