Photophoresis

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This article is about the physical phenomenon photophoresis. For the medical treatment of blood see: photopheresis.

In physics, photophoresis is the phenomenon that intense light induces motion in particles suspended in a gas. The causes are generally distinct from radiation pressure. Just like in Crookes radiometer, light can heat up one side and gas molecules bounce from that surface with greater velocity, hence push the particle to the other side.

One should note that under certain conditions (for example with particles of length comparable to the wavelength of light), the far side from the light source may heat up more, and the particle moves towards the source. This phenomenon is called negative photophoresis.

Discovery is usually attributed to Felix Ehrenhaft, though earlier observations were made by others including Augustin-Jean Fresnel.