Why is it easier to walk on wet sand then on dry sand?
(Lansing State Journal, August 12, 1992)



Question submitted by:  Dave Hou

It is easier to walk on wet sand than it is to walk on dry sand because the water acts, in a way, like a glue.  When you step on dry sand each grain is free to move and they shift under the weight.  This makes the ground shift and difficult to walk on.

When sand is wet, each grain is separated from the surrounding grains by a thin layer of water.  Both the sand and the water are made up of molecules.  The water molecules are attracted to each other and resist being separated.  The tendency of like molecules to stick together is called cohesion.

Additionally, the water molecules are attracted to the sand molecules.  The tendency of different molecules to be attracted to each other is called adhesion.

The forces that cause cohesion and adhesion are weak.  In the case of the wet sand, however, there are a large number of sand molecules sticking to a large number of water molecules and the forces add up.

The cohesion between the water molecules and the adhesion between this layer of water and the grains of sand make each grain stick to its neighbor.

When a person steps on the sand, instead of each grain moving freely they are stuck together.  This makes the sand more firm and easier to walk on.

Another way to see both adhesion and cohesion is to use two identical dinner plates.  Put a small amount of water in the bottom of one plate and stack the other on top.

The water should spread out to form a thin film between the two plats.  There are not many wwater molecules trying to adhere to many of the platesí molecules.

When you slowly try to lift the top plate you will find that it seems stuck due to the adhesion and cohesion of the molecules.


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