Surface Tension

Surface tension is the energy, or work, required to increase the surface area of a liquid due to inter-molecular forces. Since these intermolecular forces vary depending on the nature of the liquid (e.g. water vs. gasoline) or solutes in the liquid (e.g. surfactants like detergent), each solution exhibits differing surface tension properties. Whether you know it or not, you already have seen surface tension at work. Whenever you fill a glass of water too far, you may notice afterward that the level of the water in the glass is actually higher than the height of the glass. You may have also noticed that the water that you spilled has formed into pools that rise up off the counter. Both of these phenomena are due to surface tension.

What are the factors affecting the surface tension?

  • Temperature
  • Oxidation
  • Surfactant

Effects of surface tension

Due to the surface tension, small objects will “float” on the surface of a fluid, as long as the object cannot break through and separate the top layer of water molecules. When an object is on the surface of the fluid, the surface under tension will behave like an elastic membrane.

Molecular Perspective

In a sample of water, there are two types of molecules. Those that are on the outside, exterior, and those that are on the inside, interior. The interior molecules are attracted to all the molecules around them, while the exterior molecules are attracted to only the other surface molecules and to those below the surface. This makes it so that the energy state of the molecules on the interior is much lower than that of the molecules on the exterior. Because of this, the molecules try to maintain a minimum surface area, thus allowing more molecules to have a lower energy state. This is what creates what is referred to as surface tension.

The water molecules attract one another due to the water’s polar property. The hydrogen ends, which are positive in comparison to the negative ends of the oxygen cause water to “stick” together. This is why there is surface tension and takes a certain amount of energy to break these intermolecular bonds. Same goes for other liquids, even hydrophobic liquids such as oil. There are forces between the liquid such as Van der Waals forces that are responsible for the intermolecular forces found within the liquid. It will then take a certain amount of energy to break these forces, and the surface tension. Water is one liquid known to have a very high surface tension value and is difficult to overcome.

Surface tension of water can cause things to float which are denser than water, allowing organisms to literally walk on water. An example of such an organism is the water strider, which can run across the surface of water, due to the intermolecular forces of the molecules, and the force of the strider which is distributed to its legs. Surface tension also allows for the formation of droplets that we see in nature.

Cohesive and Adhesive Forces

There are several other important concepts that are related to surface tension. The first of these is the idea of cohesive & adhesive force. Cohesive forces are those that hold the body of a liquid together with minimum surface area and adhesive forces are those that try to make a body of a liquid spread out. So if the cohesive forces are stronger than the adhesive forces, the body of water will maintain its shape, but if the opposite is true than the liquid will be spread out, maximizing its surface area. Any substance that you can add to a liquid that allows a liquid to increase its surface area is called a wetting agent.

In the lab there are also several important points to remember about surface tension. The first you’ve probably noticed before. This is the idea of a meniscus (Figure a​). This is the concave (curved in) or convex (curved out) look that water or other liquids have when they are in test tubes. This is caused by the attraction between the glass and the liquid. With water, this causes it to climb up the sides of a test tube. This attraction is amplified as the diameter of the tubes increases; this is called capillary action. This can be seen if you take a tube with a very small diameter (a capillary tube) and lower it into a body of water. The liquid will climb up into the tube, even though there is no outside force. You may have also seen this when you put a straw into a drink and notice that the liquid level inside the straw is higher than it is in your drink. All of this however, requires that the adhesive forces (between the liquid and the capillary surface) be higher than the cohesive forces (between the liquid and itself), otherwise there will be no capillary action or the opposite can even happen. Mercury has higher cohesive forces than adhesive forces, so the level of the liquid will actually be lower in the capillary tubes than compared to the rest of the mercury (Figure b).

Examples of surface tension :-

Beading of water on a waxy surface: Since they are attracted in a more uniform or consistent direction, they form a more stable arrangement at the surface called surface tension. Water’s attraction to itself and its strong surface tension causes water to bead up instead of spreading out on the wax paper.

Why bubbles are round: The surface tension of water provides the necessary wall tension for the formation of bubbles with water. The tendency to minimize that wall tension pulls the bubbles into spherical shapes. The secret to making bubbles is surfacetension. … This is because the surfacetension—the forces holding the molecules of a liquid together—of water is too high. When detergent is added to water, it lowers the surfacetension so that bubbles can form.

Antiseptics : Antiseptics like Dettol have low surfacetension, so that they spread faster.

Umbrella : Surfacetension prevents water from passing through the pores of an umbrella.

Walking on water: Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface.

Floating a needle: A carefully placed small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink.

Float on water :A duck is able to float on water as its feathers secrete oil that lowers the surfacetension of water.

Don’t touch the tent: Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension, and the rain will drip through.

Clinical test for jaundice: Normal urine has a surface tension of about 66 dynes/centimetre but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulphur is sprinkled on the urine surface. It will float on normal urine but will sink if the surface tension is lowered by the bile.

Surface tension disinfectants: Disinfectants are usually solutions of low surface tension. This allows them to spread out on the cell walls of bacteria and disrupt them.

Soaps and detergents: These help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas.

Detergent and Soap Break Surface Tension: The end of the detergent molecule which attaches to fat (grease) repels water molecules. This weakens the hydrogen bonds holding the water molecules together at the surface. The result is a break in the surfacetension of the water.

Surface tension and droplets: Surface tension is responsible for the shape of liquid droplets. Although easily deformed, droplets of water tend to be pulled into a spherical shape by the cohesive forces of the surface layer.

Surface Tension related experiments:-

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