Picture an imperfect plane of loose particulate matter like sand or silt. Are there materials where if I shine light (not isotropically) on the plane I could get the particles to vibrate with large enough magnitudes that you could move (at least in a random walk) the particles around? If so what types of materials exhibit this, and how would that work, microscopically?

I can picture this being done with photo-evaporation on certain materials, but I was wondering more so if there are any non-destructive processes that could achieve macroscopic vibrations on a macro-scale. Something probably more analogous to phonon mode excitations as oppose to literally blasting the source particle around as a little thrust engine.

  • $\begingroup$ There are materials that can vibrate due to differential heating (by the light). Does that count? I don't know of ny materials that vibrate due to a direct conversion of the light to acoustic energy. Whether there are such materials, and by what mechanism they could work, strikes me as an interesting question. $\endgroup$ – John Rennie Dec 6 '15 at 8:50

Actually there are several options based on radiation pressure and some very current implementations address your question directly:

Optical tweezers and as yet rudimentary "tractor beams", use the radiation pressure distribution of Bessel beams and helical beams to deliberately move and manipulate microscopic particles. There already is abundant literature on the subject. NASA actually envisions the future use of such "tractor beams" for retrieving dust samples from various outer space environments (comet tails?).

Another idea may be the photoacoustic effect: It initially referred to acoustic vibrations, or sound waves, produced by absorption of light in a solid material, but any modulated electromagnetic radiation can produce a photoacoustic effect, and the material can be gaseous or liquid as well. I imagine if properly configured sound waves can move small particles around, the same goes for photoacoustically generated ones.


There is an effect called "Radiation pressure". Briefly when particles of light hit a surface and get absorbed they transfer their momentum to the surface.

This effect is also responsible for the reason that all the comets' tails are on their opposite side that is towards the sun; sun light hits the particles and push them away to the opposite side.

The other effect is laser cooling. See some details here

However, to answer your question, I should say that this effect needs very low friction and light weight. Another problem is steering your propulsion, which here is always opposite to the light source. For instance if you want to steer a model plane with this technique, then you will have hard time to direct it in the sky, given that your plane is small and light enough, which is a constraint considering the windy days.

Good luck

  • $\begingroup$ Interesting. So photoevaporation and radiation pressure are two different effects you observe when sunlight strikes a comet? That makes sense actually, since if there was just photoevaporation taking place the cloud of ejected material would be in front and behind the comet. $\endgroup$ – Skyler Dec 6 '15 at 8:15
  • $\begingroup$ That being said here I was kind of hunting for an effect thats probably a bit more violent, specifically since it needs to overcome friction between particulate matter. $\endgroup$ – Skyler Dec 6 '15 at 8:21
  • $\begingroup$ in Photo-evaporation the molecules gain energy by absorbing photons and then enough energy to escape. But radiation pressure propel the particles by the transfer of momentum. It is a tricky question and a good one. I should look into literature but I believe they are close phenomena. $\endgroup$ – Amin R. Dec 6 '15 at 8:23
  • $\begingroup$ Thanks for that answer though, I wasn't anticipating learning a bit more about comet dynamics while asking about sand. Physics always is ready to surprise me. $\endgroup$ – Skyler Dec 6 '15 at 8:25
  • $\begingroup$ I don't think you can utilize these methods unless you got a huge source of light or very light stuff. Good luck. $\endgroup$ – Amin R. Dec 6 '15 at 8:26

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