  We present observations of the formation process of a small-scale filament on
the quiet Sun during 5-6 February 2016 and investigate its formation cause.
Initially, a small dipole emerged and its associated arch filament system was
found to reconnect with overlying coronal fields accompanied by numerous EUV
bright points. When bright points faded out, many elongated dark threads formed
bridging the positive magnetic element of dipole and external negative network
fields. Interestingly, an anti-clockwise photospheric rotational motion (PRM)
set in within the positive endpoint region of newborn dark threads following
the flux emergence and lasted for more than 10 hours. Under the drive of the
PRM, these dispersive dark threads gradually aligned along the north-south
direction and finally coalesced into an inverse S-shaped filament. Consistent
with the dextral chirality of the filament, magnetic helicity calculations show
that an amount of negative helicity was persistently injected from the
rotational positive magnetic element and accumulated during the formation of
the filament. These observations suggest that twisted emerging fields may lead
to the formation of the filament via reconnection with pre-existing fields and
release of its inner magnetic twist. The persistent PRM might trace a covert
twist relaxation from below photosphere to the low corona.
