Surface Tension of Liquids – Importance and Definition

What is the Surface Tension of Liquids?

Surface tension illustration.
Image Source: Researchgate.com.

Surface tension is a very important physical phenomenon that we see lots of examples of it. The physical cause of this physical phenome is very simple. Here, we will explain these topics about this tension of liquids;

  • What is surface tension? What are the causes of it?
  • What are the examples of this phenomenon?
  • How to calculate it?
  • What are the surface tension values of common fluids?

What is the Surface Tension?

Surface tension is a physical phenomenon that we can see in liquids. There is a very thin surface on the liquid. So, this surface has a tension such as a membrane. This is the main mechanism of a tension.

The main cause of this physical phenomenon is the attractive forces of the liquid molecules. Liquid molecules are applying attractive forces to each other. Beneath surface, these forces are balancing each other. But at the surface, there is no force attracting the molecules above. So, a membrane-like surface layer takes place because of the forces from the sides.

The sizes of the droplets depend on the surface tension of liquids. With the increasing tension, the droplets get bigger. Because the tension at the surface is very big enough to hold molecules inside it.

Also for liquids that have very high tensions such as mercury, the droplets will be ball-like.

Unit of the Surface Tension

We are talking about stress. And the unit of this stress is N/m or lbf/ft in English units.

There is stretching work that is done by this stress. We can call this phenomenon surface energy also. So, we can use the surface energy units such as N.m/m2 or lbf.ft/ft2.

The tension on the surface effect tends to minimize the surface area of the liquid. Every time, the surface area tries to be minimum.

How to Measure the Surface Tension?

Bubble frame.

Measuring the tension of the surface for liquids is made with a quite simple experiment. Think about a wire frame that is immersed inside a foam. There will be a very thin bubble surface on this frame and the two sides of the bubble surface area open to the atmosphere.

Also, one side of this wireframe is movable. The surface tension of the bubble will try to take this movable side to make the surface smaller. So, there will be a force on that movable side. We can find the opposite balancing force by holding the movable side.

So, to find this tension, we need to divide this balancing force by the length of the movable side;

Formula of surface tension

As you understand that in the calculation of the surface tensions of liquids, we can use this method.

Effect of Temperature and Pressure on Surface Tension

If we think about the effect of the temperature on the surface tension, there is a strict effect. With the increasing temperatures, the tension of liquids decreases. Also at the critical point, it becomes zero.

But there is no strict correlation between the pressure and tension of liquids.

Effects of Impurities

Impurities in fluids have tremendous effects on tensions. In general, impurities are decreasing the surface tension of liquids. For example, they are adding surfactants to decrease the surface tension of the liquids.

The most important application of the impurities is detergents for cleaning. Detergents are added to heated liquid is extremely decreases the surface tension of the water. So, water can penetrate small openings to clean the dirt.

Surface tension is very important in the working of heat pipes. So, if there will be impurities in the liquids of heat pipes, it will be a destroying effect.

Examples of Surface Tension

You can observe different examples of surface tension of liquids around you. Take a look at these examples below.

Bumps of Fluids

Mercury.
Mercury.

If you drip liquids on a flat surface, you will see that they form bump shapes. This is because of the attractive forces between the molecules. There are no attractive forces for the outer surface of the bump and they tend to obtain minimum surface area.

With the increased surface tension of fluids, they tend to become more ball-like shapes. For example, if drip mercury has a very big tension value, you will see that the drip will be a ball-like shape. The tension of water is not high as mercury. And the water drip will not take a ball-like shape.

Fuel Injected to Engine Cylinders

In most, modern internal combustion engines, the fuel is injected with high pressures. With this injection, the liquid is divided into very small mist particles. If we take a look at these mist fuel particles, we will see that they are in a sphere shape. This sphere shape is because of the internal attractive forces between the molecules of the fuel.

Detergent Use in Cleaning

This is an example of the practical use o surface tension. Water has surface tension and this value changes with the changing temperature. If the temperature increases, the tension decreases. Also, detergent decreases the tension of the water.

So, if we add, detergent to hot water, we obtain a liquid with very low tension. This liquid can penetrate nearly all narrow places. The cleaning capability of this liquid is very high.

