What is Surface Tension?

Surface Tension is Due to Cohesive Forces

A cohesive force is the attractive forces between similar particles. Liquid molecules have a tendency not to separate because of cohesive forces between. The magnitude of cohesive forces may vary with different liquids. Because of the unbalanced cohesive forces between surface water molecules and molecules below the surface, drops of water have a tendency not to separate. This is a property of liquids called surface tension.

The net inward cohesive force on the surface molecules of a liquid is balanced by the resistance of the liquid to compression. This means the resistance of a liquid to be compressed limits the surface area of the liquid. Surface tension holds water drops together. If you squeeze a tennis ball, there is a limit to how much the ball can be compressed.  This helps me to understand the resistance of liquid molecules to compression.

In the diagram, each surface molecule has cohesive forces between molecules below it. There are no water molecules above the surface water molecules. Thus, the summation of all the down forces on produces a net force (unbalanced force) on each surface molecule toward the center of the liquid.

Discover for Yourself
Place a drop of water on a piece of wax paper. Insert one end of a toothpick in the water drop. Use the toothpick to slowly drag the water drop across the paper. The water drop may tend to spread out as you pull the drop, but surface tension holds the water drop together. In other words, because of surface tension, water drops resist separating.

Generally, rain falls as separate drops instead of fine mist. This is because water droplets are attracted  to each other. When two water droplets touch, the attractive cohesive forces between the water molecules in each drop pulls the drops together forming one larger drop.

Discover for Yourself
Place two or more small drops of water on the  piece of wax paper. Use a toothpick to  move two drops close but not touching.  The cohesive forces of attraction between the two drops will pull the drops together forming one larger drop.

A property of surface tension is that a liquid’s surface tends to contract and have properties much-like those of a stretched elastic membrane. This contraction causes water to form drops. 

Discover for Yourself

How many drops of water can you place on the surface of a coin (penny)?
Using an eyedropper, carefully squeeze one drop of water at a time on the surface of the coin. Count the drops.  Like the previous investigation, each drop of water is attracted to the water on the surface of the coin. Thus the drop of water on the coin enlarges with each added drop.