- Is it possible to walk on water?
- Why do some materials get wet and some don't?
- Why shouldn't bird feathers come into contact with oil?
The surface of the water tries to shrink to the smallest area possible
If you carefully place a paper clip on a glass filled with water, the paper clip will float on the surface. When looking at the paper clip closely, it might even seem like a plastic film was put on the water to hold it up. However, when the glass is shaken, the paper clip sinks to the bottom.
For the paper clip to sink to the bottom of the container, because of gravity, the paper clip must push the water molecules out of its way. We know that molecules are bound together in the water. In water, each molecule of water can grab a friend from the right or left, top or bottom. On the surface layer, however, there are no equivalent neighbours outside the water to pull. When a water molecule is forced to move away from the surface, its neighbours try to pull it back. This phenomenon is called surface tension. Due to surface tension, the water surface holds its shape like it was covered with a plastic film that the paper clip cannot penetrate. The surface tension also keeps the water droplet together, more or less rounded.
Small insects called water striders use surface tension to walk and run on the water surface without falling in. They move along the water surface on their two pairs of legs like speed skaters. The legs of the water strider are covered with an oil-like coating.
- Gravity
- Surface tension
- Expansion
- Freezing
Think!
- Do water striders feet get wet in the water?
Bonus: What is the shape of a raindrop?
A falling raindrop is not tear-shaped or pear-shaped, as the artists like to draw it, but rather flat. The air resistance is to blame. If you ride a bike fast, you know well how the air presses against you. For the same reason, the shape of the falling water droplet also changes. The larger the drop, the flatter the air compresses it. The largest droplets (over 4 mm in diameter) break apart due to the air resistance.
Surface tension can also be reduced
A paper clip, carefully placed in a glass filled with water, floats on the surface of the water. When you add a drop of dishwashing detergent into the glass, the paper clip sinks to the bottom. When doing laundry, water does not penetrate the tiny fabric fibres because of surface tension. If washing powder is added to the water, surface tension will decrease, and the fabric will wet. Surface tension is also lower at higher temperatures.
- Adding washing powder to water
- Increasing water temperature
- Decreasing water temperature
- Adding dishwashing detergent to water
Wetting
Water and many other liquids have the property of wetting other bodies. This means that the molecules in the liquid are attracted to the molecules on the solid surface and the liquid spreads on the surface. This phenomenon is called wetting.
But not every surface wets. You can see this by applying cooking oil on half of the surface of a pocket mirror and then dripping water on the mirror. On a clean glass surface, water spills out because the molecules of the water and the surface layer of the glass are attracted to each other. There is no such attraction between the oil and water molecules, and water forms round droplets due to the surface tension. Water cannot spread out when the surface is greasy.
The feathers of aquatic birds do not get wet because they are covered with a thin layer of fat that repels water. This fat is produced by the glands that are at the tail end of the birds. The fur of aquatic animals does not get wet either. Like aquatic birds, they smear their coats with greasy secretions. An otter or a beaver that has climbed out of the water only has to shake its fur once, and it is dry again. However, oil or petrol that gets on the bird's feathers dissolves the greasy protective layer, and cold water penetrates the feathers. This is life-threatening for the bird.
People have also thought about how not to get wet in the rain. Thick fabrics made of artificial fibres do not wet, but clothes made of natural materials can also be made non-wetting with oils and fats.
Think!
- Why are the water slider's feet covered in an oil-like coating?
- Why is it necessary to apply care products on footwear from time to time?
Watera water bird's feathers.
Water a clean glass surface.
Water a raincoat.
Water a cotton T‑shirt.
Bonus: Why is oil dangerous for water birds?
The plumage of a water bird is like a waterproof diving suit. If the plumage comes in contact with oil fuel, the waterproof greasy coating gets dissolved, and the feathers wet; the water penetrates the feathers and reaches the bird's body, and the bird dies. Oil from ships sometimes spills into the sea. It is a disaster for sea birds, and people will then have to take responsibility and help. However, they must be mindful that if the bird's feathers are washed completely clean, they will also get completely wet. Therefore, it is challenging to save oil-contaminated water birds, but very necessary. This is one reason why people have to try to stop using oil.
- The oil breaks the waterproof layer of the feathers.
- The bird's feathers get wet.
- Cold water cools down the wet birds.
- The wet bird adjusts the feathers with its beak to keep it warmer. This is how it swallows toxic oil.
Important terms
- surface tension – a phenomenon in which the surface layer of a liquid acts as a film
- wetting – a phenomenon in which liquid molecules are attracted to solid surface molecules and the liquid spreads on the solid surface
I now know that…
Due to the surface tension, the surface layer of water behaves like a film. The surface tension also holds the water drop together. Water and many other liquids wet other bodies, but not every surface wets.