Insects on Water

Some insect species such as water striders can stay on the surface of the water. So, they are using surface tensions of water to stay at the surface.

Floating Steel Needle on Water

Normally, steel sinks in the water. Because the density of steel is very high. But, if you achieve to hold a steel needle on the tension of water, you will obtain a steel piece floating on water.

Capillary Effect of Surface Tension

Capillarity.

The capillary effect is a very important phenomenon in physics. This effect is the ability of fluids to climb the capillary thin tubes. The main reason for this effect is the surface tension.

For example, if you hold a cotton piece to water, water will climb the other portions of the cotton piece. We generally call this event ‘cotton is absorbed water’. The capillary effect plays an important role in this situation.

Contact Angle

To understand the mechanism of this phenomenon, we need to learn about the contact angle. If you put a fluid inside a glass, you will see that there is a twist of water surface near the walls. This twist is caused by the surface tension.

For water, this twist is upward orientation. Also for mercury, this twist is downward. So, we call the angle of this twist a contact angle or wetting angle.

In a capillary tube, the contact angle of water becomes zero. Because of this reason, there is an upward surface tension force occurs. This surface tension force takes the water upward direction. Once the weight of the water in the capillary tube becomes equal to this force, the rise of the water stops.

Adhesive and Cohesive Forces

The wetting and capillary capabilities of liquids depending on the adhesive and cohesive forces of liquids.

Adhesive forces are the tendency of the liquid to cling to other materials such as glass. Also, cohesive forces are the intermolecular forces between the same molecules of water.

If the adhesive forces are bigger than the cohesive forces in solid-liquid interfaces, the wetting capability is very high. So, the capillary effect will be also very high. Adhesive forces are also very important in the capillary effects.

For example, the cohesive forces between the molecules of mercury are very high. So, the wetting capability of mercury is very low.

The most important example of the capillary effect in nature is the partial effect of the rise of the water to the top of the plants. The capillary effect between the bodies of plants and trees, and water, make the water rise to the top of the trees.

Surface Tension Values of Certain Liquids

You can have a general idea about the tension of different liquids with these values below.

  • Mercury: The surface tension of the mercury is 465 mN/m or dyn/cm at 20°C. So, this is the highest tension value around the fluids. Because of this high value, the droplet of mercury on a flat surface is a sphere.
  • Water: With the increasing temperature, this value decreases for water. For example at 0°C, this value is 75,6 mN/m or dyn/cm. Also at 20°C, the surface tension is 72.8 mN/m or dyn/cm. If you compare it with mercury, the tension is very low.
  • Liquid Oxygen: Oxygen is liquid at around -193°C. At this temperature, this value is 15.7 mN/m or dyn/cm. So, this is also very low if we compare it with other liquids.
  • Liquid Helium: Helium is liquid at -269°C and the surface tension of the liquid helium is 0.12 mN/m or dyn/cm which is one of the lowest ones among liquids.
  • Soap Solution: As we stated above, detergents and soaps are used to decrease the surface tension of water for better penetration. The tension of the soap solution is 25 mN/m or dyn/cm. So, soap decreases the value from 72.8 to 25.

Conclusion

So, surface tension is a very important phenomenon in physics. There are various kinds of examples of this tension in nature. Also, there are practical applications of it.

Capillary effects is an also a very important natural emergence of this phenomenon. The surface tension values of different fluids change. For example, mercury has one of the highest tension values.

So, do not forget to leave your comments and questions below about this topic.

Your precious feedbacks are very important to us.

FAQs About Surface Tension

Does surface tension increase with temperature?

No. With the increasing temperatures, this value decreases. Because of it, the wetting capabilities of hot water are better than cold water.

Does surface tension affect viscosity? 

There is a relation between tension and viscosity. With the increasing viscosities, the surface tensions are increasing. For example, Mercury has a very high density and the tension of mercury is also very high.

How surface tension is created?

This phenomenon takes place because of the intermolecular attractive forces. On the surface, there are no attractive forces from above. So, the lateral forces balance each other to form a thin surface layer.

When does surface tension increase?

With the increasing temperature and density, the surface temperature of liquids generally increases. But there may be different situations which is not increasing with the increasing density and temperature.

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