 | Capillary action: Encyclopedia - Capillary action
Capillary action
Capillary action or capillarity (also known as capillary motion) is the ability of a narrow tube to draw a liquid upwards against the force of gravity. It occurs when the adhesive intermolecular forces between the liquid and a solid are stronger than the cohesive intermolecular forces within the liquid. The effect causes a concave meniscus to form where the liquid is in contact with a vertical surface. The same effect is what causes porous materials to soak up liquids.
A common apparatus used to demonstrate capillary action is the capillary tube. When the lower end of a vertical glass tube is placed in a liquid such as water, a concave meniscus forms. Surface tension pulls the liquid column up until there is a sufficient weight of liquid for gravitational forces to overcome the intermolecular forces. The weight of the liquid column is proportional to the square of the tube's diameter, but the contact area between the liquid and the tube is proportional only to the diameter of the tube, so a narrow tube will draw a liquid column higher than a wide tube. For example, a glass capillary tube 0.5 mm in diameter will lift a theoretical 2.8 cm column of water. Actual observations show shorter total distances.
With some pairs of materials, such as mercury and glass, the interatomic forces within the liquid exceed those between the solid and the liquid, so a convex meniscus forms and capillary action works in reverse.
A plant makes use of capillary action to draw liquid water into its system, although larger plants require transpiration to move a sufficient quantity of water to where it is required.
Capillary action - Formula
With notes on the dimension in SI units, the height h of a liquid column (m) is given by:
...where:
T = surface tension (J/m²)
θ = contact angle
ρ = density of liquid (kg/m3)
g = acceleration due to gravity (m/s²)
r = radius of tube (m)
For a water-filled glass tube in air at sea level,
T = 0.0728 J/m² at 20 °C
θ = 20° (0.35 rad)
ρ = 1000 kg/m3
g = 9.8 m/s²
...and so the height of the water column is given by:
.
Dunk (biscuit), Washburn's equation
Capillary action - Trivia
Albert Einstein's first paper submitted to Annalen der Physik was on capillarity. It was titled Folgerungen aus den Capillaritätserscheinungen, which translated is Consequences of the observations of capillarity phenomena, found in volume 4, page 513. Submitted in late 1900, it was published in 1901. In 1905 Einstein published five seminal papers in the same journal.
See also
- Dunk (biscuit)
- Washburn's equation
Category: Fluid dynamics
Other related archivesAlbert Einstein, Annalen der Physik, C, Dunk (biscuit), Fluid dynamics, Surface tension, Washburn's equation, acceleration, adhesive, air, apparatus, area, cohesive, concave, contact, contact angle, convex, density, diameter, force, glass, gravitational forces, gravity, intermolecular forces, liquid, m, materials, meniscus, mercury, mm, move, plant, porous, proportional, quantity, rad, radius, sea level, solid, square, sufficient, surface tension, transpiration, tube, vertical, water, weight
 Adapted from the Wikipedia article "Capillary action", under the G.N U Free Docmentation License. Please also see http://en.wikipedia.org/wiki |
