  We have developed an implementation for the momentum transfer force in
numerical two fluid hydrodynamics. This form of the frictional coupling between
gas and grains is consistent with the microscopic interactions between the two
components. The coupling force gives rise to a drift velocity of the grains
with respect to the gas. We apply this mechanism to the outflow of (Post-) AGB
objects. Our numerical hydrodynamics code calculates self consistently the
dynamics of these outflows, as well as the nucleation and growth of grains and
equilibrium chemistry of the gas. Grain nucleation and growth are processes
that depend strongly on the rate of gas-grain collisions. Hence, the drift
velocity becomes an important variable. The tight connection between grain
chemistry and drift causes the system to become extremely sensitive to small
changes in almost any parameter. This may be a cause for deviation from
(spherical) symmetry and structure.
