  Context: Aligned dust grains are commonly exploited to probe the magnetic
field orientation. However, the exact physical processes that result in a
coherent large-scale grain alignment are far from being constrained. Aims: In
this work, we aim to investigate the impact of a gas-dust drift leading to a
mechanical alignment of dust (MAD) and to dust polarization. Methods: We
explore fractal dust aggregates to statistically analyze the average alignment
behavior of distinct grain ensembles. The spin-up efficiencies for individual
aggregates are determined utilizing MC simulations. These efficiencies are
analyzed to identify stable points for the grain alignment in direction of the
gas-dust drift and along the magnetic field lines. Finally, the net dust
polarization is calculated per grain ensemble. Results: The mechanical spin-up
within the CNM is sufficient to drive grains to a stable alignment. A likely
mechanical grain alignment is parallel to the drift direction. All grains can
align at subsonic conditions. Here, we predict a polarization efficiency in the
order of unity for the MAD. A supersonic drift may result in a rapid rotation
where dust grains may become rotationally disrupted and the polarization
becomes drastically reduced. In the presence of a magnetic field, the drift
required for the alignment of elongated grains is roughly one order of
magnitude higher compared to the pure MAD. Here, the dust polarization
efficiency is 0.6-0.9 indicating that a drift can provide the prerequisites to
probe the magnetic field. The alignment is inefficient when the direction of
the drift and the field lines are perpendicular. Conclusions: We find that MAD
has to be taken into consideration as an alternative driving mechanism where
the standard RAT alignment theory fails to account for the full spectrum of
available dust polarization observations.
