Photophoretic force on aerosol particles in the free-molecule regime

Numerical results have been obtained for the photophoretic force on spherical particles in the free-molecule regime using the Lorenz–Mie solution for the electromagnetic field inside the particle to obtain the heat-source function. The size parameter ranges up to α = 2πa/λ = 7 for various values of the complex refractive index. The force is initially positive, then becomes negative, and again becomes positive with increasing size parameter. The effect of the complex refractive index is discussed. For the parameters explored here, the photophoretic force may become as large as 4% of the gravitational force.