A key ingredient for single particle diffractive imaging experiments is the
successful and efficient delivery of sample. Current sample-delivery methods
are based on aerosol injectors in which the samples are driven by fluid-dynamic
forces. These are typically simulated using Stokes' drag forces and for
micrometer-size or smaller particles, the Cunningham correction factor is
applied. This is not only unsatisfactory, but even using a temperature
dependent formulation it fails at cryogenic temperatures. Here we propose the
use of a direct computation of the force, based on Epstein's formulation, that
allows for high relative velocities of the particles to the gas and also for
internal particle temperatures that differ from the gas temperature. The new
force reproduces Stokes' drag force for conditions known to be well described
by Stokes' drag. Furthermore, it shows excellent agreement to experiments at 4
K, confirming the improved descriptive power of simulations over a wide
temperature range
