Cohesion and Adhesion in Liquids: Surface Tension and Capillary Action
Learning Objectives
By the end of this section, you will be able to:- Understand cohesive and adhesive forces.
- Define surface tension.
- Understand capillary action.
Cohesion and Adhesion in Liquids

Children blow soap bubbles and play in the spray of a sprinkler on a hot summer day. (See Figure 1.) An underwater spider keeps his air supply in a shiny bubble he carries wrapped around him. A technician draws blood into a small-diameter tube just by touching it to a drop on a pricked finger. A premature infant struggles to inflate her lungs. What is the common thread? All these activities are dominated by the attractive forces between atoms and molecules in liquids—both within a liquid and between the liquid and its surroundings.
Attractive forces between molecules of the same type are called cohesive forces. Liquids can, for example, be held in open containers because cohesive forces hold the molecules together. Attractive forces between molecules of different types are called adhesive forces. Such forces cause liquid drops to cling to window panes, for example. In this section we examine effects directly attributable to cohesive and adhesive forces in liquids.
Cohesive Forces
Adhesive Forces
Attractive forces between molecules of different types are called adhesive forces.
Surface Tension
Surface Tension
Cohesive forces between molecules cause the surface of a liquid to contract to the smallest possible surface area. This general effect is called surface tension.
Making Connections: Surface Tension
Forces between atoms and molecules underlie the macroscopic effect called surface tension. These attractive forces pull the molecules closer together and tend to minimize the surface area. This is another example of a submicroscopic explanation for a macroscopic phenomenon.

.
Liquid | Surface tension γ(N/m) |
---|---|
Water at 0ºC | 0.0756 |
Water at 20ºC | 0.0728 |
Water at 100ºC | 0.0589 |
Soapy water (typical) | 0.0370 |
Ethyl alcohol | 0.0223 |
Glycerin | 0.0631 |
Mercury | 0.465 |
Olive oil | 0.032 |
Tissue fluids (typical) | 0.050 |
Blood, whole at 37ºC | 0.058 |
Blood plasma at 37ºC | 0.073 |
Gold at 1070ºC | 1.000 |
Oxygen at −193ºC | 0.0157 |
Helium at −269ºC | 0.00012 |
,
where r is the radius of the bubble.

Thus the pressure inside a bubble is greatest when the bubble is the smallest. Another bit of evidence for this is illustrated in Figure 4. When air is allowed to flow between two balloons of unequal size, the smaller balloon tends to collapse, filling the larger balloon.

Example 1. Surface Tension: Pressure Inside a Bubble
Calculate the gauge pressure inside a soap bubble 2.00 × 10-4 m in radius using the surface tension for soapy water in Table 1. Convert this pressure to mm Hg.
We use a conversion factor to get this into units of mm Hg:
Strategy
The radius is given and the surface tension can be found in Table 1, and so P can be found directly from the equation.
Solution
Substituting r and γ into the equation, we obtain
.
We use a conversion factor to get this into units of mm Hg:
.
Discussion
Note that if a hole were to be made in the bubble, the air would be forced out, the bubble would decrease in radius, and the pressure inside would increase to atmospheric pressure (760 mm Hg).

Making Connections: Take-Home Investigation
(1) Try floating a sewing needle on water. In order for this activity to work, the needle needs to be very clean as even the oil from your fingers can be sufficient to affect the surface properties of the needle. (2) Place the bristles of a paint brush into water. Pull the brush out and notice that for a short while, the bristles will stick together. The surface tension of the water surrounding the bristles is sufficient to hold the bristles together. As the bristles dry out, the surface tension effect dissipates. (3) Place a loop of thread on the surface of still water in such a way that all of the thread is in contact with the water. Note the shape of the loop. Now place a drop of detergent into the middle of the loop. What happens to the shape of the loop? Why? (4) Sprinkle pepper onto the surface of water. Add a drop of detergent. What happens? Why? (5) Float two matches parallel to each other and add a drop of detergent between them. What happens? Note: For each new experiment, the water needs to be replaced and the bowl washed to free it of any residual detergent.
Adhesion and Capillary Action
Contact Angle
The angle θ between the tangent to the liquid surface and the surface is called the contact angle.

