Capillary action is fluid flow through a narrow tube or space from surface tension, cohesion, and adhesion. For example, if you place a thin tube into water, the water flows up the the tube. Other names for the phenomenon are capillarity, capillary motion, and wicking. Capillary action does not require the force of gravity. In fact, liquids often rise in narrow tube in opposition to gravity.
Forces in Capillary Action – How It Works
The three forces largely responsible for capillary action are surface tension, cohesion, and adhesion.
- Cohesion occurs when liquid molecules stick to each other. In the case of water molecules, cohesion is high because of hydrogen bonding between the molecules.
- Adhesion describes how well liquid molecules stick to surfaces. Water molecules stick to glass and plastic surfaces. In contrast, mercury atoms stick well to each other, but are not as attracted to a container surface.
- Surface tension is the tension on a liquid at its interface with air that minimizes the liquid surface area. Water and mercury both have high surface tension. In a narrow tube, the meniscus formed by these two liquids is curved. However, water curves toward the container wall, while mercury forms a rounded shape in the center of a capillary.
- Gravity affects how far a liquid rises within a capillary, as it exerts a downward pull on the liquid in a vertical tube.
Many liquids act like water and rise in a capillary tube. However, liquids like mercury rise to a level that is lower than that of the liquid surrounding the tube.
A convex meniscus forms when molecules are more attracted to each other than they are to the container. For example, mercury forms a convex meniscus in glass. A concave meniscus forms when molecules are more attracted to the container than they are to each other. For example, water forms a concave meniscus in glass. The meniscus shape depends on both the composition of the liquid and that of the container. For example, the meniscus of water in plastic is nearly flat.
Capillary Action Examples
There are many familiar examples of capillary action in everyday life:
- When you place a straw in a glass of water, the liquid level within the straw is higher than the height of the water in the glass.
- Capillary action causes the rise of damp in concrete and drywall.
- The lacrimal ducts (tear ducts) of the eyes continuously drain tears from the eye surface.
- A candle wick absorbs liquid wax that constantly supplies a candle flame. Paint brushes and lamp wicks pick up liquid the same way.
- Paper towels wick up water using capillary action.
- Fabrics that wick away perspiration also use capillary action.
The movement of water up the stems and trunks of plants (transpiration) involves capillary action, but it also relies on evaporation from leaves and osmotic pressure from roots.
Capillary Action Uses
Capillary action has several uses. For example:
- Fountain pens draw up ink using capillary action
- Thin layer and paper chromatography apply capillary action.
- Capillary tubes are thin tubes in science and medicine that draw small samples, such as blood.
Formula for the Height of the Meniscus
Be sure you measure the liquid level in a capillary tube, test tube, or buret at the meniscus line. There is a formula for calculating the height of the meniscus in a liquid column that accounts for capillary action:
h = 2γcosθ / ρgr
Here:
- h is the height of the meniscus in the column of liquid
- γ is the surface tension in the liquid-air environment
- θ is the angle of contact between the liquid and the column wall
- ρ is the liquid density
- g is the acceleration due to gravity
- r is the tube internal radius
Using this formula, note the effect of the radius of the tube. The thinner the tube, the smaller the radius, and the further the liquid travels from capillary action.
Formula for Volume of Liquid Transport
A dry porous medium, such as a paper towel, absorbs liquid at a rate that slows over time. There is a formula for calculating the volume absorbed over time:
V = SA√t
Here:
- V is the volume of liquid
- S is the sorptivity or the medium’s capacity for absorption via capillary action
- A is the cross-section of the wet area
The rate of liquid absorption depends on several factors, including temperature, permeability, and humidity.
References
- Batchelor, G.K. (1967). An Introduction to Fluid Dynamics. Cambridge University Press. ISBN 0-521-66396-2.
- de Gennes, Pierre-Gilles; Brochard-Wyart, Françoise; Quéré, David (2004). Capillarity and Wetting Phenomena. Springer New York. ISBN 978-1-4419-1833-8. doi:10.1007/978-0-387-21656-0
- Freeman, Scott (2014). Biological Sciences. United States of America: Pearson. ISBN 978-0-321-74367-1.
- Liu, M.; et al. (2016). “Evaporation limited radial capillary penetration in porous media”. Langmuir. 32 (38): 9899–9904. doi:10.1021/acs.langmuir.6b02404