  The charge on micron-sized dust grains plays a crucial role in the structure
and evolution of forming aggregates within the dust population during the
coagulation process. The manner in which the charge is arranged on developing
irregular structures can affect the fractal dimension of aggregates formed
during collisions, which in turn influences the coagulation rate and size
evolution of the dust cloud. Preliminary models for the charge evolution on
fractal aggregates immersed in a plasma environment calculated using a
modification to the orbital-motion-limited (OML) theory are presented in this
paper. The model calculates currents to each point on the aggregate surface
using a line-of-sight (LOS) approximation: only those electron or ion
trajectories which are not blocked by another grain within the aggregate
contribute to the charging current. Both the total charge and the dipole moment
are calculated for the dust aggregate. While most coagulation theories assume
that it is difficult for like-charged grains to coagulate, the OML_LOS
approximation indicates that the electric potentials of aggregate structures
are often reduced enough to allow significant coagulation.
