Photophoresis

The applications of photophoresis expand into the various divisions of science, thus physics, chemistry as well as in biology. Photophoresis is applied in particle trapping and levitation,[3] in the field flow fractionation of particles,[4] in the determination of thermal conductivity and temperature of microscopic grains[5] and also in the transport of soot particles in the atmosphere.[6] The use of light in the separation of particles aerosols based on their optical properties, makes possible the separation of organic and inorganic particles of the same aerodynamic size.[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. If developed the technique would be orders of magnitudes faster than currently established ultracentrifugation techniques.

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. Payload capacity is 300 mg and could support bidirectional radio communication at over 10 Mb/s and some navigational control. By upscaling the structure it might carry payloads of a few grams. They suggest uses for telecommunications, and deployment on Mars.[11]

Direct photophoresis is caused by the transfer of photon momentum to a particle by refraction and reflection.[12] Movement of particles in the forward direction occurs when the particle is transparent and has an index of refraction larger compared to its surrounding medium.[7] Indirect photophoresis occurs as a result of an increase in the kinetic energy of molecules when particles absorb incident light only on the irradiated side, thus creating a temperature gradient within the particle. 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. The component of the photophoretic force responsible for this phenomenon is called the radiometric force.[13] This comes as a result of uneven distribution of radiant energy (source function within a particle). Indirect photophoretic force depends on the physical properties of the particle and the surrounding medium.

For pressures ${\displaystyle p}$, where the free mean path of the gas is much larger than the characteristic size ${\displaystyle r_{0}}$ of the suspended particle (direct photophoresis), the longitudinal force is [14]

${\displaystyle \mathbf {F} _{\text{phot}}=-{\frac {\pi }{3}}\,\alpha \,\alpha _{\text{m}}{\frac {p}{\sqrt {{\overline {T_{\text{gas}}^{\text{out}}}}\,T_{\text{gas}}^{\text{in}}}}}\,r_{0}^{2}\,{\frac {I\,J_{1}}{{\frac {k}{r_{0}}}+4\sigma _{\text{SB}}\varepsilon \,T_{\text{black body}}^{3}}}\,\mathbf {e} _{z}}$

where the mean temperature of the scattered gas is (thermal accommodation coefficient ${\displaystyle \alpha }$, momentum accommodation coefficient ${\displaystyle \alpha _{\text{m}}}$)

${\displaystyle {\overline {T_{\text{gas}}^{\text{out}}}}=T_{\text{gas}}^{\text{in}}+\alpha \left(T_{\text{black body}}-T_{\text{gas}}^{\text{in}}\right)}$

and the black body temperature of the particle (net light flux ${\displaystyle I=\varepsilon \,I_{0}}$, Stefan Boltzmann constant ${\displaystyle \sigma _{\text{SB}}}$, temperature of the radiation field ${\displaystyle T_{\text{opt}}}$)

${\displaystyle T_{\text{black body}}={\sqrt[{4}]{{\frac {I_{0}}{4\sigma _{\text{SB}}}}+T_{\text{opt}}^{4}}}}$.

${\displaystyle k}$ is the thermal conductivity of the particle. The asymmetry factor for spheres ${\displaystyle J_{1}}$ is usually ${\displaystyle 1/2}$ (positive longitudinal photophoresis). For non-spherical particles, the average force exerted on the particle is given by the same equation where the radius ${\displaystyle r_{0}}$ is now the radius of the respective volume-equivalent sphere.[15]

• Photophoresis in the context of astrophysics
• Negative photophoresis