Capillary Action
The tendency of a fluid to be raised or suppressed in a narrow tube, or capillary tube, is called capillary action.

Interface | Contact angle Θ |
---|---|
Mercury–glass | 140º |
Water–glass | 0º |
Water–paraffin | 107º |
Water–silver | 90º |
Organic liquids (most)–glass | 0º |
Ethyl alcohol–glass | 0º |
Kerosene–glass | 26º |
.
If we look at the different factors in this expression, we might see how it makes good sense. The height is directly proportional to the surface tension γ, which is its direct cause. Furthermore, the height is inversely proportional to tube radius—the smaller the radius r, the higher the fluid can be raised, since a smaller tube holds less mass. The height is also inversely proportional to fluid density ρ, since a larger density means a greater mass in the same volume. (See Figure 9.)

Example 2. Calculating Radius of a Capillary Tube: Capillary Action: Tree Sap
Can capillary action be solely responsible for sap rising in trees? To answer this question, calculate the radius of a capillary tube that would raise sap 100 m to the top of a giant redwood, assuming that sap’s density is 1050 kg/m3, its contact angle is zero, and its surface tension is the same as that of water at 20.0º C.
Strategy
The height to which a liquid will rise as a result of capillary action is given by, and every quantity is known except for r.
Solution
Solving for r and substituting known values producesDiscussion
This result is unreasonable. Sap in trees moves through the xylem, which forms tubes with radii as small as 2.5 × 10-5 m. This value is about 180 times as large as the radius found necessary here to raise sap 100 m. This means that capillary action alone cannot be solely responsible for sap getting to the tops of trees.
Section Summary
- Attractive forces between molecules of the same type are called cohesive forces.
- Attractive forces between molecules of different types are called adhesive forces.
- Cohesive forces between molecules cause the surface of a liquid to contract to the smallest possible surface area. This general effect is called surface tension.
- Capillary action is the tendency of a fluid to be raised or suppressed in a narrow tube, or capillary tube which is due to the relative strength of cohesive and adhesive forces.
Conceptual Questions
1. The density of oil is less than that of water, yet a loaded oil tanker sits lower in the water than an empty one. Why?
2. Is surface tension due to cohesive or adhesive forces, or both?
3. Is capillary action due to cohesive or adhesive forces, or both?
4. Birds such as ducks, geese, and swans have greater densities than water, yet they are able to sit on its surface. Explain this ability, noting that water does not wet their feathers and that they cannot sit on soapy water.
5. Water beads up on an oily sunbather, but not on her neighbor, whose skin is not oiled. Explain in terms of cohesive and adhesive forces.
6. Could capillary action be used to move fluids in a "weightless" environment, such as in an orbiting space probe?
7. What effect does capillary action have on the reading of a manometer with uniform diameter? Explain your answer.
8. Pressure between the inside chest wall and the outside of the lungs normally remains negative. Explain how pressure inside the lungs can become positive (to cause exhalation) without muscle action.
Problems & Exercises
1. What is the pressure inside an alveolus having a radius of 2.50 × 10-4 if the surface tension of the fluid-lined wall is the same as for soapy water? You may assume the pressure is the same as that created by a spherical bubble.
2. (a) The pressure inside an alveolus with a 2.00 × 10-4 -m radius is 1.40 × 103, due to its fluid-lined walls. Assuming the alveolus acts like a spherical bubble, what is the surface tension of the fluid? (b) Identify the likely fluid. (You may need to extrapolate between values in Table 1.)
3. What is the gauge pressure in millimeters of mercury inside a soap bubble 0.100 m in diameter?
4. Calculate the force on the slide wire in Figure 3 (shown again below) if it is 3.50 cm long and the fluid is ethyl alcohol.
Figure 3. Sliding wire device used for measuring surface tension; the device exerts a force to reduce the film’s surface area. The force needed to hold the wire in place is F = γL = γ(2l), since there are two liquid surfaces attached to the wire. This force remains nearly constant as the film is stretched, until the film approaches its breaking point.

