Negative photophoresis

<img src="photophotesis2.png" alt="photophoresis">

Photophoresis is the phenomenon which means transporting particle by light. you maybe think that the particles are pushed by radiation pressure of light. But sometimes NOT, Because some kind of particles show negative photophoresis which means particles move toward the light source. The curious phenomenon not caused by light directly, but caused by local heating of light as same as Crookes radiometer. The heated particles gets reaction force by suspending heat to ambient gas molecules. For example, light wave length size particles and crystal particle would be heated far-side better than the near-side of light source. Of course, these particles are also pressed by the radiation force by absorbed light, but it is much less than radiometric force.

Negative photophoresis of smoke particle.

Smoke particle is useful airosol to visualize light beam. In convergent laser beam, the particles show negative photophoresis and are traped at the focal point simultaneously.

How curious! Laser trapping theory explains that trapping force is caused by pure electromagnetical interaction, i.e. radiation force or dielectric interaction. But generally thought, electromagnetical interaction is negligible for negative photophoretic particles. There have been no theory unified laser trapping and photophoresis yet.

So we measured the photophoretic force field, analizing video image of photophoresis of smoke particles on a small convergent laser beam. To avoid buoyancy-driven air flow, we experimented under the micro-gravity condition on the parabolic-flighting airplain.

Finally, we get the new result, "Photophoretic force scale as the 0.48 power of radiation intensity." Smoke particle is liquid state. So we took Marrangoni convectional drive heat conduction into the recent photophoresis theory, we derived a half power. The result means that smoke particles are subject to the direct electromagnetical interaction near the focal point and subject to the photophoretic force far from the focal point.

  • Appendix of the article 'negative photophoresis of smoke particle'