At the start of X-Peri-Mas we looked at what makes things sink or flat – density. If the density is higher than the liquid it is in, then your object will sink. Density is just how much stuff you have crammed into one space – so a metal object has more stuff crammed together inside it than, say, foam where there are loads of gaps filled with air.
If we could change the density of something that normally sinks – if we could lower it a bit – then we could make it float. If we could raise the density a bit then something that would normally float will start to sink!
Once you’ve dropped it into the water, there’s not much you can do to change the amount of stuff you’ve added – it’s in the water now. There are two ways we can look at to change the density of your object without touching it.
Raising density – making a floater sink
This is also known as a Cartesian Diver. We can use pressure to compress your object a bit and then it takes up less space – but there’s the same amount of stuff there so it has a higher density (more mass in the same space).
You need to have something that has a little air bubble trapped inside it – a really easy one is a ketchup packet. In the GIF below I’ve used a plastic pippette that I’ve cut the end off and wrapped some wire around it to make it just heavy enough that it nearly sinks.
For a sauce packet… drop it into a bottle of water and see whether it floats. Depending on the size of the air bottle in that sauce packet it might float already. If it does, great, you’re all set. If it sinks, add salt to the water until it starts to float.
Now, add the lid to the bottle and squeeze it. You’re increasing the pressure inside the bottle but the water can’t be compressed – so instead the air bubble inside your Cartesian Diver or sauce packet gets squashed instead! Now it takes up a smaller space, but has the same mass so the density has increased. Job done, it will sink.
Lowering density – making a sinker float
This one is a bit trickier. We can use the Cartesian Diver trick if you have a way to lower the pressure in the bottle and make the air bubble expand – if you had a bottle that you could stretch instead of squeeze or a syringe that you could pull air out of the bottle.
The other way is exactly what we do when people learn to swim. We lower their density so that they float more easily. We wear armbands! These are filled with air – air has a really low density – so when you attach a bag of air to yourself, you are lowering your overall density. You stay the same, but there is now a big bag of low density air pulling you up towards the surface. We can use this trick to make some objects that normally sink, float.
Drop some raisins into a fizzy drink. They will sink at first but bubbles will start to form all over the surface of the raisin. Because of all the wrinkles on the raisins surface, the bubbles will get trapped. The raisin is now wearing armbands! The little trapped bubbles lower the density of the raisin-bubble combo and so they float together.
The fun bit here is that when the raisins reach the surface, the bubbles pop! This takes away the only thing that helped the raisins float so now their density is too high for the object to float, and it will sink again! Keep watching as they dance away, up and down in your bottle.
I’ve made an even better way to see this, though. I’ve used a 3D printer to make a shape with a really rough surface and wrinkles just like the raisins except that it is much bigger! That makes it easier to see what’s going on. Unfortunately, I made an error with my maths and got the density wrong so it didn’t quite sink to start with… I fixed that with the usual trick of adding some weight (increasing density) by wrapping some copper wire around it.
Here’s the result… a model raisin!