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Photophoresis
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.〔Shahram Tehranian et al. 2001. Photophoresis of micrometer-sized particles in the free-molecular regime. International Journal of Heat and Mass Transfer, 44, 1649.〕 Separately from photophoresis, in a fluid mixture of different kinds of particles, the migration of some kinds of particles may be due to differences in their absorptions of thermal radiation and other thermal effects collectively known as thermophoresis. In laser photophoresis, particles migrate once they have a refractive index different from their surrounding medium. The migration of particles is usually possible when the laser is slightly or not focused. A particle with a higher refractive index compared to its surrounding molecule moves away from the light source due to momentum transfer from absorbed and scattered light photons. This is referred to as a radiation pressure force. This force depends on light intensity and particle size but has nothing to do with the surrounding medium. 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. Under certain conditions, with particles of diameter comparable to the wavelength of light, the phenomenon of a negative indirect photophoresis occurs, due to the unequal heat generation on the laser irradiation between the back and front sides of particles, this produces a temperature gradient in the medium around the particle such that molecules at the far side of the particle from the light source may get to heat up more, causing the particle to move towards the light source.〔Hitoshi WATARAI et al. 2004.Migration Analysis of Micro-Particles in Liquids Using Microscopically Designed External Fields. Analytical Sciences, Vol 20, p 423.re〕
Discovery of photophoresis is usually attributed to Felix Ehrenhaft in the 1920s, though earlier observations were made by others including Augustin-Jean Fresnel.
==Applications of 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,〔M. Rosenberg, D.A. Mendis, D.P. Sheehan.1999. Positively charged dust crystals induced by radiative heating, IEEE Trans. Plasma Sci.27, 239-242〕 in the field flow fractionation of particles,〔V.L. Kononenko, J.K. Shimkus, J.C. Giddings, M.N. Myers. (1997). Feasibility studies on photophoretic effect in field flow fractionation of particles, J. Liquid Chromatogr. Related Technol. 20, 2907-2929〕 in the determination of thermal conductivity and temperature of microscopic grains〔X.F. Zhang, E. Bar-Ziv. (1997). A novel approach to determine thermal conductivity of micrometre-sized fuel particles, Combust. Sci.Technol.130, 79-95.〕 and also in the transport of soot particles in the atmosphere.〔H. Rohatschek. (1997). Levitation of stratospheric and mesospheric aerosols by gravito-photophoresis, J. Aerosol Sci. 27,467-475.〕 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.〔C. Helmbrecht, C. Kykal, C. Haisch. Photophoretic Particle Separation in `Institute of Hydrochemistry, Annual report, 2006. p11〕
Recently, photophoresis has been suggested as a chiral sorting mechanism for single walled carbon nanotubes.〔D. Smith. (2014). Photophoretic separation of single-walled carbon nanotubes: a novel approach to selective chiral sorting, Phys. Chem. Chem. Phys., 2014,16, 5221-5228.〕 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.
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