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
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.
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,
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.