Scale and Cilia

[Johnny Pneumatic]

I'm quite fascinated by ciliated microorganisms. They are like little jet fighters underwater. Question: Can cilia be scaled up and still work like they do at micrometer scales? For instance, a robot the size of a soft ball that is used to explore tight spaces underwater?

 

[Capsid]

I don't think this will work because as you scale up there is a weight issue and the cilia will have to be very large to support their own weight. It's not a linear relationship.

The example I read used Arthur C Clarke's novel called The Fountains of Paradise in which he describes a space elevator. The problem with this is that the structure needed to support this would be very heavy and hence collapse under its own weight. We don't have materials yet that are both strong and light enough.

 

[Johnny Pneumatic]

We don't have materials yet that are both strong and light enough.

Yeah we do. They just currently cost $300 to make a gram of it.(hint: it's carbon nanotubes; a.k.a. fullerene tubes)

 

[Johnny Pneumatic]

I don't think this will work because as you scale up there is a weight issue and the cilia will have to be very large to support their own weight.

Why would they have to support their own weight? The hypothetical robot is in water and has gas cells inside it so it can vary its buoyancy from positive to negative. The cilia are for steering and rapidly changing depth.

 

[Capsid]

Yes, you have a point, the bouyancy would certainly help. But it's still a mass/inertia problem. As the robot gets bigger it has more mass and a thin rope like propulsion unit may not exert enough force to overcome inertia.

Why did this method of propulsion not appear in the evolution in higher organisms? Large animals all use paddles for propulsion.

 

[Goshawk]

But maybe that's only because organisms are under pressure to evolve the most efficient means of propulsion, and maybe it's just that cilia wouldn't be the most efficient means of propulsion for a large organism like, say, a fish, and so no fish have evolved them.

I'm visualizing a fish propelled by cilia: I think it would work, but it would be verrr....rrryyyy...sloooooooooow.... Mass will yield eventually to even the feeblest propulsion, it just takes a while--look at barges on the Mississippi river, they're being pushed by tugboats with relatively small engines (compared to something like an aircraft carrier). The tugboat that went under the bridge was designed to push six barges with a total of 24 million pounds of cargo. It's all about momentum, and being patient.

And any organism that was using cilia for propulsion would have to also evolve some kind of defense system for predators, since "speedy escape" wouldn't be an option. Possibly either a shell like a turtle, or toxicity of some kind.

I don't think that any large organism that was any shape other than totally streamlined would have a hope of making cilia propulsion work.

Even water turtles are streamlined, if you think about it. At least, they're *more* streamlined than box turtles.

 

[SGT]

There is a problem in scaling that was not mentioned:
The mass is proportional to the cube of the dimension, while the propulsion force obtained by a vibrating paddle (or cilion) is proportional to its surface (the square of the dimension).
That's why big animals use fins instead of cilia.