  Cell crawling crucially depends on the collective dynamics of the acto-myosin
cytoskeleton. However, it remains an open question to what extent cell
polarization and persistent motion depend on continuous regulatory mechanisms
and autonomous physical mechanisms. Experiments on cell fragments and
theoretical considerations for active polar liquids have highlighted that
physical mechanisms induce motility through splay and bend configurations in a
nematic director field. Here, we employ a simple model, derived from basic
thermodynamic principles, for active polar free-surface droplets to identify a
different mechanism of motility. Namely, active stresses drive drop motion
through spatial variations of polarization strength. This robustly induces
parity-symmetry breaking and motility even for liquid ridges (2D drops) and
adds to splay- and bend-driven pumping in 3D geometries. Intriguingly, then,
stable polar moving and axisymmetric resting states may coexist, reminiscent of
the interconversion of moving and resting keratocytes by external stimuli. The
identified additional motility mode originates from a competition between the
elastic bulk energy and the polarity control exerted by the drop surface. As it
already breaks parity-symmetry for passive drops, the resulting back-forth
asymmetry enables active stresses to effectively pump liquid and drop motion
ensues.
