  The azimuthal anisotropies of the collective transverse flow of charged
hadrons are investigated in a wide range of heavy-ion collision energies within
the microscopic Parton-Hadron-String Dynamics (PHSD) transport approach which
incorporates explicit partonic degrees-of-freedom in terms of strongly
interacting quasiparticles (quarks and gluons) in line with an
equation-of-state from lattice QCD as well as the dynamical hadronization and
hadronic collision dynamics in the final reaction phase. The experimentally
observed increase of the elliptic flow $v_2$ of charged hadrons with collision
energy is successfully described in terms of the PHSD approach. The analysis of
higher-order harmonics $v_3$ and $v_4$ in the azimuthal angular distribution
shows a similar tendency of growing deviations between partonic and purely
hadronic models with increasing collision energy. This demonstrates that the
excitation functions of azimuthal anisotropies reflect the increasing role of
quark-gluon degrees of freedom in the early phase of relativistic heavy-ion
collisions. Furthermore, the specific variation of the ratio $v_4/(v_2)^2$ with
respect to bombarding energy, centrality and transverse momentum is found to
provide valuable information on the underlying partonic dynamics.
