  We investigate thermal and isothermal symmetric liquid-vapor separations via
a FFT-Thermal Lattice Boltzmann (FFT-TLB) model. Structure factor, domain size
and Minkowski functionals are employed to characterize the density and velocity
fields as well as to understand the configurations and the kinetic processes.
Compared with the isothermal phase separation, the freedom in temperature
prolongs the Spinodal Decomposition (SD) stage and induces different
rheological and morphological behaviors in the thermal system. After the
transient procedure, both the thermal and isothermal separations show power-law
scalings in domain growth; while the exponent for thermal system is lower than
that for isothermal system. With respect to the density of field, the
isothermal system presents more likely bicontinuous configurations with
narrower interfaces, while the thermal system presents more likely
configurations with scattered bubbles. Heat creation, conduction and lower
interfacial stresses are main reasons for the differences in thermal system.
Different from the case with isothermal phase separation, the release of latent
heat causes the changing of local temperature which results in new local
mechanical balance. When the Prandtl number becomes smaller, the system
approaches thermodynamical equilibrium more quickly. The increasing of mean
temperature makes lower the interfacial stress in the following way:
$\sigma=\sigma_{0}[(T_{c}-T)/(T_{c}-T_{0})]^{3/2}$, where $T_{c}$ is the
critical temperature and $\sigma_{0}$ is the interfacial stress at a reference
temperature $T_{0}$, which is the main reason for lower growth exponent in
thermal case.
