This week’s question was asked by a friend who plays with his food.
QUESTION: Why do two fresh Cheerios rapidly pull together when placed near each other in a bowl of milk?
ANSWER: You are very observant. Cheerios is the O-shaped breakfast cereal from General Mills. Two important natural phenomena are at play: capillary action and surface tension.
Capillary action, sometimes called capillarity, is the rising of a liquid in narrow tubes or porous material. Examples of capillary action are the uptake of water in tissue paper, paper towels, plaster, the wicking of paint between the hairs of a paintbrush, the movement of water through sand and the rise of water in the xylem of trees.
Capillary action is caused by the combination of adhesion and cohesion. Adhesion is the force of attraction between unlike molecules, such as water and glass. Set a drinking glass in water, pick it up and notice water clinging to the bottom and sides of the glass. That’s adhesion in action.
Cohesion is the force of attraction between like molecules, such as water and water. Pour some water out on a smooth surface and note how it beads up. Cohesion is the same as surface tension and always wants to make as small a surface area as possible.
In a small diameter tube, adhesion draws water up along the side. Cohesion draws water up in the center to make less surface area. Those two forces act together and against gravity. The spaces between the fibers of a paper towel are ideal for the rising of a liquid. Try this yourself. Take a piece of paper toweling and dip it a fraction of an inch into water. Hold it steady and note how quickly the paper is wettened, usually an inch in less than 30 seconds.
We see capillary action in other ways in the everyday world. When burning a candle, the melted wax rises up to the flame. Blood is pumped throughout our body, it is this process which distributes blood in the smallest blood vessels which are called, appropriately, capillaries.
Surface tension, as mentions before, is the same as cohesion, the force of attraction between like molecules. Surface tension is caused by intermolecular forces that hold the water molecules together. Surface tension wants to make the least surface area possible. It makes the surface of water to act much like a thin rubber membrane. Surface tension allows insects, such as water striders or skeeter bugs, usually denser than water, to stride on a water surface. It allows us to set a sewing needle down on water, even though the needle is seven or eight times as dense as water.
Try this really neat experiment to observe the surface tension of water. Fill a bowl or plate with about an inch of water. The amount is not critical. Sprinkle or shake some pepper on the surface of the water. Drop a single soap drop in the middle and observe how the pepper all darts to the edge of the bowl.
What is going on here? Soap breaks down the surface tension of water. The water molecules still want to keep the surface tension going, so they pull back away from the soap and carry the pepper particles with them. Note that some pepper sinks to the bottom of the vessel because surface tension no longer supports them.
The idea that soap breaks down surface tension is the reason we use soap to wash clothes. The soap can get down into the fabric and carry the dirt away. Surface tension is what makes a soap bubble.
What about the Cheerios in milk? Milk rises into the two Cheerios because of capillary action.
The surface tension of milk, mostly water, has a horizonal component that will pull the two Cheerios together. Science in a cereal bowl!
Send questions and comments to: firstname.lastname@example.org.