5. Figure 9(a) (shown again below) shows the effect of tube radius on the height to which capillary action can raise a fluid. (a) Calculate the height h for water in a glass tube with a radius of 0.900 cm—a rather large tube like the one on the left. (b) What is the radius of the glass tube on the right if it raises water to 4.00 cm?
Figure 9. (a) Capillary action depends on the radius of a tube. The smaller the tube, the greater the height reached. The height is negligible for large-radius tubes. (b) A denser fluid in the same tube rises to a smaller height, all other factors being the same.

6. We stated in Example 2 above that a xylem tube is of radius 2.50 × 10-5 m. Verify that such a tube raises sap less than a meter by finding h for it, making the same assumptions that sap’s density is 1050 kg/m3, its contact angle is zero, and its surface tension is the same as that of water at 20.0º C.
7. What fluid is in the device shown in Figure 3 (shown again below) if the force is 3.16 × 10-3 and the length of the wire is 2.50 cm? Calculate the surface tension γ and find a likely match from Table 1 (above).
Figure 3. Sliding wire device used for measuring surface tension; the device exerts a force to reduce the film’s surface area. The force needed to hold the wire in place is F = γL = γ(2l), since there are two liquid surfaces attached to the wire. This force remains nearly constant as the film is stretched, until the film approaches its breaking point.

8. If the gauge pressure inside a rubber balloon with a 10.0-cm radius is 1.50 cm of water, what is the effective surface tension of the balloon?
9. Calculate the gauge pressures inside 2.00-cm-radius bubbles of water, alcohol, and soapy water. Which liquid forms the most stable bubbles, neglecting any effects of evaporation?
10. Suppose water is raised by capillary action to a height of 5.00 cm in a glass tube. (a) To what height will it be raised in a paraffin tube of the same radius? (b) In a silver tube of the same radius?
11. Calculate the contact angle θ for olive oil if capillary action raises it to a height of 7.07 cm in a glass tube with a radius of 0.100 mm. Is this value consistent with that for most organic liquids?
12. When two soap bubbles touch, the larger is inflated by the smaller until they form a single bubble. (a) What is the gauge pressure inside a soap bubble with a 1.50-cm radius? (b) Inside a 4.00-cm-radius soap bubble? (c) Inside the single bubble they form if no air is lost when they touch?
13. Calculate the ratio of the heights to which water and mercury are raised by capillary action in the same glass tube.
14. What is the ratio of heights to which ethyl alcohol and water are raised by capillary action in the same glass tube?
Glossary
- adhesive forces:
- the attractive forces between molecules of different types
- capillary action:
- the tendency of a fluid to be raised or lowered in a narrow tube
- cohesive forces:
- the attractive forces between molecules of the same type
- contact angle:
- the angle θ between the tangent to the liquid surface and the surface
- surface tension:
- the cohesive forces between molecules which cause the surface of a liquid to contract to the smallest possible surface area
Selected Solutions to Problems & Exercises
1. 592 N/m23. 2.23 × 10-2 mm Hg
5. (a) 1.65 × 10-3 m (b) 3.71 × 10-4 m
7. 6.32 × 10-2 N/m. Based on the values in table, the fluid is probably glycerin.
9.
Alcohol forms the most stable bubble, since the absolute pressure inside is closest to atmospheric pressure.
11. 5.1º. This is near the value of θ=0º for most organic liquids.
13. -2.78. The ratio is negative because water is raised whereas mercury is lowered.
11. 5.1º. This is near the value of θ=0º for most organic liquids.
13. -2.78. The ratio is negative because water is raised whereas mercury is lowered.
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