Olgert Lindau

Collapse and rebound of a laser-induced cavitation bubble

A strong laser pulse that is focused into a liquid produces a vapor cavity, which first expands and then collapses with subsequent rebounds. In this paper a mathematical model of the spherically symmetric motion of a laser-induced bubble is proposed. It describes gas and liquid dynamics including compressibility, heat, and mass transfer effects and nonequilibrium processes of evaporation and condensation on the bubble wall. It accounts also for the occurrence of supercritical conditions at collapse. Numerical investigations of the collapse and first rebound have been carried out for different bubble sizes. The results show a fairly good agreement with experimental measurements of the bubble radius evolution and the intensity of the outgoing shock wave emitted at collapse. Calculations with a small amount of noncondensable gas inside the bubble show its strong influence on the dynamics.

Bubble dynamics, shock waves and sonoluminescence

Sound and light emission by bubbles is studied experimentally. Single bubbles kept in a bubble trap and single laser–generated bubbles are investigated using ultrafast and high–speed photography in combination with hydrophones. The optical observation at 20 million frames per second of the shock waves emitted has proven instrumental in revealing the dynamic process upon bubble collapse. When jet formation is initiated by a non–spherically symmetric environment, several distinct shock waves are emitted within a few hundred nanoseconds, originating from different sites of the bubble. The counterjet phenomenon is interpreted in this context as a secondary cavitation event. Furthermore, the light emission of laser–generated cavities (termed cavitation bubble luminescence) is studied with respect to the symmetry of collapse. The prospects of optical cavitation and multibubble trapping in the study of few–bubble systems and bubble interactions are briefly discussed. Finally, the behaviour of bubble clouds, their oscillations, acoustic noise and light emission are described. Depending on the strength of the driving sound field, period doubling and chaotic oscillations of the collective bubble dynamics are observed.

Cinematographic observation of the collapse and rebound of a laser-produced cavitation bubble near a wall

Collapse and rebound of a cavitation bubble near a wall are revisited with modern experimental means. The bubble is generated by the optical breakdown of the liquid when a strong laser pulse is focused into water. Observations are made with high-speed cinematography; framing rates range between several thousand and 100 million frames per second, and the spatial resolution is in the order of a few micrometres. After formation the bubble grows to a maximum size with a radius of 1.5 mm at the pulse energy used, and in the subsequent collapse a liquid jet evolves on the side opposite the wall and penetrates through the bubble. Using a shadowgraph technique and high framing rates, the emission of shock waves, which is observed at minimum bubble size, is resolved in detail. For a range of stand-off distances between the bubble centre and the wall, a counterjet forms during rebound. The counterjet is clearly resolved to consist of cavitation micro-bubbles, and a quantitative measure of its height evolution is given. Its emergence might be caused by a shock wave, and a possible connection of the observed shock wave scenario with the counterjet formation is discussed. No counterjets are observed when the stand-off distance is less than the maximum bubble radius, and the bubble shape becomes toroidal after the jet hits the wall. The jet impact on the wall produces a pronounced splash, which moves radially outwards in the space between the bubble and the wall. The volume compression at minimum bubble size is found to depend strongly on the stand-off distance. Some of the results are compared to numerical simulations by Tong et al. (1999), and the material presented may also be useful for comparison with future numerical work.

Dynamics of laser-induced cavitation bubbles

Abstract Single cavitation bubble luminescence induced by laser in contrast to single bubble sonoluminescence has no need in a sound field for a strong collapse and for light emission. The cavitation bubbles are produced by focused laser light and make the single strong collapse. As shown experimentally, the number of emitted photons from cavitation luminescence is much greater than it was observed in sonoluminescence due to the large bubble size during the final stage of collapse. To describe the process of laser-induced bubble collapse a mathematical model is used, which is based upon the spherically symmetric motion including compressibility, heat and mass transfer effects. The basic results of the numerical solution are presented for the bubbles with maximum radii of about 1 mm. According to the observed results the minimum bubble radius in collapse is about 15 μm, and the mass decreases up to 5% of the initial value. Calculations with a small amounts of noncondensable gas inside the bubble predict its strong influence on the dynamics. As shown numerically the theoretical model gives a good agreement with experimental measurements.

