Why Does Cereal In a Bowl Of Milk Move Toward the Sides of the Bowl and What Causes Surface Tension?

The force that moves cereal to the sides of the bowl is due to an imbalance in the pull from the surface tension of the liquid around the sides of the floating piece of cereal.

A simple experiment explains what is happening.

You need tap water and two polystyrene cups plus two small pieces taken from a third cup.

Fill the first cup up to within 1 centimetre of the rim, fill the second cup to the top and then carefully add more to the second cup until the water is up over the top of the cup but not spilling over, that is until the water is held in a convex bulge above the top of the cup by surface tension.

Now place the small circles of polystyrene in the middle of each.

The piece floating in the partially filled cup will, with a little prompting move to the side of the cup and be held there. By contrast, the piece floating on the convex bulge of the water in the second cup will remain near the center.

Furthermore, if you push the piece to the edge of the cup, say with the tip of a pencil, the edge repels the small piece towards the centre with considerable force.

This is all caused by the surface tension of the water.

In the partially filled cup the water surface curves up to meet the polystyrene. This occurs because water molecules are more attracted to polystyrene than to each other.

The water forms the convex bulge at the top of the second cup because the surface tension constrains the liquid surface to the smallest area possible, which similarly accounts for the spherical shape of liquid drops.

The water also curves up to meet the small circle of polystyrene on all sides.

Where the water meets the polystyrene of the small circle, the surface tension pulls on each contact point in a down and outward direction provided by the angle of contact with the water.

When the circle is in the middle of the cup the pull on the circle on one side is directly balanced by the pull on the opposite side, because the water curves up to meet the circle equally at all points.

However, if the piece is moved towards the side of the partially filled cup, the upward curve of the water surface near the cup side reduces the curve of the surface in contact with the circle. This increases the outward pull on the side of the circle nearest to the cup edge, resulting in a net force towards the side of the cup.

The effect also accounts for the clumping together of cereal pieces on the surface of milk in your bowl and similar behavior of leaves and twigs on ponds and lakes.

Maybe it is a defensive strategy, they huddle together like bison, to protect each other from the predator. Or maybe it’s just the surface tension in the milk.

The fact that rice and wheat, or any grain for that matter, can gravitate towards its companions in this fashion, depends upon their being able to “sense” their way towards the common centre of mass.

This ability is known as the Grain of Common Sense.

Research has shown that when human beings are dropped into a large bowl of milk this flocking or aggregation potential is entirely lacking, thus proving that they don’t have a single grain of common sense at all.