Laser-induced Cavitation Bubble Research Articles

Energy partition in laser-induced cavitation bubbles near the rigid wall with a gas-containing hole

Energy partition in laser-induced cavitation bubbles near the rigid wall with a gas-containing hole

Energy partition in laser-induced cavitation bubbles near the rigid wall with a gas-containing hole

Energy partition in laser-induced cavitation bubbles near the rigid wall with a gas-containing hole

Experimental Study on Near-Wall Laser-Induced Cavitation Bubble Micro-Dimple Formation on 7050 Aluminum Alloy

To investigate the feasibility and formation laws of fabricating micro-dimples induced by near-wall laser-induced cavitation bubble (LICB) on 7050 aluminum alloy. A high-speed camera and a fiber-optic hydrophone system were used to capture pulsation evolution images and acoustic signals of LICB. Meanwhile, a three-dimensional profilometer was employed to examine the contour morphology of the surface micro-dimple on the specimen. The results show that at an energy level of 500 mJ, the total pulsation period for the empty bubble is 795 μs, with individual pulsation periods of 412.5 μs, 217 μs, and 165 μs for the first, second, and third cycles, respectively, with most energy of the laser and bubble being consumed during the first evolution period. Under the synergy of the plasma shock wave and collapse shock wave, a spherical dimple with a diameter of 450 μm is formed on the sample surface with copper foil as the absorption layer. A model of micro-dimple formed by LICB impact is established. As the energy increases, the depth of the surface micro-dimple peaks at an energy of 400 mJ and then decreases. The depth of the surface micro-dimple increases with the increase in the number of impacts; the optimal technology parameters for the micro-dimple formation by LICB impact are as follows: the absorption layer is copper foil, the energy is 400 mJ, and the number of impacts is three.

Experimental Study of Laser-Induced Cavitation Bubbles near Wall: Plasma Shielding Observation

To investigate the plasma shielding of laser-induced cavitation bubbles near a wall, a pulsed laser with different energies was selected to induce cavitation bubbles on the surface of 7050-T7451 aluminum alloy. A high-speed camera captured the evolution of the cavitation bubble, while a fiber-optic hydrophone system collected the acoustic signals during the evolution. Finally, a confocal microscope was used to view and analyze the surface morphology of 7050 aluminum alloy. The experimental results indicate that as the laser energy increases, the diameter, the evolution time, the pressure of the bubble, and both the pit diameter and depth all increase. Beyond an energy level of 1.4 J, the maximum diameter and the evolution time of the laser-induced cavitation bubble begin to decrease; the maximum diameter decreases by 2.04%, and the first evolution time decreases by 3.26%. Plasma shielding was observed in this experiment. Considering that the essence of a laser-induced cavitation bubble is the interaction between a high-energy laser and a liquid medium, the abnormal decrease in the maximum diameter, evolution time, and sound pressure epitomizes the manifestation of plasma shielding.

Experimental Investigations on the Cavitation Bubble Dynamics near the Boundary of a Narrow Gap

Cavitation bubbles near narrow gaps widely exist within microfluidic control devices. In the present paper, a laser-induced cavitation bubble is arranged in a narrow gap composed of two parallel plates. The inception position of the bubble is set to be at the same distance from the two plates so that the dynamic behaviors of the bubble are symmetrical. The collapse and rebound dynamics of the bubble near the boundary of a narrow gap are investigated through high-speed photography. The bubble behaviors (e.g., shape deformation, translational movement, and jet characteristics) are analyzed while considering the influence of the dimensionless distance between the bubble and the boundary and the dimensionless gap width. The principal findings include the following: (1) When the dimensionless distance is small, a violent jet towards the gap is generated during the bubble collapse stage, along with a weak counter-jet towards the boundary appearing during the rebound stage. (2) As the dimensionless distance increases, the translational distance of the bubble during the collapse stage initially decreases, then increases, and finally decreases to zero. (3) Within the parameter range considered in this paper, the dimensionless width mainly affects the expansion degree and movement direction of the bubble cloud during its rebound and subsequent stages. The above research findings can provide experimental support for bubble-driven flow control, pumping, and liquid mixing in microfluidic channels.

