  In earlier studies we have proposed that most parton $v_2$ comes from the
anisotropic escape of partons, not from the hydrodynamic flow, even for
semi-central Au+Au collisions at $\sqrt {s_{NN}}=200$ GeV. Here we study the
flavor dependence of this escape mechanism with a multi-phase transport model.
In contrast to naive expectations, we find that the charm $v_2$ is much more
sensitive to the hydrodynamic flow than the lighter quark $v_2$, and the
fraction of $v_2$ from the escape mechanism decreases strongly with the quark
mass for large collision systems. We also find that the light quark collective
flow is essential for the charm quark $v_2$. Our finding thus suggests that
heavy quark flows are better probes of the quark-gluon-plasma properties than
light quark flows.