Laser-Induced Bubbles in Cavitation Research

The strength of shock waves is measured with a fiberoptic hydrophone in dependence on bubble radius for ns laser pulses. The peak pressures at the bubble wall upon collapse range from 10 kbar to 25 kbar for maximum bubble radii between 0.5 mm and 3 mm. Cavitation bubble luminescence is determined in dependence on hydrostatic pressure. A maximum in light output is obtained at elevated hydrostatic pressure for constant laser energy input. The dynamics and luminescence properties of strongly elongated bubbles from fs laser pulses are investigated.

Laser-produced cavitation—studied with 100 million frames per second

Shock wave emission of laser produced cavitation bubbles is investigated. For spherical bubbles the precise shape of the pressure pulse emitted at collapse is obtained using a new fiber optic type of hydrophone. The aspherical bubble collapse near a rigid boundary is observed by ultra high-speed photography. Multiple shock wave emission is made visible with a slightly divergent light source.

Luminescence from spherically and aspherically collapsing laser‐induced bubbles

Luminescence from laser‐induced cavitation bubbles (SCBL) is investigated experimentally. This experimental technique offers several advantages over the acoustical driving technique for luminescence studies: The optical resolution of the imaging system allows the investigation of the shape of the luminescence spot. This technique facilitates the study of the spherical asymmetries during the bubble collapse phase. Finally, complications due to gas diffusion and Bjerknes forces are avoided. Comparison with theory gives good agreement between the size of the luminescence spot and the observed minimum bubble size for spherical symmetry. A rigid boundary greatly influences the light output; SCBL is only observed for a mildly aspherical bubble collapse.

Temperature dependency of luminescence from laser‐produced cavitation bubbles

Single laser‐produced bubbles emit short light pulses in the collapse phase. This phenomenon is called single‐cavitation bubble luminescence (SCBL) in analogy to single‐bubble sonoluminescence (SBSL) where the bubbles levitate in an acoustic field. Laser bubbles are produced using a Q‐switched Nd:YAG laser. The laser beam is focused with an aberration‐minimized lens system into a temperature controlled cuvette filled with bidistilled water. A photomultiplier collects the light emitted in the collapse phase of the bubble. Bubble size is monitored with a hydrophone. As laser‐produced bubbles are bigger in size than their acoustically driven counterparts, they emit stronger light pulses. The light energy per pulse in dependence on bubble size is reported. SBSL shows a strong dependency on temperature and a similar behavior is expected for SCBL.

Shock waves from cavitation bubbles

Cavitation bubbles are produced in water by focused laser light with pulse durations in the 100‐fs and 10‐ns range. Special attention is paid to the shock waves during breakdown and first collapse of the bubble or bubbles generated. The special focusing properties of femtosecond pulses lead to cylindrical shock waves from elongated breakdown channels and to more complicated shock wave patterns in the pre‐breakdown region. Bubble collapse leads to spherical outgoing shock waves in the case of spherical shrinking of the bubble and to a sequence of distinct shock waves in the case of aspherical shrinking. The shock waves are made visible with shadowgraph techniques, including femtosecond pulses for illumination, and are measured with a fiberoptic hydrophone. [Work supported by the Deutsche Forschungsgemeinschaft.]

Laser induced cavitational luminescence in a bipolar acoustic pulse wave

Laser induced bubble dynamics (200–300 μs life time) under the influence of a convergent bipolar acoustic pulse (100 bar, 2 μs) in water is investigated experimentally. The amplitude of both an emitted shock and luminescence from the bubble collapse are recorded by a pvdf pressure probe and a photomultiplier tube (which integrates the spectral range from 260 to 530 nm). Different delay time (phase) between collapse and pressure pulse is found to influence the luminosity. For certain phases the average amplitude of the luminescence signal is increased by an order of magnitude in comparison with the average signal from an undisturbed collapse (i.e., outside the additional pressure pulse). Photorecordings show that bubbles re‐expand after the first collapse nonspherically if an acoustic pulse was applied. They form a nose on the shadow side of the pulse wave which is possibly indication for jetting. Results for a collapse of a laser cavitation bubble and a bubble produced by shock cavitation are compared. [W...