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    Surface Tension: Definition, Causes, Measurement & Formula

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    • 0:04 Surface Tension
    • 1:52 Measuring Surface Tension
    • 2:40 Examples
    • 5:33 Lesson Summary
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    Lesson Transcript
    Instructor: Laura Foist

    Laura has a Masters of Science in Food Science and Human Nutrition and has taught college Science.

    In this lesson, we will look at a couple examples of surface tension. We will learn the definition of surface tension, what causes it, how to measure surface tension, and how to use the surface tension formula.

    Surface Tension

    Have you ever looked at a water droplet on a leaf after a rain shower? The water forms beautiful round droplets. Or have you ever watched a water strider walking across the top of water almost magically? These are both possible due to the surface tension of water.

    Surface tension is the attractive force in liquids that pulls surface molecules into the rest of the liquid, minimizing the surface area. These attractive forces are due to electrostatic forces. We typically refer to this cohesion at the gas-liquid surface (not liquid-solid or liquid-liquid surfaces). We often see this occur with water, but it occurs with all other liquids to some degree.

    Electrostatic forces are the forces that attract molecules to each other. With water the slight dipole makes the molecules particularly attracted to each other, making it able to form a tight knit cohesive unit.

    When in the liquid state this force doesn't make much of a difference because each molecule is being pulled all directions towards other molecules. But on the surface of the liquid, the molecules aren't being pulled up at all because there are no molecules there to pull the others up. There is a strong force between the molecules, pulling them down. This creates a strong barrier at the liquid-gas interface. It takes a lot more force to break through the surface of a liquid than it does to go through the liquid.

    So, the water droplets form because the water molecules are trying to interact with each other. If the water forms a flat surface there are more molecules that need to interact with the air instead of with each other. Once it forms a drop the more water molecules can interact with each other on the inside. This pulls the molecules together so the outside surface has a strong surface tension which keeps the drop in place.

    Measuring Surface Tension

    The general formula for measuring surface tension is: gamma equals force divided by length.

    Surface tension formula

    Gamma represents surface tension, F represents force, and d represents the length along which the force is felt. The units for surface tension are Newtons per meter (N/m) or dyne per centimeter (dyn/cm).

    One thing to note is that as temperature increases the surface tension of water decreases. This is the reason hot water is so much better at cleaning clothes than cold water. The lower surface tension of hot water allows it to interact with the soil in the clothes more than cold water. As stated previously the high surface tension of water is due to the dipoles found in water molecules. This dipole interaction decreases as temperature increases because the molecules are moving faster.

    Examples

    So, let's look at water. If we know that the surface tension of water at 25 degrees Celsius is 72 mN/m, how much force can a 5 cm area of the surface withstand before breaking? When we rearrange the surface tension equation we see that force is equal to gamma * length. 5 cm is equal to 0.05 m.

    Force equation

    F = 72 mN/m * 0.05 m

    F = 3.6 mN

    If the only acceleration is gravity (9.8 m/s2), we can determine what mass this 5 cm area of water can hold before the surface tension will break.

    Converting mN to N: 3.6 mN = 0.0036 N.

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