Oscillations, Waves, and Interactions - GWDG

Dynamics of pulsed laser tissue ablation 227
Figure 7. Pressure vs. specific volume projection of the thermodynamic phase diagram
including the spinodal curve and equilibrium and van der Waals isotherms. Points 1–4
correspond to those shown in Fig. 11. The importance of points 4 ′ , 5, 5 ′ and the isoenthalp
is discussed in Sect. 5.4.
4.2 Surface vaporization
Equilibrium vaporization at a liquid–vapour interface is associated with an increase of
specific volume at constant temperature that requires the latent heat of vaporization.
This process can occur at any single location along the binodal in the p–T diagram
of Fig. 6. In the p–v diagram in Fig. 7, surface vaporization is represented by a path
following an equilibrium isotherm that connects a state of saturated liquid with a
state of saturated vapour. Thus vaporization does not occur at a pre-determined temperature
and theoretical models that adopt a fixed “vaporization temperature” [49]
violate the basic physics of the process [50]. The actual surface temperature is dictated
by the rate of equilibrium vaporization that balances the irradiance supplied
to the system.
However, during laser ablation, rates of vapour formation in excess of that predicted
by equilibrium vaporization are often achieved. This occurs because the increased
equilibrium vapour pressure corresponding to the increased temperature of the liquid
surface is not established instantaneously. This results in an increased mass flux of
vapour transported into the surroundings known as non-equilibrium interphase mass
transfer and can be estimated using arguments from the kinetic theory of gases [51,52].
Nevertheless, even this increased non-equilibrium interphase mass transfer provides
a very small ablation rate. Efficient ablation is achieved at laser irradiances that
deposit energy at a rate that cannot be balanced by surface vaporization processes
alone. This results in material removal via volumetric processes.