(MICHAEL KEARNEY, STAFF WRITER)
Today we’re going to do three simple experiments that have to do with surface tension and water. You might be surprised at some of the results!
What you will need:
PHOTO: MICHAEL KEARNEY, TAIPEI TIMES
A paper towel or some tissue
A plug for the sink
EXPERIMENT ONE 實驗一
► Fill the cup to the very top with water. The water should be flat and look like it’s about to overflow.
► Guess how many coins you can add before the water overflows.
► Now start adding coins one by one. Drop the coins in carefully. You don’t want to disturb the water too much.
► How many coins did you add? Was your guess close? How does the surface of the water look?
EXPERIMENT TWO 實驗二
► Fill a cup about three-quarters full with water. Drop in a paperclip. What happens?
► Now tear off a small piece of the paper towel. Put a paper clip on top of it and carefully place it on the water surface.
► Wait for the paper towel to be saturated with water. It should sink to the bottom. If it doesn’t, give it a slight nudge with the tip of a pencil or some other pointy object. Be careful not to disturb the paperclip.
► The paperclip should float on the surface.
► Add a drop or two of soap to the water. Don’t hit the paperclip. What happens?
EXPERIMENT THREE 實驗三
► Fill the sink about halfway full of water. Make sure there’s no soap in the sink.
► Make a small boat using the aluminum foil. Place the boat in the water. Wait for it to stop moving.
► Drop a few drops of water behind the boat. What happens?
► Now try dropping a few drops of soap behind the boat. What happens?
The term “surface tension” is used to describe the tendency of liquid molecules in contact with a gas to “stick” together. Surface tension causes liquids like water to form a sort of skin at their surfaces.
When you added coins to the water, did you notice the water ends up having a curved surface before it overflows? That curved surface forms because of surface tension — the water “sticks” together rather than overflows.
The paperclip is denser than water, so it should sink. But when you carefully placed it onto the surface of the water, it floated. The surface tension holds it up. The soap drops you added to the water broke the surface tension and caused the paperclip to sink.
When you dropped drops of water behind the boat, not much happened. The water doesn’t break the surface tension. But when you added drops of soap, the boat moved forward. The surface tension behind the boat was broken, so the surface tension in the water in front of the boat pulled it forward.
Some insects, like the water strider, are able to stand and move on top of water because of surface tension. The bugs are very light, so when they stand on water, they only slightly dent the surface and their legs repel each other. That repulsion lifts their bodies.
A group of scientists at Carnegie Mellon University in Pittsburgh, Pennsylvania, studied insects like water striders and then built a small robot using ultra-lightweight carbon fiber that is also able to walk on water. But they still have some improvements to make — they said the water strider is able to move about 20 times as fast.
1. surface tension n.
表面張力 (biao3 mian4 zhang1 li4)
2. overflow v.i./v.t.
溢出 (yi4 chu1)
3. saturated adj.
滲透的 (shen4 tou4 de5)
4. nudge n.
輕推 (qing1 tui1)
5. water strider n.
水黽 (shui2 min3)
6. dent v.i./v.t.
使凹下 (shi3 ao1 xia4)
7. repel v.i./v.t.
互相排斥 (hu4 xiang1 pai2 chi4)
8. carbon fiber n.
碳纖維 (tan4 xian1 wei2)
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