I found a way to ensure that even if either the Japanese Self-Defense Force or the U.S Army invades my fantasy world, its inhabitants would still have a fighting chance.
Hall thought of it as a nanotechnological replacement for car seatbelts. The robots would be microscopic, with extending arms reaching in several different directions, and could perform three-dimensional lattice reconfiguration. Grabbers at the ends of the arms would allow the robots (or foglets) to mechanically link to one another and share both information and energy, enabling them to act as a continuous substance with mechanical and optical properties that could be varied over a wide range. Each foglet would have substantial computing power, and would be able to communicate with its neighbors.
In the original application as a replacement for seatbelts, the swarm of robots would be widely spread out, and the arms loose, allowing air flow between them. In the event of a collision the arms would lock into their current position, as if the air around the passengers had abruptly frozen solid. The result would be to spread any impact over the entire surface of the passenger's body.
While the foglets would be micro-scale, construction of the foglets would require full molecular nanotechnology. Hall suggests that each bot may be in the shape of a dodecahedron with twelve arms extending outwards. Each arm would have four degrees of freedom. The foglets' bodies would be made of aluminum oxide rather than combustible diamond to avoid creating a fuel air explosive.
Hall and his correspondents soon realised that utility fog could be manufactured en masse to occupy the entire atmosphere of a planet and replace any physical instrumentality necessary to human life. By foglets exerting concerted force an object or human could be carried from location to location. Virtual buildings could be constructed and dismantled within moments, enabling the replacement of existing cities and roads with farms and gardens. While molecular nanotech might also replace the need for biological bodies, utility fog would remain a useful peripheral with which to perform physical engineering and maintenance tasks. Thus, utility fog also came to be known as ″the machine of the future".
The "Magic-users" in my setting are a small group of people, who posses barriers (city-sized, amorphous blobs) made of utility fog. They have full control over their barriers, and usually hide inside them.
One of the nine Magic-users, Raoh (I'm still working on a better name), is particularly fond of the "air-fuel explosive" part, so he turned the hurdle of the combustibility of the foglets into a feature, and dedicated the entire barrier to explode (eventually). Whoever tries to attack him will soon find themselves surrounded by fast-moving blobs of explosive bots (that usually keep some distance to avoid setting off a chain reaction).
So, the foglets are destroyed in the process, as they're the "fuel" part in the air-fuel explosive.
So, my question is, how would foglets be designed in terms of size (they're at around a hundred micrometers, try to stick to that order of magnitude), composition and shape, if I'm trying to optimize them for travel speed and high TNT-equivalence with some stability (not a lot, but a simple fire shouldn't detonate an entire batch)?