Making a dome out of ... water!
What is surface tension – and why don’t water meters sink?
Safety precautions
None.
Equipment
- coin;
- syringe or pipette;
- glass;
- marbles.
Step-by-step instructions
Carefully fill a glass to the brim with water. Obviously, the water won't spill out just yet; it can’t get over the rim of the glass. What do you think – how many marbles can you add without spilling any water? When you add marbles one by one, you’ll notice that a dome of water gradually forms above the glass as each marble displaces a certain amount of water. It’s incredible how many marbles you can add before the water spills over the edge! To better understand this process, use a syringe or pipette to gradually place drops of water on the surface of a coin. A “cap” of water will grow on the coin! The explanations for both of these effects are grounded in surface tension.
Process description
In liquids, molecules are located relatively close to each other. Each molecule in a liquid is attracted to its neighbors on all sides. The molecules on the surface, however, don’t have any neighbors above them. The attractive forces acting on them are all directed inward, and as a result, the surface of the liquid as a whole tends to contract. This effect leads to the force of so-called surface tension, which acts along the surface of the liquid and leads to the formation of an invisible, thin, elastic “film”. Consequently, it takes mechanical work to increase a liquid’s surface area, as these surface tension forces must be overcome. If the liquid is not subject to other forces, it will strive to minimize its surface area, adopting a spherical shape. This is why dewdrops collect in almost-spherical droplets on the grass. Moreover, water striders skillfully use surface tension to walk along bodies of water. They don’t sink because they weigh less than the forces of surface tension they rely on.
