Activities Action and reaction Clingy surface tension Diffusion "Floating" a paper-clip Getting cold at night Insulation for living things Oil spills and birds Paper fish Pepper on water Surface tension "flotation" Water jets cling Other pages on this site

How to do it This web page involves a grab bag of observations, experiments and demonstrations, all related to the problems physics can make for different life forms. Surface tension 1 You will need a fluffy dish mop or a paint brush, and a small container of water. Notice that the brush or mop, which is fluffy when dry, is held together when it is wet. Look at it first in its dry form, and then when it is all wet and clingy. Can you see what problems this would cause for a fish that wanted to go onto the land? Or seals, otters and other water-mammals? This will help you understand Surface tension 2 You will need some copper wire, a cork, a sharp knife and a pair of pliers, a nail, and possibly some Plasticine or modelling clay (it will depend on what sort of English you speak). Cut a piece of the wire about 25 cm long, and bend it into a circle. Twist the ends neatly together, and fix two more pieces of wire, about 10 cm long, to the opposite sides of the circle, and twist the other ends together, so these wires meet at about 90 degrees, making a cone shape. Keep the circle as flat as possible, although this is not important. Cut a slice of cork, poke a hole through the centre with the nail, and push the twisted wire ends through for several centimetres, so it looks like the illustration, although the cork is shown too thick there. If you use a large cork, you will need a counterweight of Plasticine or modelling clay Now wrap the wire "tail" around the nail, and test the whole construction. It should float upright, but only just floating -- the weight of the nail will keep it stable, helped by the Plasticine or modelling clay. Adjust the flotation by trimming the cork with the knife, or trimming the nail with the pliers, until it floats with only a tiny bit of the cork above the water. Sink the "floater": it should now be held under water by surface tension. This will help you understand Surface tension 3: Forming a coalition Cut the top of an aluminium can off: use a nail to make a small hole first, then use scissors to cut the can around. Then use the nail to make five holes in the side of the can. The holes should be made close to the base of the can and should be spaced about 5 mm apart. Hold the can under a kitchen tap or hose and fill it with water. Pinch the streams of water together with your thumb and forefinger. What happens? Can you explain what happens? Work out why this is called "Forming a coalition". This will help you understand Back to the index Surface tension 4: Live fish? Draw a fish shape on a piece of paper and cut it out. Make a small hole as shown, with a track leading down to a point near the tail. Set the paper fish in a pan of water. Drop a tiny amount of vegetable oil or detergent into the hole in the fish. What happens? This will help you understand Back to the index Surface tension 5: Cast your pepper upon the waters Fill a dish with water. Sprinkle pepper all over the top of the water. Put several drops of dish detergent into the centre of the dish. What happens to the pepper? Is the same thing happening over the whole surface of the water, or just in the middle? There are some more surface tension things to be found here This will help you understand Back to the index Surface tension 6: Floating a paper clip Bend a paper clip into an "L" shape. Use this clip to gently lay another B on the top of the water in a glass. What happens? Why? This will help you understand Back to the index Insulation and life it's about time I did this one :-) This will help you understand Oilspill problems If you take an ordinary feather and push it into clean water, you will see a silvery layer over the feather where a layer of air has been held, trapped against the feather by the bird's preening oil which it spreads over the feather. This is clearest in the flight feathers of water birds. If you spill a small amount of cooking oil onto the surface of the water, and push the feather in again, you will see some changes. Now there will be no silvery layer, because the cooking oil has mixed in with the bird's natural oils. In nature, when a bird is caught in an oil spill, this is what causes the feathers to clog together, cutting down on insulation, and often killing the bird with cold. So do what the rescuers do -- clean the feather with detergent and try again. Whoops! No silver film! Why is that? This will help you understand The cold cold ground You will need a piece of aluminium foil, a foam cup, a sensitive thermometer and some sticky tape. Line the foam cup with foil, poke a hole in the bottom, and insert the thermometer bulb into the cup from underneath. Use the tape to fix it in place, so the thermometer does not fall out and break. In daylight, you can use this set-up to show that the gadget is a neat heat-catcher, just by pointing it at the sun. Now wait for a clear, crisp night, and take the heat-catcher outside. Point it at the sky, wait a minute or two, and read the thermometer with a torch. Then point it at the ground for a minute or two, and read the thermometer again. This will help you understand Never shoot an elephant from a skateboard Kids! Don't try this at home, as elephants damage your parents' flower beds when they fall over, and your parents will get mad, and the elephant will be none too pleased, either. Umm, sorry. The meaning of the heading is that an elephant has an immensely strong skull, and so an elephant gun has to fire a huge lead slug that will crash through and damage the brain. To see what this leads to, you will need some sort of a platform on rollers or castors, sitting on a hard smooth floor, a heavy weight to throw, and a good sense of balance. When you fire an elephant gun, the force that sends a heavy lead slug in one direction also pushes the gun and the shooter in an opposite direction. If you stand on your platform and throw a brick or a shot, there will be a backwards force on you in just the same way. What has this got to do with biology? This will help you understand All good mixers Herbicides, we are assured are applied selectively, only to their targets. When people say this, they are relying on your ignorance of entropy, the natural cussedness of matter. One of the most basic laws of nature is matter spreads. To see this happening, you need a deep glass container like a measuring cylinder or a long vase, a piece of fairly stiff tubing (glass is ideal, but you can substitute here), possibly a small funnel, and some coloured material: either dye, or food colouring, or old-fashioned fountain-pen ink, or crystals such as Condy's crystals (potassium permanganate). If you are using Condy's crystals, a bit of adult supervision would be a very good idea, because if something goes wrong, the adult will take the blame. Food colouring is a lot safer! Fill the container almost to the top with water, and put it in a place where it can be left for a week or two. Then put the tube in, and pour your coloured material down the inside of the tube, so that it drops to the bottom of the container. Pour some fresh water down to wash the last of the colour out, then take the tube gently out. You should now have a layer of colour at the bottom of the container, but over the next few weeks, random movement of the atoms and molecules will cause the coloured zone to expand and get lighter as the dye spreads in a process called diffusion. This will help you understand

