I heard about this on a Science 360 broadcast this morning. A laser pointed at a graphene sponge propels it forward. (not quite the same as skysail) The researchers have several theories as to why. They do note the sponge emits electrons after being charged with the laser. My read says the electrons are only emitted from one side, the side the laser hits. Hmm? Also works with focused sunlight and different wavelengths.
My question and it is probably to simple for the researches not to have pointed out; Does a laser beam have a charge? If yes, wouldn't the like charges repel, causing the movement? (negatively charged laser beam negatively charged graphene sponge repel each other)
They did discover that a graphene sheet (read one molecule thick) has a lattice far larger than a Helium atom, yet no Helium will pass through it. They are talking about using it to line balloons with.
Perhaps the light impinges in a similar way yet will not pass.
On a sunny day (Sat, 13 Jun 2015 07:12:19 -0500) it happened amdx wrote in :
Without looking I do remember that burning of material by laser from an object in space causes the action - reaction to push uit away. That is very old and tried. Ahy should sponges be different?
They say it is not burning, 1 watt laser on graphene that has been prepped at 800C. It's an easy read, not that I have a great understanding of the subject. Take a minute and check it out. Mikek
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So,you are saying, all the electrons in the graphene will be depleted then you're coasting?
That's logical, but, I did not see that the graphene fell in the vertical vacuumed tube because of depletion. See page 4 illustration for Vertical tube, no mention of running out of charge.
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No, it'll peter out well before that. A 1-cm cube in free space has a capacitance of a bit under 1 pF (a 1-cm radius sphere is 1.12 pF iirc).
Saying 1 pF to be generous, it will charge to a million volts after emitting 1 microcoulomb of electrons, which weighs about 5 femtograms.
If it's photoemission from visible light that's causing the effect, it'll shut down much earlier than that, at about 2 volts, because the emitted electrons can't have more kinetic energy larger than 2 eV, and so can't escape from the potential well.
The mass of an electron (mc**2) is 511 keV, or 9E-31 kg. If KE = 0.5 mv**2 = 2 eV = 3.2E-19 J, the momentum is at most
p = mv = sqrt(2 m KE) = sqrt(2 * 9E-31 kg * 3E-19 J) = 8e-25 kg m/s per electron, or 4e-6 kg m/s per coulomb.
If the sponge weighs 10 mg (1E-5 kg), that sounds medium zippy--a coulomb of electrons would get the sponge moving at
v = p/m = (4E-6 kg m/s)/( 1e-5 kg) = 0.4 m/s.
However, at 2 volts that 1 pF only gets you 2 pC, i.e. enough electrons to get the sponge moving at 0.8 pm/s, i.e. one hydrogen atom diameter every two minutes.
And it's still rocket propulsion.
There is no way in the world that electron emission from an insulated solid body can support its weight. It's off by many orders of magnitude (see above). The total specific impulse available would support its mass against gravity for 0.8 pm/s / 9.8 m/s**2 = 80 femtoseconds.
The classic spinning pinwheel radiometer was assumed to spin from light pressure, until someone noticed that the direction was wrong. It turned out to be a thermal effect against the gas in the bulb.
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John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
As Phil notes, the numbers don't work. As far as space propulsion goes, it's just a very inefficient electron gun. Ions work a lot better. A xenon atom has a lot more mass than an electron. To keep the spacecraft from charging up positive, and sucking the electrons back, you need to fire out some protons anyhow.
I hope the graphene fad ends soon, along with buckyballs and nanotubes and stuff. I wonder what's next.
Fig 1 looks to me like a piston in a cylinder, pressurized from below by crud boiled out from the graphene sponge.
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John Larkin Highland Technology, Inc
picosecond timing laser drivers and controllers
My guess would be the same as JL's, i.e. that they're boiling water out of the pores of the sponge. That sucker must have a _lot_ of surface area. Hanging a residual gas analyzer on that vacuum system would tell the tale in pretty short order, I expect.
I read a little farther and see he did his own math on page 15. Starting with; With all these experimental results, the remaining question is whether the kinetic energy generated by the ejected electrons is large enough to move/propel the sample.
I haven't read it all through. If they had a wire on the sample, so they didn't run out of electrons in 80 fs, it could be somewhat better. However, electrons are way too light to make good reaction mass for a rocket--it takes way too much energy to get a given amount of momentum. (Photons are dramatically worse, of course.)
("Bulk graphene" was the tipoff. Sheesh. As though we didn't all use that stuff in pencils as kids.)
It says they are using "stacked sheets" of graphene, so many 2D sheets of graphene making a cuboid, with a small non-zero bandgap between each sheet, and that this structure is why the propulsion effect occurs, and with other materials that don't maintain the 2D structure, ie carbon nanotubes, there is no propulsion effect.
"our bulk graphene material could be treated as a macroscopic collection of many electronically isolated individual graphene sheets, thus a bulk scale sum of such a photoresponse should be observed due to the macroscopic addition of many individual and suspended graphene sheets in this unique graphene material"
I guess each sheet attenuates the light so there are diminishing returns with more thickness.
Also they weighed the sample before and after (to check for ablation, but would also work to check for water loss) and noticed no weight change.
If they can do a test with the laser at constant power, and a frequency sweep past the threshold to trigger a photoelectron emission, and notice the levitation occuring at that point, I think that would settle it maybe, but they did do multiple laser frequency tests too.
The mechanism can't possibly be what they're claiming. A whole coulomb gets them 0.4 m/s specific impulse with a 10-mg mass, so in order to levitate that 10 milligrams they'd need a current of
I = g/specific impulse per coulomb = 9.8 m/s**2 / 0.4 m/s/C = 40 amps.
That would be a pretty spectacular show, briefly. Not to mention needing a wire large enough to support all of the authors' weight.
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