# Surface tension

Have you ever played soap bubbles? Soap bubble round. Funny. Can be blown. After flying, soap bubbles burst. Wow, it’s fun, a childhood game. Why is soap bubble round?

Please wake up in the morning, then notice the leaves around the house. Observe the droplets of dew attached to the leaves. It’s strange; dew drops are round. How can it be like that?

Surface tension occurs because the surface of the liquid tends to tighten so that the surface looks like a thin membrane. This is influenced by the cohesion force between water molecules. To better understand this explanation, consider the following illustration. We review the liquid in a container. Inside the liquid, each liquid molecule is surrounded by other molecules on each side; but on the surface of the liquid, there are only liquid molecules below. At the top, there are no other liquid molecules. Because the liquid molecule attracts one another, there is a total force of zero in the molecule that is in the inside of the liquid. In contrast, the liquid molecules located on the surface are pulled by the particles of fluid which are on the bottom. As a result, on the surface of the liquid, there is a total force that is directed downwards. Because of the net force downward, the liquid located on the surface tends to reduce its surface area, by shrinking as hard as possible.

This causes the liquid layer on the surface to appear as if it is covered by a thin elastic membrane. This phenomenon is known as Surface Tension.

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When the clip is carefully placed on the surface of the water, the water molecules located on the surface are slightly pressed by the gravity of the clip, so that the water molecules located below give the restoration force up to support the clip.

In reality, it’s not just clips or paper clips, but also other things like needles. If we carefully place the needle above the surface of the water, the needle will float. The surface tension of the liquid is also the reason why insects can float on water.

The Equation of Surface Tension

In the previous discussion, we have studied the concept of surface tension qualitatively (there is no mathematical equation). Now we review the surface tension quantitatively. To help us derive the surface tension equation, we look at a wire that is bent to form the letter U. Another straight wire is attached to the two legs of the U wire, where the straight wire can be moved. If this wire is inserted into the soap, then after removing it will form a layer of soapy water on the surface of the wire. It’s like when you play soap bubbles. Because the straight wire can be moved and the mass is not too large, the layer of soapy water will give the surface tension force on the wire straight so that the wire goes straight up. To keep the straight wire from moving (the wire is in equilibrium), a total force downward is needed, where the amount of the total force is F = w + T. In balance, F = the force of the surface tension carried by the layer of soap on the wire straight.

Suppose the length of a straight wire is l. Because the layer of soapy water that touches a straight wire has two surfaces, the surface tension force generated by the layer of soapy water works along 2l. Surface tension in the soap layer is a comparison between the Surface Tension Force (F) and the surface length at which the force works (d). In this case, the surface length is 2l. Mathematically, written: Because surface tension is the ratio between the surface tension force and length, the surface tension unit is Newton per meter (N / m) or dyne per centimeter (dynes/cm).

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Following are some of the Surface Tension values obtained based on experiments.  Based on the Surface Tension data, the temperature affects the value of the fluid surface tension. Generally when there is an increase in temperature, the value of surface tension decreases (Compare the surface tension of water at each temperature. See table). This is because when the temperature increases, the liquid molecules move faster so that the effect of interaction between the fluid molecules decreases. As a result of the surface tension value also decreased.

Application of Surface Tension Concepts in everyday life

Have you ever asked, why should we wash clothes with soap? The problem is so that the clothes we wash are really clean; the water must pass through a very narrow gap in clothing fibers. For this reason, it is necessary to increase the water surface area. Well, this is very difficult to do because of the surface tension. Inevitably the surface tension value must be lowered first. We can reduce surface tension by using hot water. The higher the water temperature, the better because, the higher the water temperature, the smaller the surface tension (see table). This is the first alternative and is a rarely used method. Another option is to use soap.

At a temperature of 20 oC, the tension value of the surface of the soap water is 25.00 mN/m. Try to compare between soap and hot water, which is the smallest surface tension value? At 100 oC, the value of hot water surface tension = 58.90. At a temperature of 20 oC, the surface tension value of soap is 25.00 mN/m. It’s more profitable to use soap. What is explained above is only one of the influencing factors?

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Why is the soap or water bubble round?

Bubbles of soap or drops of water are round because they are affected by surface tension. First, we discuss soap bubbles. Soap bubbles have two thin membranes on the surface and between them are thin layers of water.

The presence of surface tension causes the membrane to contract and tends to reduce its surface area. When the soapy water membrane contracts and tries to minimize its surface area, there is a difference in the air pressure on the outside of the membrane (atmospheric pressure) and air pressure on the inside of the layer. The air pressure that is outside the layer (atmospheric pressure) also pushes the membrane of the soapy water when it is contracting because the air pressure on the inside of the membrane is smaller.

After the membrane has contracted, the air inside (air trapped between the two membranes) is suppressed, increasing the air pressure inside the layer until there is no contraction. In other words, when there is no contraction again, the amount of air pressure between the membranes is the same as atmospheric pressure + the force of the surface tension that wrinkles the layer.

Then what about dewdrops or drops of water coming out of the faucet? The same because the primary cause is surface tension. If soap bubbles have two thin membranes on two surfaces, then the drops of water have only one thin layer, which is on the outside of the water drops. The inside is full of water. Due to the cohesion force, surface tension arises. The outer part of the water drop is pulled in. As a result, water contracts and tends to reduce its surface area. The atmospheric pressure that is outside also suppresses water drops. The contraction will stop when the pressure on the inside of the water is equal to atmospheric pressure + the force of the surface tension that constricts the water membrane.

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