The science behind the enquiries

Surface tension 1
The threads of the mop or brush are held together because surface tension in the water pulls the water together, so as to reduce the surface volume. Gills in water are fine, because they float free in the water but on land they are useless because the filaments cling together, and so we land animals need lungs. Aquatic mammals need water-proof coats, or they get cold when they come out of the water.

The problem comes from the way that water molecules attract each other strongly. When you have a drop of water, it acts as though it has a skin, because the water molecules inside are all pulling on the outside ones, and the water around the mop fluff (or brush bristles) acts to pull all the threads together.

There is a related experiment on this.

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Surface tension 2
This is a serious problem for any small animal that falls into the water and sinks through: it can't get out again! On the other hand, water striders and a few other small invertebrates can skitter around on a pond surface, held up by the surface tension of the water. Woe betide them if they fall thorough, or are sunk by a wave!

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Surface tension is the name we give to an effect caused by particles attracting each other. All of the water molecules attract each other, and this means that water gets pulled into a round shape. To push it out of a spherical shape you have to do work on it, so unless the paperclip can exert enough force, it won't be able to stretch the water surface enough to let the paperclip slip through.

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Why did the pepper "run away"?
The pepper sprinkled on the water stays in place because the water is pulling on the pepper evenly in all directions. When the detergent comes into contact with the water in the centre, it reduces the water's pulling action on the pepper and the pepper appears to run away from the detergent. But the water around the edges (untouched by the detergent) still has its pulling strength.
You will find some more enquiries into surface tension through this link.

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Why do the streams knot together?
The streams of water are held together, once they have been joined, by the water's "stickiness," or surface tension. Surface tension is the tendency of the surface of a liquid to behave as though covered with a skin. This is caused by the cohesive forces between the molecules at and near the surface.

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Why does the fish move across the water?
All liquids have a certain amount of surface tension, a property that causes a liquid surface to behave like an elastic skin. The vegetable oil or detergent decreases the water's surface tension. If different parts of the fish have contact with different surface tensions, the fish will be moved along.
What happens if you use the same fish or the same water when you use oil after detergent, or detergent after oil? Do you get the same effect? Why or why not?

You can also make a small boat instead of a fish, and you can use camphor instead of oil or detergent. If you try making a boat, you may like to investigate what happens when you make the slit to one side or the other, rather than straight down the middle.

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Insulation problems
Sorry, still thinking . . .

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Oilspill!
The detergent that takes away the polluting oil also takes off the natural oils, so the rescued bird cannot survive without help. This is why rescuers have to keep the birds they have saved until they have preened their feathers back into a naturally oily state.

Probe

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One of the odder facts about gravity is that, inside a hollow sphere in free fall, the gravitational pull anywhere inside the hollow space is zero. So if you ever get trapped in a situation like that, make sure you are carrying a spanner to throw in the opposite direction to the way you want to go.

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Diffusion effects
Diffusion happens because of random events. Imagine you have a paddock full of rabbits, and another paddock with no rabbits. This is my thought experiment, so I have blindfolded all of the rabbits, and they are wandering around at random, bumping into each other, tripping over carrots, falling off fence posts and whatever else demented rabbits may do.

Look, don't argue. I don't like rabbits, so I can do what I like to them in my thought experiment -- even put them in 200 litre oil drums and blow Rugby whistles at them if I like. Stop arguing and pay attention!

Now that I have your attention again, what is the probability of rabbits wandering into the rabbit paddock? The answer: none at all, because there are no rabbits outside to wander in.

The probability of rabbits wandering out, though, is very good. After a while, there will be about the same number of rabbits in each paddock, and now the flow of rabbits from A to B is about the same as from B to A. But on the far side of the paddock that once had no rabbits, there is another paddock with no rabbits, and the rabbits are still wandering, aimlessly . . .

We say that diffusion is always from areas of high concentration to areas of low concentration.
PS: about the oil drums and Rugby whistles -- somebody did that once, while testing to see if stressed rabbits survive longer. It was not, so far as I can tell, meant to imply that Rugby players are bunnies.

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