I read Michael Crichton’s (and Richard Preston’s!) Micro last week and it’s not spoiling anything to say that it involves shrinking human beings ala Fantastic Voyage or Honey I Shrunk the Kids or Incredible Shrinking Man or etc…
Anyway, they’re 1/100 scale and there’s a scene where one of the characters makes a small fire using chopped up candlenuts. Now I don’t have any of those to play with but I have played with fire a lot (even making fire from scratch in the scouts) and it seems to me that a fire (in terms of an actual flame) can only be so small. The book didn’t describe the fire in very much detail but I would imagine that it would have had to have been huge to these half-inch tall humans, more like a bonfire of a single flame.
So is there a limit or must there be some minimum size to a flame?
if it’s unsustainable (a one-off combination between oxygen and a volatile carbon-based substance) then it’s down to the molecular level.
but one’s basic notion of a flame would be a sustainable combustion of volatiles under normal atmospheric conditions. no cheating allowed like increasing ambient oxygen, or a forced/pressurized induction of air and combustibles. i would say a 3mm diameter flame such as the kind that one sees as a diposable lighter runs out or the flames that disappear from a burning piece of wood that runs out of volatiles, leaving just the carbon to glow red. it’s very difficult to make a contraption to produce a flame smaller than 3mm because the fuel you use (say methane or alcohol) will expand at STP to produce a sustained flame. i don’t know if you can produce a small flame using hydrogen gas.
Yeah, that’s pretty much what I thought given the parameters of the story i.e. plant fuel (candle-nut), current atmospheric makeup, etc. If this ever becomes a movie, I hope that they get that detail right: that a single 3mm flame would look pretty strange to an 18 mm person.
I’m not certain about the size question, but surely the length of time the fire keeps burning would also be an issue. If they just chop up tiny fire materials they are essentially making kindling, and I would thing the fuel would be burned up in a few seconds unless they had some method to artificially suppress the size of the fire.
your better question is how an 18mm-tall man would start a fire. supposing he had a match or lighter that fits his hand. those things would light up very briefly (like a flash) or will result in a flame out of proportion to the size of his hand (or his entire body.)
Yeah, I don’t have any candlenuts to play with. ATW (according to wiki) one nut will burn for fifteen minutes or so and was even used as a time piece. Presumably, a chopped up nuts would burn proportionally quicker but the story made it sound like throwing logs on a fire. Yeah, probably not much point in nitpicking a story where people get shrunk 100X.
Short answer is tools. It’s probably a small spoiler so since the book was just released… the character who starts the fire is a hermit who has been surviving in the micro world as they call it for years. He has tools that were also shrunk, including a gas torch he used to start his fireplace. Now, fluid dynamics at 10^-2 scale would also be problematic I would suppose. Things just don’t scale like that.
The strength of the story was the biology and chemistry more than the physics and mechanics I suppose.
Sustainable fire exists as a balanced system where the heat input and heat output are matched. Below a certain threshold, this can’t happen as the system can’t generate enough heat to keep the fire sustained and it will flicker out.
Well, if it makes you feel any better, a human body probably wouldn’t scale down that well either. I know that if a human body were scaled up beyond a certain point it’d be crushed by its own weight.
Yeah, I think a fire can’t scale down indefinitely. Rough estimate is that the heat escaping is going to scale with the surface area (square of the length), while the heat produced scales with the volume (cube of the length). So a 1/10 scale fire would lose heat at 1/100 the rate of the full scale, but produce heat at 1/1,000 the rate. As you scale down, eventually the fire can’t stay hot enough to sustain a flame.
There may be some fluid dynamics effects as well, as air gets relatively more viscous as you scale down. But I 'd guess the heat problem is the limiting factor.
I used to use a lot of butane lighters back when I smoked. If I had a lighter that was unlightable due to low fuel, I could take off the metal cover around the valve and still get a small flame. The smallest flame I could get was about half the diameter of a BB.