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Surface Tension

Introduction: In a body of water, like what is shown to the left, certain objects and insects, like water striders, can move along the water surface, even though the object or the insect is actually more dense than water is. Why is this? Conventionally, we understand that denser objects sink more easily, while less dense objects tend to float more easily. This phenomenon can be explained by the concept of surface tension.

Surface tension refers to the tendency of liquids to be elastic and, in a sense, "stretch", causing them to have as little surface area as possible. Surface tension is related to the strong attraction of water molecules to one another because of cohesion (forces between like particles). This attraction of water molecules to one another is greater than that of water molecules and the surrounding air, known as adhesion (forces between unlike particles). As a result, water acts elastic.

Surface tension can be measured in units of force per unit length. Some examples of these units include Newtons per meter (N/m), milliNewtons per meter (mN/m), dyne per centimeter (dyn/cm), and Joules per square meter (J/$"m"^2$). Despite all of the mentioned units for surface tension, the official SI unit for surface tension is Newtons per meter (N/m).

Surface tension plays an important role in our everyday lives. Oil and water will not dissolve in one another, partly due to differences in their individual surface tensions. Mercury can be used in a thermometer because it does not experience adhesion to the walls of the thermometer, so it can be read accurately. Without mercury's surface tension, it would not be possible to make viable interpretations of temperature.

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