Representation and calculation of photophoretic forces and torques

For the free molecule regime analytical expressions for the photophoretic force and torque can be derived which do not explicitly involve the surface temperature but directly contain the heat sources. This makes it possible to easily calculate photophoretic forces and torques, even for non-spherical particles. For the expressions certain weight functions are required, which are either explicitly known or can be computed numerically. It is shown how the approach carries over to the continuum limit, where the medium surrounding the particle is assumed to satisfy the Stokes equations of fluid mechanics.