Tag: ammonites

An experiment in buoyancy

Here is a little science experiment anyone can do to help understand the physics of marine animals and their buoyancy. It helps give insight into how animals such as ancient ammonites and now cuttlefish can move up and down at will just by changing the density of internal fluids.* (See Ammonite propulsion of underwater robots). It also shows how marine robots could do the same with a programmed ammonite brain.

First take a beaker of water and a biro pen top. Put a small piece of blu tack over the the top of the pen top (to cover the holes that are there to hopefully stop you suffocating if you were to swallow one – never chew pen tops!). Next, put a larger blob of blu tack round the bottom of the pen top. You will have to use trial and error to get the right amount. Your aim is to make the pen top float vertically upright in the water, with the smaller blu tack just floating above the surface. Try it, by carefully placing the pen top vertically into the water. If it doesn’t float like that, dry the blu tack then add or remove a bit more until it does float correctly.

It now has neutral buoyancy. The force of gravity pulling it down is the same as the buoyancy force (or upthrust) pushing it upwards, caused by the air trapped in the top of the lid… so it stays put, neither sinking nor rising.

Now fill a drink bottle with water all the way to the top. Then add a little more water so the water curves up above the top of the bottle (held in place by surface tension). Carefully, drop in the weighted pen top and screw on the top of the bottle tightly.

The pen top should now just float in the water at some depth. It is acting just like the swim bladder of a fish, with the air in the pen top preventing the weight of the blue tack pulling it down to the bottom.

Now, squeeze the side of the bottle. As you squeeze, the pen top should suddenly sink to the bottom! Let go and it rises back up. What is happening? The force of gravity is still pulling down the same as it was (the mass hasn’t changed), so if it is sinking the buoyancy force pushing up must be less that it was.

What is happening? We are increasing the pressure inside the bottle, so the water is now compressing the air in the pen top, reducing its volume and increasing its density. The more dense your little diving bell is, the less the buoyancy force pushing up, so it sinks.

That is essentially the trick that ammonites evolved, many, many millions of years ago, squeezing the gas inside their shell to suddenly sink to get away quickly when they sensed danger. It is what cuttlefish still do today squeezing the gas in their cuttlebone so the cuttlefish becomes denser.

So, if you were basing a marine robot on an ammonite (with movement also possible by undulating its arms, and by jet propulsion, perhaps) then your programming task for controlling its movement would involve it being able to internally squeeze an air space by just the right amount at the right time!

In fact, several groups of researchers have created marine robots based on ammonites. For example, a group at Utah have been doing so to better understand the real but extinct ammonites themselves, including how they did actually move. For example, the team have been testing different shell shapes to see if some shapes work better than others, and so just how efficient ammonite shell shapes actually were. By programming an ammonite robot brain, you could similarly, for example, better understand how they controlled their movement and how effective it really was in practice (not just in theory).

Science can now be done in a completely different way to the traditional version of just using discovery, observation and experiment. You can now do computer and robotic modelling too, running experiments on your creations. If you want to study marine biology, or even fancy being a Palaeontologist with a difference, understanding long extinct life, you can now do it through robotics and computer science, not just by watching animals or digging up fossils (but understanding some physics is still important to get you started).

– Paul Curzon, Queen Mary University of London

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*Thanks to the Dorset Wildlife Trusts at the Chisel Beach Visitor Centre, Portland where I personally learnt about ammonite and cuttlefish propulsion in a really fun science talk on the physics of marine biology, including demonstrating this experiment.

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This blog is funded by EPSRC on research agreement EP/W033615/1.

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Ammonite propulsion of underwater robots

Ammonite statue showing creature inside its shell
Image by M W from Pixabay

Intending to make a marine robot that will operate under the ocean? Time to start learning, not just engineering and computing, but the physics of marine biology! And, it turns out you can learn a lot from ammonites: marine creatures that ruled the ocean for millennia and died out while dinosaurs ruled the earth. Perhaps your robot needs a shell, not for protection, but to help it move efficiently.

If you set yourself the task of building an underwater robot, perhaps to work with divers in exploring wrecks or studying marine life, you immediately have to solve a problem that is different to traditional land-based robotics researchers. Most of the really cool videos of the latest robots tend to show how great they are at balancing on two legs, doing some martial art, perhaps, or even gymnastics. Or maybe they are hyping how good they are running through the forest like a wolf, now on four legs. Once you go underwater all that exciting stuff with legs becomes a bit pointless. Now its all about floating not balancing. So what do you do?

The obvious thing perhaps is to just look at boats, submarines and torpedoes and design a propulsion system with propellers, maybe using an AI to design the most efficient propellor shape, then write some fancy software to control it as efficiently as possible. Alternatively, you could look at what the fish do and copy them!

