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In addition to the revision notes for Adhesive and Cohesive Forces. Surface Tension and Capillarity on this page, you can also access the following Density and Pressure learning resources for Adhesive and Cohesive Forces. Surface Tension and Capillarity
| Tutorial ID | Title | Tutorial | Video Tutorial | Revision Notes | Revision Questions | |
|---|---|---|---|---|---|---|
| 9.2 | Adhesive and Cohesive Forces. Surface Tension and Capillarity |
In these revision notes for Adhesive and Cohesive Forces. Surface Tension and Capillarity, we cover the following key points:
Cohesion is the property of like particles to stick to each other. These like particles belong to the same substance. To make possible such a mutual attraction, particles interact with each other through certain attraction forces known as cohesive forces. For example, water molecules are pulled with each other through cohesive forces making water droplets take a spherical shape (occupying thus the smallest possible volume).
On the other hand, when substances are different, they stick to each other because of adhesive forces. Thus, we can define adhesion as the ability of different substances or surfaces to cling to each other. For example, water wets the inner part of a metal container because water molecules cling to the molecules of the metal container by means of adhesive forces.
In solids (which usually have high densities), cohesive forces prevail over adhesive ones as molecules of solids are closer (and as a result, stronger bonded) to each other than to the molecules of the other substance they are in contact. On the other hand, gases show more adhesive than cohesive properties. This is why gases stick easily to other surfaces.v
Liquids, as an intermediate category, show a dualistic behaviour depending on the density of the substance they are in contact with. They manifest cohesive behaviour when it is in contact with less dense materials and adhesive behaviour when they are in contact with less dense materials.
Molecules of the upper layer of a liquid are pulled by other molecules below, and as a result, a non-zero resultant force in the inward direction is produced. This force is known as the force of surface tension, F, and it causes liquids to shrink and take a spherical shape.
Do not confuse surface tension and surface tension force. Surface tension, γ is an intrinsic property of the liquid itself (like density, freezing temperature, colour, texture, etc.) It depends neither on environmental conditions nor on the material structure but only on the surface tension force F and the length L of the part of liquid surface in contact with the surrounding substance. The equation of surface tension is:
Obviously, the unit of surface tension γ is [N/m]. From the above formula, we obtain for the surface tension force,
The force of surface tension is very small. However, when the surface increases this force helps heavy objects such as ships to float more easily on water due to the compactness that water molecules produce during their shrinkage.
Capillarity is the phenomenon of a liquid's raise through a narrow tube due to the change in air pressure.
Examples of capillarity include: the raise of mercury in barometer columns, the raise of water from the roots to the highest parts of a tree, the circulation of blood throughout the human body even when we are standing up, the use of straws to drink juices by removing the air in the upper part of the straw, etc.
When liquids are in narrow tubes, their level increases until the weight of the raised liquid balances the force of surface tension. This helps us calculate the height of a liquid inside a narrow tube.
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