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.
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