What do fish do? They don’t have propellors! The most obvious thing is they have tails and fins and wiggle a lot. Perhaps your marine robot could be streamlined like a fish and well, swim, its way through the sea. That involves the fish using its muscles to make waves ripple along its body pushing against the water. In exerting a force on the water, by Newton’s Laws, the water pushes back and the fish moves forward.

Of course, your robot is likely to be heavy so will sink. That raises the other problem. Unlike on land, in water you need to be able to move up (and down) too. Being heavy, moving down is easy. But then that is the same for fish. All that fishy muscle is heavier than water so sinks too. Unless they have evolved a way to solve the problem, fish sink to the bottom and have to actively swim upwards if they want to be anywhere else. Some live on the bottom so that is exactly what they want. Maybe your robot is to crawl about on the sea floor too, so that may be right for it too.

Many, many other fish don’t want to be at the bottom. They float without needing to expend any energy to do so. How? They evolved a swim bladder that uses the physics of buoyancy to make them naturally float, neither rising or sinking. They have what is called neutral buoyancy. Perhaps that would be good for your robot too, not least to preserve its batteries for more important things like moving forwards. How do swim bladders do it? They are basically bags of air that give the fish buoyancy – a bit like you wearing a life jacket. Get the amount of air right and the buoyancy, which provides an upward force, can exactly counteract the force of gravity that is pushing your robot down to the depths. The result is the robot just floats under the water where it is. It now has to actively swim if it wants to move down towards the sea floor. So, if you want your robot to do more than crawl around on the bottom, designing in a swim bladder is a good idea.

Perhaps, you can save more energy and simplify things even more though. Perhaps, your robot could learn from ammonites. These are long extinct, dying out with the dinosaurs and now found only as fossils, fearsome predators that evolved a really neat way to move up and down in the water. Ammonites were once believed to be curled up snakes turned to stone, but they were actually molluscs (like snails) and the distinctive spiral structure preserved in fossils was their shell. They didn’t live deep in the spiral though, just in the last chamber at the mouth of the spiral with their multi-armed octopus like body sticking out the end to catch prey. So what were the rest of the chambers for? Filled with liquid or gas, they would act exactly like a swim bladder providing buoyancy control. However, it is likely that, as with the similar modern day nautilus, the ammonite could squeeze the gas or liquid of its spiral shell into a smaller volume, changing its density. Doing that changes its buoyancy: with increased density the buoyancy is less, so gravity exerts a greater force than the lift the shell’s content is giving and it suddenly sinks. Decrease the density by letting the gas or liquid expand and it rises again.

You can see how it works with this simple experiment.

You don’t needs a shell of course, other creatures have evolved more sophisticated versions. A cuttlebone does the same job. It is an internal organ of the cuttlefish (which are not fish but cephalopods like octopus and squid, so related to ammonites). They are the white elongated disks that you find washed up on the beach (especially along the south and west coasts in the UK). They are really hard on one side but slightly softer on the other. They act like an adjustable swim bladder. The hard upper side prevents gas escaping (whilst also adding a layer of armour). The soft lower side is full of microscopic chambers that the cuttlefish can push gas into or pull gas out of at will with the same effect as that of the ammonites shell.

This whole mechanism is essentially how the buoyancy tanks of a submarine work. First used in the original practical submarine, the Nautilus of 1800, they are flooded and emptied to make a submarine sink and rise.

Build the idea of a cuttlebone or ammonite shell into your robot and it can rise and sink at will with minimal energy wasted. Cuttlefish, though, also have another method of propulsion (aside from undulating their body) that allows it to escape from danger in a hurry: jet propulsion. By ejecting water stored in their mantle through their syphon (a tube), they can suddenly give themselves lots of acceleration just like a jet engine gives a plane. That would normally be a very inefficient form of propulsion, using lots of energy. However, experiments show that when used with negative buoyancy such as provided by the cuttlebone, this jet propulsion is actually much more efficient than it would be. So the cuttlebone saves energy again. And a rare ammonite fossil with the preserved muscles of the actual animal suggests that ammonites had similar jet propulsion too. Given some ammonites grew as large as several metres across, that would have been an amazing sight to see!

To be a great robotics engineer, rather than inventing everything from scratch, you could do well to learn from biological physics. Some of the best solutions are already out there and may even be older than the dinosaurs, You might then find your programming task is to program the equivalent of the brain of an ammonite.

– Paul Curzon, Queen Mary University of London

More on …

Thanks to the Dorset Wildlife Trusts at the Chisel Beach Visitor Centre, Portland where I personally learnt about ammonite and cuttlefish propulsion in a really fun science talk on the physics of marine biology.

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This blog is funded by EPSRC on research agreement EP/W033615/1.

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