Deformation Hide and seek

The Cheerios Effect

Let’s conduct a fun experiment where a chopstick piece and a thumb tack chase each other across the surface of water!

You will need:

  • A bamboo chopstick piece (around 1 centimeter in length);
  • A glass cup (6.5 centimeters in height, with a diameter of 4.5 centimeters)
  • A plastic dropper
  • A plastic cup
  • A thumb tack (a traditional flat bottom tack pin)

Step-by-step instructions:

  1. Fill 90% of the glass cup with water and place the wet chopstick and thumb tack into the water.
  2. The chopstick should stay at the edge of the cup, while the thumb tack should stay away from the edges.
  3. Use the dropper to try and push the chopstick away from the edge of the cup. The chopstick should return to the edge almost immediately. By contrast, when you push the thumb tack towards the edge of the cup, the thumb tack should immediately return to the center of the cup.
  4. Use the dropper to gently drip water into the glass cup. Continue to do so until the water level slightly exceeds the brim of the cup, when the chopstick and the thumb tack should switch places.

《Little tip for you》

Bleach or preservatives are often added to disposable chopsticks during the manufacturing process, making the surface of the chopsticks hydrophobic. As this will affect the experiment, we must moisten the chopsticks first.

How it works:

When you fill the glass cup with water, the attractive force between the water and the glass is strong, which is why the water level has a slight “moon-like shape” (meaning that the water level at the edge of the cup is slightly higher than the water level in the center of the cup), as shown in diagram 1.This is why the chopstick sticks to the edge of the cup, while the thumb tack moves towards the center of the cup, where the water level is lower.

When you continue adding water to the glass cup, until it is full to the brim, the water surface will curve slightly upwards, as shown in diagram 2, due to the surface tension. As the center of the cup now has a higher water level, the chopstick will “rise” towards the center of the cup. Likewise, the thumb tack will “slide” towards the edge of the cup. As a result, the chopstick now stays at the center of the cup, while the thumb tack stays at the edge of the cup, as shown in diagram 3.

*Resource come from: NTCU Science Game Lab.

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