4.8 Shock wave propagation in water
Shadowgraph images are recorded using a CCD camera (pco. imaging sensicam, Germany) in its single frame mode. An LED flash lamp with pulse duration of τ ≈8 ns (FWHM) is applied for back illumination. The short duration of the flash lamp pulses provides the possibility of capturing the fast traveling shock waves in a medium like water. Although the exposure time of the camera is much longer than this time interval, the effective exposure time is limited to the pulse duration of the flash lamp.
In order to illustrate the shock wave passage in water, shadowgraph pictures are shown in Figure 4.12. The Mach stem (see chapter 3) is visible in the frames where the shock wave arrives in the focus position. Some small bubbles are seen at the bottom of the lower frames (about 1.2 µs after passage of the shock wave from its focus position). They begin to growth and their population is increasing by time. Radii of these cavitation bubbles range fromR ≈20µm to ≈240 µm.
The acoustic field at the moment of t ≈ 15 µs after the passage of the lithotripter shock wave (LSW) from the focus position is shown in Figures 4.13 and4.14. These images are related to application of deionized, filtered, partially degassed water and also tap water, respectively. The effect of water impurity and gas content can be seen by comparing these two Figures. The population of cavitation bubbles is enhanced by using tap water. The bubbles in this field have maximum radius of R ≈400 µm at the moment of recording of the corresponding images. The pressure pulse amplitude at distance ofd ≈1.8mm above the focus location is P = 108.1±2.5MPa.
Figure 4.15 displays bubble clouds in the acoustic field at the moment of t ≈ 9 µs after the passage of the shock wave at its focus position. It can be seen that the spherical collapse of individual cavitation bubbles enhances their neighboring bubble implosions. Also some shock induced liquid jets are visible. The maximum bubble radius in this case is ≈ 310 µm. This image shows bubble – bubble interaction as well as shock wave – bubble interaction.
Figure 4.12: Image sequence of the shock wave propagation in water. The Mach stem is visible in the frames that the shock front is nearly focused. After passage of the the shock wave secondary cavita-tion takes place.
4.8 Shock wave propagation in water
Figure 4.13: Acoustic field at the moment t ≈15 µs after the passage of the shock wave when using deionized, partially degassed and filtered water. The pulse amplitude is P = 108 MPa.
Figure 4.14: Acoustic field at time t ≈ 15 µs after the passage of the shock wave, by application of tap water. The bubble population is enhanced and therefore, the acoustics field appears to be more noisy compared to Figure 4.13. The pulse amplitude is P = 108 MPa. The radius of the initiated cavitation bubbles is as large as R≈400 µm.
Figure 4.15: Acoustic field at momentt≈9µs after the passage of the shock wave, using tap water. The pulse amplitude is P = 108 MPa.
The maximum radius of the bubbles is R ≈ 310 µm at this moment.
Chapter 5
Interaction between lithotripter shock waves and laser-generated single cavitation bubbles
In this work, the behavior of laser-induced single cavitation bubbles in response to impact of a lithotripter shock wave is investigated by means of high-speed photography and acoustic field measurements. The effect of variation of the shock wave energy is considered. A shock wave impinges on the bubbles at different moments of their oscillation phase, which provides the possibility of studying the influence of the initial bubble oscillation phase on the interaction.
Enhancement of the pressure wave emitted from bubble forced implosion as well as reduction of the collapse time relative to the inertial cavitation bubble oscillation, have been explored. Shock wave-induced liquid jetting has been considered for different shock wave energies. The effect of the presence of a solid interface nearby to the cavity parallel to the direction of shock wave propagation is studied in the next chapter using different wall distances.
5.1 Method
In order to study shock wave – bubble interaction, we need to have precise temporal and spatial information about cavitation bubbles. The single cav-itation bubbles have to be well localized and the bubble generation process to be synchronized with the rest part of the experiment. For this purpose, a Q-switched laser source (Brio, Quantel) with a second harmonic generation (SHG) module at wavelength of λ= 532nm and with pulse duration of≈4ns is used. The laser beam has a diameter of ≈ 4 mm. It is widened by an 8×
beam expander in order to reduce the spot size and also increase the numerical aperture of the optical system. An aberration minimized lens system with focal length of inner lens as≈50mm in water has been applied (numerical aperture (NA) is ≈0.3).
The quality of the laser focus plays an important role in reproducibility of the induced cavitation bubbles in different trials. It has also a major effect on the sphericity of the generated bubbles. The shock focus position is located on the center axis of a water filled tank with the dimensions of (16 cm ×16 cm
×12 cm) right at the location of the laser focus. The laser energy has been reduced to a few mJ using a combination of a λ/2 wave plate and a cubic polarizing beam splitter. For back illumination, a bright flash lamp (Metz Mecablitz 60 CT-4) is applied. In order to record the images, a long distance microscope (K2 Infinity) attached to a very high-speed camera (Imacon 468, DRS Hadland Ltd.) has been used. The camera consists of 8 independent intensified CCDs, and is capable of taking pictures with frame rates of up to 100 million frames/sec. Delay time between individual channels can be varied by the help of an electronic box inside the camera.
A piezoelectric shock wave generator (PiezoSon 100, FB 12 G5, Richard Wolf Germany) described in chapter4, produces lithotripter shock wave pulses which are focused at the center of the water filled container (the place where cavitation bubbles are generated via optical breakdown). For recording of the acoustic field at top of cavitation bubbles, a fiber optic probe hydrophone (FOPH300, Univ. Stuttgart, Germany [82]) as discussed in chapter 4, has been utilized.
In order to synchronize the laser source, shock wave generator, flash lamp and camera, two delay generators (DG535, Stanford research systems and a workshop made 10 Hz clock with a trigger selection box electronic circuit) have been applied. Figure 5.1 shows a schematic view of the synchronization procedure. Figures 5.2 and 5.3 display schematic layouts of the experimental setup.
Figure 5.1: Schematic view of the synchronization procedure of the experi-mental setup.
5.1 Method
Figure 5.2: Schematic picture of the experimental setup for the study of inter-action between lithotripter shock waves and laser-induced single cavitation bubbles (top view).
Figure 5.3: Schematic layout of the experimental setup for investigation of shock wave – bubble interaction (side view).