PURPOSE: To illustrate the surface tension of water.

DESCRIPTION: The bowl photographed above contains wet sand. When tapped on its side, the grains of sand settle with their sides so close that water cannot flow into the cracks due to its surface tension. The surface of the sand looks wet. When you push down on the sand with a foot, as though walking on the beach, the spacing between sand grains opens up sufficiently that water can flow down into the space. The sand in the vicinity of the foot appears dry. This effect causes the sand to appear lighter in color by your footprints at the beach.

This effect is known as "dilatancy."

Click on the links below to see a variety of videos of the effect:

• foot stepping in coarse wet sand
• foot stepping and leaving in coarse wet sand
• foot stepping in fine wet sand
• finger pushing into wet sand using blue water (easier to see)

SUGGESTIONS:

See Question of the Week #171 for information on using this demonstration to enhance class involvement.

REFERENCES: (PIRA unknown.)

EQUIPMENT: Bowl with sand, plywood feet.

SETUP TIME: 5 min.