Oscillations, Waves, and Interactions - GWDG

230 A. Vogel, I. Apitz, V. Venugopalan
Figure 8. Variation of the critical bubble radius required for spontaneous vapour bubble
growth with superheat temperature. Note that the critical bubble radius goes to zero at the
critical temperature.
tension disappears and this occurs at the critical point. To account for the influence
of statistical fluctuations on the stability limit, Kiselev has introduced the concept
of a kinetic spinodal. The kinetic spinodal is defined as the locus of thermodynamic
states where the time for spontaneous formation (driven by superheat temperature)
of vapour nuclei becomes smaller than the characteristic time for their decay to local
equilibrium (driven by surface tension) [61]. The superheat temperatures defined by
the kinetic spinodal are much lower than the critical temperature and slightly lower
than the classical spinodal and represent the physical limits of the metastable liquid
states that can be achieved prior to spinodal decomposition.
Thus, in general, the transformation of superheated (metastable) liquid to an equilibrium
state of mixed phase may involve both bubble nucleation (large density fluctuations
extending over a small spatial extent) and spinodal decomposition (small
density fluctuations extending over a large spatial extent). We refer to the collective
phase transition process as a phase explosion. A more detailed consideration of nucleation
theory and spinodal decomposition as it relates phase transitions and tissue
ablation can be found in our earlier review [6].
Thus far we have focused on processes tracing a path indicated by 1 → 3 → 4 ′ →
5 → 5 ′ in Figs. 6 and 7. This path corresponds to the extreme case in which no
vapour nuclei are present in the liquid. When the heating occurs very rapidly at
high radiant exposures, the liquid experiences a recoil pressure from surface vaporization/interphase
mass transfer that can be substantial due to the non-equilibrium
conditions produced during the beginning of the laser pulse. Thus spinodal conditions
are reached at an elevated pressure somewhere between point 3 and the critical
point C. Because the resulting phase explosion occurs at elevated temperature and