Oscillations, Waves, and Interactions - GWDG

244 A. Vogel, I. Apitz, V. Venugopalan
gelatin surface and rapid particle ejection are observed only after a further pressure
build-up through confined boiling (Sect. 4.5). The material ejection during skin ablation
is also characterized by a phase explosion followed by confined boiling. However,
the higher mechanical strength of skin causes a further delay of material ejection
compared to gelatin. It is important to note that both for skin and gelatin targets
fragments are ejected in the form of solid particles. The absence of droplet-like ejecta
indicates that gelatin exposed to temperatures near the spinodal limit does not melt
within 200 µs, even though it melts at 60 ◦ C for sufficiently long heat exposures. This
finding is consistent with the strong increase in denaturation temperature for very
short exposures that was discussed in Sect. 2.
Initial material ejection velocities observed for microsecond laser ablation are
roughly one order of magnitude lower than those reported for nanosecond ablation
[103]. For free-running pulses, an increase of the radiant exposure results in an
earlier onset of the material ejection but does not change the ejection velocity significantly.
It is only for very large radiant exposures in which the first intensity spike
of the free-running pulse provides a dose in excess of the ablation threshold that an
increase of the ejection velocity is observed. By contrast, for nanosecond exposures
an increase of the radiant exposure is always coupled with an increase of the volumetric
energy density that translates directly into a higher temperature, pressure,
and ejection velocity.
In both Q-switched and free-running laser ablation of soft tissues, material ejection
continues for a considerable time following laser irradiation that can last up to
several milliseconds [78–80,82,99,104]. In general, post-pulse ablation lasts longer for
mechanically weaker tissues, larger radiant exposures, and larger laser beam diameters.
One possible driving force for the continuation of the ablation process after
the end of the laser pulse is the heat retained in the tissue. A progressive weakening
of the tissue matrix through thermal denaturation enables a propagation of the
ablation front until the vapour pressure in the residual tissue drops below the UTS
of the weakened tissue matrix. Another very important source of post-pulse ablation
are hydrodynamic phenomena such as recoil stress-induced material expulsion.
5.3 Recoil stress and secondary material ejection
Both the rapidly expanding vapour plume and the ejected particles generate recoil
stresses that impart momentum to the tissue. The linear momentum per unit area
of the ablated material l is the time integral of the recoil stress σrec at the target
surface
� ∞
l = σrec(t) dt . (12)
0
A derivation of the peak recoil stress requires assumptions on the nature and duration
of the ablation process. Various authors have presented solutions for the peak
stress amplitude produced by a continuous vaporization process [40,66,105,106], and
by explosive ablation where the entire laser pulse is deposited prior to the onset of
material removal [41,107].
Experimental values for the recoil stress produced by nanosecond laser ablation
have been obtained through direct pressure measurements using piezoelectric trans-