  The time evolution of both proton and anti-proton $v_2$ flows from Au+Au
collisions at $\sqrt{s_{NN}}$=7.7 GeV are examined by using both pure cascade
and mean-field potential versions of the UrQMD model. Due to a stronger
repulsion at the early stage introduced by the repulsive potentials and hence
much less annihilation probabilities, anti-protons are frozen out earlier with
smaller $v_2$ values. Therefore, the experimental data of anti-proton $v_2$ as
well as the flow difference between proton and anti-proton can be reasonably
described with the potential version of UrQMD.
