  We study the QCD phases and their transitions in 2+1 flavour NJL model, with
focus on the interface effects such as the interface tension, the interface
entropy and critical bubble size in the coexistence region of the first order
phase transitions. Our results show that, under the thin-wall approximation,
interface contribution to total entropy density changes its discontinuity scale
in the first order phase transition. However, the entropy density of dynamical
chiral symmetry (DCS) phase is always greater than that of the dynamical chiral
symmetry broken (DCSB) phase in both heating and hadronization processes. To
address this entropy puzzle, the thin-wall approximation is evaluated in the
present work. We find that the puzzle can be attributed to a drastic
overestimate of the critical bubble size at low temperature in the
hadronization process. With an improvement on the thin-wall approximation, the
entropy puzzle is well solved with the total entropy density of the hadron-DCSB
phase exceeding apparently that of the DCS-quark phase at low temperature.
