What is surface tension cohesion and adhesion?

Cohesion is the property of like molecules (of the same substance) to stick to each other due to mutual attraction. Adhesion is the property of different molecules or surfaces to cling to each other. For example, solids have high cohesive properties so they do not stick to the surfaces they come in contact. On the other hand, gases have weak cohesion. Water has both cohesive and adhesive properties. Water molecules stick to each other to form a sphere. This is the result of cohesive forces. When contained in a tube, the water molecules touching the surface of the container are at a higher level (see Meniscus). This is due to the adhesive force between the water molecules and the molecules of the container.

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Capillary action
Applications

Adhesion versus Cohesion comparison chart

	Adhesion	Cohesion
Constituents	Dissimilar molecules	Similar molecules
Effect	Capillary action, meniscus	Surface tension, capillary action and meniscus

Surface tension is the result of cohesive forces between adjacent molecules. The molecules in the bulk of a liquid are pulled equally in all directions by the neighboring molecules. But the surface molecules do not have molecules on all sides. Hence they are pulled inwards causing the liquid to shrink to form a surface with minimum area, a sphere. Hence water droplets are spherical.

Water molecules bead together on a wax paper because surface tension is greater than the adhesive forces between the paper and water molecules.

The surface tension of water allows objects heavier than it to float across it. When water molecules do not stick to the object (non-wettable) and the weight of the object is less than the forces due to surface tension.

Meniscus

Concave and Convex Meniscus. The meniscus is concave when adhesive forces are stronger than cohesive forces. e.g. water. It is convex when cohesion is stronger. e.g. mercury

The curved surface of a liquid inside a container is the meniscus.

When the cohesive forces between the liquid molecules are greater than the adhesive forces between the liquid and the wall s of the container, the surface of the liquid is convex. For example, Mercury in a container.
When the cohesive forces between the liquid are less than the adhesive forces between the liquid and the container, the surface curves up. For example, water in a glass container.
When both adhesive and cohesive forces are equal, the surface is horizontal. For example, distilled water in a silver vessel.

Capillary action

Capillary action is the result of cohesive and adhesive forces. When a liquid flows through a narrow space, the cohesive and adhesive forces act together to lift it against the natural force of gravity. Wetting of a paper towel, water flowing up from the roots to the tip of a plant are a few examples of capillary action.

Mercury exhibits more cohesion than adhesion with glass.

Cohesion causes water to form drops, surface tension causes them to be nearly spherical, and adhesion keeps the drops in place.

Applications

Adhesion is used for the proper functioning of glue, paint, tar, cement, ink etc. Adhesive and cohesive forces together induce capillary action which is the principle used in lamp wicks. Synthetic fibers use wicking to remove sweat from the skin.

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Learning Objectives

Describe the cohesive and adhesive properties of water.

Have you ever filled a glass of water to the very top and then slowly added a few more drops? Before it overflows, the water forms a dome-like shape above the rim of the glass. This water can stay above the glass because of the property of cohesion. In cohesion, water molecules are attracted to each other (because of hydrogen bonding), keeping the molecules together at the liquid-gas (water-air) interface, although there is no more room in the glass.

Cohesion allows for the development of surface tension, the capacity of a substance to withstand being ruptured when placed under tension or stress. This is also why water forms droplets when placed on a dry surface rather than being flattened out by gravity. When a small scrap of paper is placed onto the droplet of water, the paper floats on top of the water droplet even though paper is denser (the mass per unit volume) than the water. Cohesion and surface tension keep the hydrogen bonds of water molecules intact and support the item floating on the top. It’s even possible to “float” a needle on top of a glass of water if it is placed gently without breaking the surface tension.

Figure \(\PageIndex{1}\): Surface Tension: The weight of the needle is pulling the surface downward; at the same time, the surface tension is pulling it up, suspending it on the surface of the water and keeping it from sinking. Notice the indentation in the water around the needle.

These cohesive forces are related to water’s property of adhesion, or the attraction between water molecules and other molecules. This attraction is sometimes stronger than water’s cohesive forces, especially when the water is exposed to charged surfaces such as those found on the inside of thin glass tubes known as capillary tubes. Adhesion is observed when water “climbs” up the tube placed in a glass of water: notice that the water appears to be higher on the sides of the tube than in the middle. This is because the water molecules are attracted to the charged glass walls of the capillary more than they are to each other and therefore adhere to it. This type of adhesion is called capillary action.

Figure \(\PageIndex{1}\): Adhesion: Capillary action in a glass tube is caused by the adhesive forces exerted by the internal surface of the glass exceeding the cohesive forces between the water molecules themselves.

Why are cohesive and adhesive forces important for life? Cohesive and adhesive forces are important for the transport of water from the roots to the leaves in plants. These forces create a “pull” on the water column. This pull results from the tendency of water molecules being evaporated on the surface of the plant to stay connected to water molecules below them, and so they are pulled along. Plants use this natural phenomenon to help transport water from their roots to their leaves. Without these properties of water, plants would be unable to receive the water and the dissolved minerals they require. In another example, insects such as the water strider use the surface tension of water to stay afloat on the surface layer of water and even mate there.

Figure \(\PageIndex{1}\): Cohesion & Adhesion: Water’s cohesive and adhesive properties allow this water strider (Gerris sp.) to stay afloat.

Cohesion holds hydrogen bonds together to create surface tension on water.
Since water is attracted to other molecules, adhesive forces pull the water toward other molecules.
Water is transported in plants through both cohesive and adhesive forces; these forces pull water and the dissolved minerals from the roots to the leaves and other parts of the plant.

adhesion: The ability of a substance to stick to an unlike substance; attraction between unlike molecules
cohesion: Various intermolecular forces that hold solids and liquids together; attraction between like molecules