A single drop of fluid can contain billions of bacteria swimming around inside it. For the most part, the movements of these bacteria are random and chaotic. But if you look at them under the microscope, you begin to see patterns emerging – swirls and vortices that come in and out of existence as groups of bacteria briefly swim in the same direction.
Scientists refer to these kinds of systems as ‘active matter’. They are made up of large numbers of active individuals that take energy from their surroundings to move around. which means the system is out of thermal equilibrium.
Because of this, interesting collective behaviour can spontaneously arise as the individuals interact with each other in complex ways, like in flocks of birds or shoals of fish.
Under the right conditions, organised collective behaviour can also be seen in bacterial suspensions. For example, if you let bacteria swim round an array of small wells, they begin to move in a synchronised way, and a regular pattern arises.
A few months ago, we recorded a podcast about this collective swimming behaviour. Professor Raymond Goldstein explained that if you make the wells the right size, you spontaneously get order out of chaos: the bacteria in each well all swim around in the same direction, and every well spins in the opposite direction to its neighbour.
Now, another team of researchers from the University of Oxford is looking at how we could harness the collective energy of these bacteria to drive an array of circular rotors.
“The human body is brilliant at extracting mechanical work from chemical energy – that’s how we move around, it’s why we’re alive,” says Professor Julia Yeomans, lead author on the paper. “We’d really like to understand how this works. So we did a computer simulation of active material surrounding little circular rotors.”
Bacterial suspensions display something that looks a lot like turbulence – the movement of the bacteria is very random, and it’s too disordered to extract any useful energy from. But if you put them into an array of rotors, they self-organise themselves to swim in a pattern like a tiny wind farm.
“Putting the bacteria in this particular geometry, they can drive the rotors,” explains Julia. “The rotors make the bacteria move in an ordered way – if they weren’t there, the movement would be random.”
In the array, neighbouring rotors spun in the opposite direction to each other. This is just like the experiments with the array of wells from Raymond Goldstein’s lab (the arrangement is called ‘antiferromagnetic’). And here too, the physical dimensions of the rotors were important.
“The rotors help the bacteria to organise themselves into vortices that have a certain length scale, or diameter,” says Julia. Once whirlpools emerge of the right size, they stay there. The whirlpools effectively stabilise the system, so the rotors keep turning.
This means that, in the future, this type of setup could be used as a kind of ‘bacterial wind farm’ that could generate electricity and power tiny devices. “It’s important to say this is not free energy – you have to feed the bacteria,” says Julia. “But having said that, these motors in cells are incredibly efficient.”
The next step would be to move away from the computer simulation to see if the array can actually be built, and whether real bacterial suspensions behave the same way.
“There are lots of possibilities – but we’re a long way from that,” says Julia. “This is just an example to show it might be possible one day to exploit nature to transform energy into useful work.”