  A methodology to calculate the friction coefficient of an aggregate in the
continuum regime is proposed. The friction coefficient and the monomer
shielding factors, aggregate-average or individual, are related to the
molecule-aggregate collision rate that is obtained from the molecular diffusion
equation with an absorbing boundary condition on the aggregate surface.
Calculated friction coefficients of straight chains are in very good agreement
with previous results, suggesting that the friction coefficients may be
accurately calculated from the product of the collision rate and an average
momentum transfer,the latter being independent of aggregate morphology.
Langevin-dynamics simulations show that the diffusive motion of straight-chain
aggregates may be described either by a monomer-dependent or an
aggregate-average random force, if the shielding factors are appropriately
chosen.
