SURFACE TENSION

Overview of basic concepts

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Surface tension, what is it?

Surface tension is an effect within the surface layer of a liquid that causes the layer to behave as an elastic sheet. Thus, the surface of any liquid behaves as if it was covered by a stretched membrane. Surface tension is measured in units of force per unit length.

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Surface Tension

See some Illustrations:

Insect walking on water

Drop of water

Can you make steel float?

What causes surface tension?

Surface tension is caused by the effects of intermolecular forces at the interface. Surface tension depends on the nature of the liquid, the surrounding environment and temperature. Liquids were molecules have large attractive intermolecular forces will have a large surface tension.

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Forces within the body of a liquid

How does temperature influence surface tension?

In general, surface tension decreases when temperature increases because cohesive forces decrease with an increase of molecular thermal activity. The influence of the surrounding environment is due to the adhesive action liquid molecules have at the interface.

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Effect of temperature on surface tension

Surface tension of water

Because intermolecular forces between water molecules are due to hydrogen bonds and these are high energy, surface tension for water is larger than many other liquids.

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Comparison of surface tension of water and other liquids

Measurement of surface tension

There are various methods to measure surface tension of a liquid. One such method consists of a platinum ring placed over the surface of the liquid. The force required to separate the ring from the surface is measured with a high precision scale.

See a diagram:

Free body Diagram of Dunoy's Ring

Surface Tension Statics

Surface tension plays an important role in systems where small forces are involved wherever there is a liquid-air interface.

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Solution of a statics problem involving surface tension

Sample Problem 1

A glass slide is in contact with a water surface along one of its sides as shown in the figure below. What is the magnitude of the force required to overcome surface tension as the slide is lifted from the water surface? The surface tension of water, for water at 20 degrees Celsius, is 0.0739 g/cm. The dimensions of the plate are 10 cm, by 2 cm, by 0.02 cm.

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Sample Problem 2

Consider the above example of a steel pin "floating" on water. What is the angle of the tangent to the water surface at the point of air-water-solid contact with respect to the horizontal? The surface tension of water, for water at 20 degrees Celsius, is 0.0739 g/cm. The volumetric weight of steel is 7.7 g/cm3. The diameter of the pin is 0.2 cm.

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Sample Problem 3

Agricultural spray equipment atomizes liquid particles to very small diameters. If surface tension for water at 20 degrees Celsius is 0.0739 g/cm, what is the pressure inside a water droplet 0.05 cm in diameter?

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