  We point out, according to the principle of entropy growth, that the volume
occupied by the system in the momentum space should expand during the
first-order phase transition. Such an expansion is visualized by the simulation
using the transport model based on a NJL-typed Lagrangian, and quantified by
several observables, some of which, proposed for the first time in this
article, could be the probes of the first-order phase transition either between
the quark gluon plasma and the hadronic phase taking place in the heavy-ion
collisions or of the other expanding systems.
