Forces on a small grain in the nonlinear plasma wake of another.

The transverse force on a spherical charged grain lying in the plasma wake of another grain is analyzed to assess the importance of ion-drag perturbation, in addition to the wake-potential-gradient. The ion-drag perturbation is intrinsically one order smaller than the wake-potential force in the ratio of grain size (r(p)) to Debye length (λ(De)). So ion-drag perturbation is important only in nonlinear wakes. Rigorous particle-in-cell calculations of the force are performed in the nonlinear regime with two interacting grains. It is found that even for quite large grains, r(p)/λ(De)=0.1, the force is dominated by the wake-potential gradient. The wake-potential structure can then help explain the preferred alignment of floating dust grains.