  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 PHSD scaling
properties of various collective observables are confronted with experimental
data as well as with hydrodynamic predictions. 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 dynamics.
