The photophoretic force in the free-molecular regime has been calculated fo
r a spherical particle using the Lorenz-Mie solution to the electromagnetic
field within the particle. The temperature distribution on the surface of
the suspended particle is calculated using a finite difference method. The
effect of the complex refractive index m = n + ik and the normalized size p
arameter defined as alpha = 2 pia/lambda on the photophoretic force and par
ticle velocity are also examined. We show that far a 1 solar constant illum
ination, the photophoretic forces might be as high as 20% of the weight of
the particles considered. (C) 2001 Published by Elsevier Science Ltd.