Distribution function P_∆s(ϑ). The gas-dust drift of Δs is antiparallel with the axis ê₁ while the dust grain is situated in the origin of the lab-frame. For a gas-dust drift of Δs = 0 the gas velocity field is isotropic concerning the trajectories of individual gas particles. With Δs > 0 the gas is more likely to approach the gain with an trajectory parallel to Δs. Note that for a drift with Δs ≥ 1 the probability to find a gas particle approaching the dust under an angle of ϑ = 180° is already virtually zero.