Fig. 2 A schematic diagram of the experimental setup for high-speed shad-owgraph imaging of bubble dynamics and shock wave–bubble interaction produced by an XL-1 lithotripter; M: mirror.
Fig. 2 shows the high-speed imaging system set up
inside the water tank of the XL-1 lithotripter.
Fig. 2 A schematic diagram of the experimental setup for
high-speed shad-owgraph imaging of bubble dynamics and shock wave–bubble
interaction produced by an XL-1 lithotripter; M: mirror.
Fig. 3 shows a selection of high-speed shadowgraphs taken at different time instants after the spark discharge, using the XL-1 lithotripter. Several important features can be observed. The incident shock wave, moving upward, consists of a concave central part corresponding to the focused shock front, and a convex part originating from the wave diffraction at the aperture of the reflector (Sturtevant B and Kulkarny VA, The focusing of weak shock waves, J. Fluid Mech. 1976, 73: 651-671.). The diffracted waves propagated laterally along the focused shock front and crossed on the central axis of the reflector. Due to nonlinear wave propagation, the diffracted and focused shock waves merged together to form a flattened shock front beyond the geometric focus of the reflector (see frame t =147 in Fig. 3). This observation is consistent with the shift of the peak positive pressure of the lithotripter shock waves, measured by the PVDF hydrophone.
Fig. 3 Representative high-speed shadowgraphs of the incident
lithotripter shock wave and subsequent dynamics of cavitation bubbles,
produced by the XL-1 lithotripter . The incident shock wave propagates
from the bottom to the top of the frame. The number on top of each image
frame indicates the time delay from the spark discharge in microseconds.
The cross symbol indicates the geometric focus (F2) of the lithotripter.
LSW: lithotripter shock wave, and DW: diffracted wave.
Because of the temporal profile of the shock wave, cavitation bubbles were induced in the wake of the shock front where the tensile pressure is high. So me bubbles were observed to expand initially to a maximum size of 3-4 mm and then collapse violently, producing secondary shock wave emission which could be visualized as spherically divergent circular rings surrounding the collapsed bubbles (see frame t =740 in Fig. 3). During the initial expansion, individual bubbles were nearly spherical and separated from each other. However, in the later stage of the expansion, significant bubble aggregation was observed in regions near the shock wave axis, leading t o the formation of large bubbles of 5~7mm in diameter (see the frame t =740 in Fig. 3) with various shapes. This aggregation appears to extend the duration of bubble expansion significantly from the corresponding values for the isolated single bubbles. Whe n the large, aggregated bubbles collapsed, much stronger secondary shock wave emission was produced (see frame t =900 in Fig. 3).
When a staged double-ellipsoidal reflector was used, three major differences
in the high-speed shadowgraphs could be noticed (Fig. 4).
First, a PSW produced by the reflection and diffraction of the incident
shock wave from the inserted reflector was observed ahead of the LSW. This
PSW consists largely of diffracted waves from both the upper and lower
rims of the annular ring reflector, with both waves crossing on the central
axis of the reflector. In addition, another weak shock wave was also observed
in between the preceding pulse and the LSW. This additional wave is speculated
to correspond to the portion of the LSW that is in the shadow of the inserted
reflector, which is accelerated while passing diagonally through the brass
reflector. Second, small inertial bubbles were induced by the PSW, and
expanded to a size of 100–200 mm, with a few
grown to a size up to 400 mm (measured directly
from enlarged images) before being collapsed in situ by the incident LSW.
Consequently, strong secondary shock wave emission was generated immediately
following the propagating lithotripter shock front, visualized as numerous
circular rings. Third, the collapse of some inertial microbubbles by the
lithotripter shock front appeared to be asymmetric, resulting in the formation
of microjets (label ‘‘J’’ in Fig. 4) along the wave-propagation
direction. In general, as the PSW becomes stronger, increased numbers of
microjets will be formed following the lithotripter shock front. After
the initial collapse, most microbubbles re-bounded and reexpanded again
under the influence of the tensile stress of the LSW. These bubbles again
reached a maximum size in several hundred microseconds and then collapsed
rapidly, generating strong secondary shock wave emission.
Fig. 4 Representative high-speed shadowgraphs of the incident
shock waves and subsequent dynamics of cavitation bubbles produced by the
XL-1 lithotripter at 25 kV, with the annual ring reflector (D6 reflector).
The incident shock wave propagates from the bottom to the top of the frame.
The number on top of each image frame indicates the time delay from the
spark discharge in microseconds. The cross symbol indicates the geometric
focus ( F2) of the lithotripter. LSW: lithotripter shock wave, PSW: preceding
shock wave, IMB: inertial microbubble, DW: diffracted wave, and J: jet.
Similar characteristics of shock wave propagation and in situ
microbubble–shock wave interaction were also observed inside the pipette
and near an aluminum foil used for the bioeffects study (Fig.
5, Fig. 6 and Fig 7). For the
series of experiments with pipette, saline solution was filled inside pippete,
and a yellow filter was used to reduce the light intensity in the surrounding
water so that a uniformly illuminated image could be recorded. Using the
modified reflectors, inertial microbubbles were induced in front of the
lithotripter shock wave (frames 1 and 2 of Fig. 5),
and a more uniformly distributed bubble cluster was produced inside the
pipette (frame 5 of Fig. 5), compared to the standard
reflector. In the later stage (>200 ms), larger
bubbles were formed both inside and on the exterior surface of the pipette
wall, with the later ones lasting longer. Therefore, a clear and complete
sequence of the bubble expansion and collapse inside the pipette could
not be obtained.
Fig. 5 Representative high-speed shadowgraphs of the incident
lithotripter shock waves and subsequent dynamics of cavitation bubbles
in pipettes, produced by the XL-1 lithotripter at 25 kV. The incident shock
wave propagates from the bottom to the top of the frame.
For the experiments with alumimum foil, we placed the sample with size
of 2''x8'' at F2. With modified reflector, a dense bubble cluster was formed
under the foil when shock wave just passed through (frames t=145ms
and 147ms of Fig. 7) and
then disappeared afterwards. Microjet with direction towords the foil was
also noticed (frame t= 165ms of Fig.
7).
Fig. 6 Representative high-speed shadowgraphs of the incident
lithotripter shock waves and subsequent dynamics of cavitation bubbles
near an aluminum foil , produced by the XL-1 lithotripter at 25 kV. The
incident shock wave propagates from the bottom to the top of the frame.
Fig. 7 Representative high-speed shadowgraphs of the incident
lithotripter shock waves and subsequent dynamics of cavitation bubbles
near an aluminum foil, produced by the XL-1 lithotripter at 25 kV. The
incident shock wave propagates from the bottom to the top of the frame.
When pure nitrocellulose blotting membranes (PNBM) were placed at F2,
the surface damage patterns were different from using original reflector
and modified reflector. More pits were noticed by using modified reflector
(Fig 8).
Fig. 8 Comparison of the surface damage patterns after 1
shock by using the Dornier XL-1 lithotripter and modified lithotripter
with adding an annular ring reflector.
See following paper for detail information.