  Dust grains emit intrinsic polarized emission if they are elongated and
aligned in the same direction. The direction of the grain alignment is
determined by external forces, such as magnetic fields, radiation, and gas flow
against the dust grains. In this letter, we apply the concept of the grain
alignment by gas flow, which is called mechanical alignment, to the situation
of a protoplanetary disk. We assume that grains have a certain helicity, which
results in the alignment with the minor axis parallel to the grain velocity
against the ambient disk gas and discuss the morphology of polarization vectors
in a protoplanetary disk. We find that the direction of the polarization
vectors depends on the Stokes number, which denotes how well grains are coupled
to the gas. If the Stokes number is less than unity, orientation of
polarization is in the azimuthal direction since the dust velocity against the
gas is in the radial direction. If the Stokes number is as large as unity, the
polarization vectors show a leading spiral pattern since the radial and
azimuthal components of the gas velocity against the dust grains are
comparable. This suggests that if the observed polarization vectors show a
leading spiral pattern, it would indicate that Stokes number of dust grains is
around unity, which is presumably radially drifting.
