Photophoresis denotes the phenomenon that small particles suspended in gas (aerosols) or liquids (hydrocolloids) start to migrate when illuminated by a sufficiently intense beam of light.
The existence of this phenomenon is owed to a non-uniform distribution of temperature of an illuminated particle in a fluid medium.
In laser photophoresis, particles migrate once they have a refractive index different from their surrounding medium.
This force depends on light intensity and particle size but has nothing to do with the surrounding medium[clarification needed].
Discovery of photophoresis is usually attributed to Felix Ehrenhaft in the 1920s, though earlier observations were made by others including Augustin-Jean Fresnel.
[7] Recently, photophoresis has been suggested as a chiral sorting mechanism for single walled carbon nanotubes.
[8] The proposed method would utilise differences in the absorption spectra of semiconducting carbon nanotubes arising from optically excited transitions in electronic structure.
In 2021 Azadi, Popov et al. report "light-driven levitation of macroscopic polymer films with nanostructured surface as candidates for long-duration near-space flight" Using a light intensity comparable to sunlight, they levitated centimeter-scale disks made of commercial 0.5-micron-thick mylar film coated with carbon nanotubes on one side.
[9] Experiments by Schafer, Kim, Vlassak and Keith suggest that photophoretic forces could levitate thin 10 centimetre-scale structures in Earth′s stratosphere indefinitely for the purpose of atmospheric science, especially monitoring high-altitude weather.
They describe in 2022 a preliminary design fabricated with available methods of a 10 cm diameter device combining a levitating structure of two membranes[10] 2 μm apart in a stiff support structure tested to have sufficient strength to withstand transport, deployment, and flight at 25 km altitude.
[11] Direct photophoresis is caused by the transfer of photon momentum to a particle by refraction and reflection.
In this situation the surrounding gas layer reaches temperature equilibrium with the surface of the particle.
Molecules with higher kinetic energy in the region of higher gas temperature impinge on the particle with greater momenta than molecules in the cold region; this causes a migration of particles in a direction opposite to the surface temperature gradient.
Indirect photophoretic force depends on the physical properties of the particle and the surrounding medium.
of the suspended particle (direct photophoresis), the longitudinal force is [14] where the mean temperature of the scattered gas is (thermal accommodation coefficient