Photophoretic forces

In the case of one-sided illumination of a strongly absorbing sphere, the photophoretic force for the slip-flow regime is given by (Reed, 1977)... 

Since the photophoretic force depends on the electromagnetic absorption efficiency Q y , which is sensitive to wavelength, photophoretic force measurements can be used as a tool to study absorption spectroscopy. This was first recognized by Pope et al. (1979), who showed that the spectrum of the photophoretic force on a 10 foa diameter perylene crystallite agrees with the optical spectrum. This was accomplished by suspending a perylene particle in a Millikan chamber with electro-optic feedback control and measuring the photophoretic force as a function of the wavelength of the laser illumination. Improvements on the technique and additional data were obtained by Arnold and Amani (1980), and Arnold et al. (1980) provided further details of their photophoretic spectrometer. A photophoretic spectrum of a crystallite of cadmium sulfide reported by Arnold and Amani is presented in Fig. 11. 

Measurements of the photophoretic force on crystalline ammonium sulfate particles were made by Lin and Campillo (1985) using an electrodynamic balance. The measurement procedure is identical to that for any such force, that is, the levitation voltage is measured in the absence of the photophoretic force and then when the force is exerted. A force balance yields an equation for the photophoretic force similar to Eq. (31) ... 

Fig. 12, Photophoretic force data of Lin and Campillo (1985) for crystalline ammonium sulfate particles levitated in an electrodynamic balance. Reprinted with permission from Lin, H.-B., and Campillo, A. J., Applied Optics 24, 244, Copyright 1985, The Optical Society of America.

The direct photophoretic force Tph for geometrically small particles can be written as a sum of the radiation pressure force and the gradient force F ... 

The practical implementation of photophoretic FFF encounters two major difficulties. First, the generation of substantial photophoretic force over the full length... 

For nonequilibrium host gas, a number of particle forces, arise. These include isothermal drag force, thermal force, photophoretic force, diffusion force, stress force, and other additional cross effects in combined flows of heat, mass, and momentum. 

In this same limit at sufficiently small host-gas pressures, heating of the particle by thermal radiation can become important and (2.85) would be incorrect. Under these conditions the particle can be said to experience also a photophoretic force. This case has been analyzed approximately by HUGHES [2.130]. The photophoretic force will not be discussed in this survey. 

In general, the analysis shows the pronounced and diverse character of the dependences of direct photophoretic forces on both the size and optical parameters of a particle. These features form the fundamental basis for FFF applications. 

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