# What would limit the magnitude of force a utility fog (aerosol swarm of tiny bots) can generate?

So, utility fogs have been around for quite a while in my world, mainly to patch up whatever holes I had in my magic system:

• If the Fireball really is just an M67 nade, then what propels it 122 meters forward?
• How can my characters summon festive hats out of nothing?
• Who killed Laura Palmer?

Utility fogs are swarms of tiny robots, called foglets. Foglets are 100 micrometers in diameter (The world's smallest insect is like 186 micrometres long on average), including their arms. These arms are used to exert force and grab other foglets.

Image is from: https://commons.m.wikimedia.org/wiki/File:Foglet.jpg

While the proposal says that they can exert any force anywhere, in the fog, but it doesn't mention the approximate magnitude. In our case, if it's a neat but harmless tool or Lucy from Elfen Lied(NSFW/Edgy).

What elements of the foglets' construction limit/determine the maximum pressure the fog could generate under standard circumstances (part of the fog can push against an immovable solid wall, placed perpendicular to the direction of the force)?

• What's a utility fog? Commented Sep 1, 2018 at 20:45
• Dude, that document was purely theoretical and far FAR from reality. The reason it glanced over certain aspects you cared about was because the author obviously didn't want to dig deep into the realities of such a system. To even attempt addressing things like force or pressure or even capability you need to address things like how they move how they get power, let alone things like what they are made of and what components they have.
– anon
Commented Sep 1, 2018 at 21:12
• You have to tell us the grip strength of these foglets, and the tensile strength of a foglet's structure. Commented Sep 2, 2018 at 1:14
• @kingledion "Thus Utility Fog as a bulk material will have a density (specific gravity) of 0.2; for comparison, balsa wood is about 0.15 and cork is about 0.25. Fog will have a tensile strength of only 1000 psi; this is about the same as low-density polyethylene (solid, not foam). The material properties arising from the lattice structure are more or less isotropic; the one exception is that when Fog is flowing, tensile strength perpendicular to the shear plane is cut roughly in half." Commented Sep 2, 2018 at 8:39
• @anon "Foglets run on electricity, but they store hydrogen as an energy buffer. We pick hydrogen in part because it's almost certain to be a fuel of choice in the nanotech world, and thus we can be sure that the process of converting hydrogen and oxygen to water and energy, as well as the process of converting energy mid water to hydrogen and oxygen, will be well understood. That means we'll be able to do them efficiently, which is of prime importance." Commented Sep 2, 2018 at 8:50

What elements of the foglets' construction limit/determine the maximum pressure the fog could generate under standard circumstances (part of the fog can push against an immovable solid wall, placed perpendicular to the direction of the force)?

Take a scissors in your left hand. Another in your right hand. Have them grab each other by the blades. Pull them apart. Easy.

That is the problem with the foglets. The foglet claws are crap for grabbing one another. They look like shears and not even pinking shears that have a little serration. There is no grip, or serration; they have smooth edges. The interface claw to claw is thin and so will have minimal friction. There is no male and female like Legos or Velcro. They cannot curl around each other like human hands. Whoever made your image (and you should cite the source for your images) intended these claws to grab soft things, like stuffed animals in a 50 cent to play stuffed animal grabber game. There is no good way these claws can interlock with one another.

Because the foglets as depicted cannot effectively interlock their arms, the force they can exert as a group will be negligible.

• I think the interlock is probably the smallest of the many larger problems in the concept. Like how they move, or how they find eachother to even engage in interlock.
– anon
Commented Sep 1, 2018 at 21:16
• These things are tiny, but the friction of their interface multiplies. Lay 2 sheets of paper over each other and you can pull them apart without a problem. Interlock all pages of 2 telephone books and you cannot pull them apart without ripping the books apart. See the MythBustets trying their hand
– Elmy
Commented Sep 2, 2018 at 6:18
• If I became agitated enough, I'd say either magnets or geckos. Commented Sep 2, 2018 at 8:42

The force is not only or exactly a function of the strength of interconnection of particles, so that

• can exert any force anywhere, in the fog

isn't that simple to answer. An example of weakly interconnected media which can exert different forces is the water. The pressure at depth, understandable, but we also use water to cut metals(not exactly directly, but even without abrasive in it water jets are strong enough to easily cut flesh and stuff)

So in terms of interaction, with a human body as they suggest some bridge between VR and reality - you may not necessarily need a strong interconnection of your actuators to achieve sufficient results as limits of what you need is the strength of another participant - the human body.

The strength also depends on the density of the stuff.

For 0.2 t/m3, they mantion 1000PSI which is about 6.9MPa. Generally, It is a believable strength for such a structure, and about 50 times less than aluminum strength, as an example.

It is more than enough to simulate any environment a human may interact, and tensile strength for muscle tissue isn't that high, and well below the number(even I failed to sort out Google results to find the numbers, even I do remember seeing some, but it needs to dig through different pdf's. It is one of the subjects for different biomedical studies.)

So as a simulation the stuff is good enough, even if for sure we could suggest some improvements in that regard from a position of our time - like a direct supply of visual information to eye's, without the needs to have sparse fog density to mix for air and be transparent and stuff.

The highest imaginary tensile strength mentioned in the fog pdf is 6000PSI which is about 42MPa. And if we imagine a non-simulating setting, considering the Lucy scenario, it is, most likely, not enough. Not that is it not enough, it may be not enough if the stuff applied in a direct fashion, hard to describe but nothing stops the thing to use some shards of materials, by gripping/wrapping around them and use them as a tool, and then if they can have 42MPa strength for action it may lead to very powerful scenarios, far from invisible force, but with respectable destructive strength.

A diamond shard(let's be exotic with examples), some cone with 1cm2 base and 3cm height, will be 1 cubic centimetre, thus 3.51g mass. Applying 42MPa to the base(pushing), for a distance 1m of acceleration "track", will result in it having a velocity of 1540 m/s Even a lesser 6.9MPa strength, may lead to 620 m/s

You really can play a walking gun machine with those guys.

Manila rope has about 69MPa strength and a lot of stuff can be done with that alone.

But all that isn't a Lucy scenario, you need something better for that I present you a Smart Matter - strains of carbon nanotubes joint like muscles, not those piny grapples of micrometer stuff, but micrometer diameter threads with 100+GPa strength and arbitrry length(meters, 10's m and more). So it 2/3 nanostructures(two dimensions are nano, one is macro) It can do whatever it is imagined fog can do, but better, and in a bit different way. Just like a side note, I do not have a better description of that stuff than the link, but it at least something.

So generally if you need to imagine some force, take a number like 40MPa at its dense state and then % the density of those things in the place multiplied by that number multiplied by the surface area of where it pushes.

7-40MPa is a reasonable strength for the potential fog material. But exact numbers - we won't have them until we design the stuff, or build it.

• That's interesting, but how are you supposed to control the threads or arrange them to be effective in a three-dimensional space? Can I have Kikiki huna like biomimetic robots move them around? Commented Sep 4, 2018 at 16:15
• @Mephistopheles similar to snakes and worms arrange themselves in 3d space. stressing certain parts of the "rope". It is not plain CNT's stuck together, it needs weaving them in some basic construction which can be controlled. cnt's diameter of tubes is small so you may have millions of them weaved in some configuration to create that basic block which can be controlled, and that basic block can be um size in diameter. As a source of forces - charge, current, ions interacting etc. Commented Sep 5, 2018 at 14:33