Characteristics of evolution and collapse in spark-induced cavitation bubbles

Numerous scientists have thoroughly researched cavitation bubble dynamics using experimental methods such as tube arrest, underwater discharge, ultrasound, and laser focusing. In this study, with the aid of high-speed photography, the formation mechanism of the electrode-contact spark-induced cavitation bubble is discovered, i.e., electrolysis results in non-condensable gases wrapping the small inter-electrode gaps, and the non-condensable gases are broken down by discharge to form plasma, which then induces the cavitation bubble. Since the cavitation bubbles already contain a certain amount of non-condensable gases during the discharge process, the differences in the spatiotemporal evolution and collapse characteristics of the cavitation bubbles with varying amounts of non-condensable gases are further analyzed. The results show that underwater electrode-contact discharge system has an optimal voltage if the capacitance and discharge electrode size remain constant, and the cavitation bubbles generated under the optimal voltage condition are not only morphologically closest to the laser-induced cavitation bubbles, but also the change in radius over time during collapse is quite consistent with the Rayleigh bubble. Furthermore, compared to cavitation bubbles generated under varying voltages, those induced by the optimal voltage have a lower amount of non-condensable gases. This leads to the minimum first contraction radius and the maximum rebound radius being close to the corresponding values of the laser-induced cavitation bubbles. These new findings are of great significance for the improvement of experimental technology in the study of cavitation bubble dynamics, obtaining precise and dependable experimental data, and validating numerical simulations.

An innovative method of pressure measurement inside a laser-induced cavitation bubble

This study proposes a novel method for measuring the pressure inside a laser-induced cavitation bubble during its expansion. Based on Paschen's theory, which relates electrical breakdown to gas pressure, applied voltage, and electrode distance, our approach uses two electrodes to estimate the cavitation bubble pressure at different expansion stages by noting the breakdown time at varying voltages. This method offers a high temporal resolution owing to its rapid electrical discharge response. The unveiled pressure during the expansion phase was approximately 0.4 atm. This value contradicts previous reports that assumed that the minimum pressure was close to the water vapor pressure.

Study on Non-Spherical Deformation Velocity of a Single Cavitation Bubble

Cavitation bubbles commonly exist in shipbuilding engineering, ocean engineering, mechanical engineering, chemical industry, and aerospace. Asymmetric deformation of the bubble occurs near the boundary and then has strong destructiveness, such as high amplitude loading. Therefore, the research on non-spherical deformation is of great significance, and the objective of this paper is to investigate the non-spherical collapse dynamics of laser-induced cavitation bubbles when near different boundaries. In this study, experimental data, such as the bubble pulsation process and bubble surface velocity distribution, were obtained by high-speed camera techniques and full-field velocity calculations. Near the different boundaries, the results show that the bubbles appeared to have different collapse shapes, such as near-hemispherical, near-ellipsoidal, near-cone, and near-pea shapes, and the surface velocity distribution is extremely non-uniform. When the bubble near the free surface or rigid boundary collapses, the smaller the stand-off r is, the more obvious the repulsive effect of the free surface or the attractive effect of the rigid boundary is. As the stand-off r decreases, the larger the Bjerknes force and the bubble surface velocity difference and the more pronounced the non-spherical shape becomes.

Enhanced mitigation of cavitation erosion by means of femtosecond pulsed laser-induced hierarchical surface structures

Hierarchical structures were fabricated on brass surfaces by means of a straightforward 2-step laser-scribing process. We tested the ability of these surface structures to withstand cavitation erosion and compared their performance to untreated brass samples. We found that hierarchical surface structures reduced the eroded volume by a factor between 7.2 and 22.7 times when compared to untreated samples after 800,000 cavitation events. Additionally, the hierarchical surface structures reduced the erosion by a factor between 1.1 and 1.5 times when compared to samples patterned with only microchannel surface structures, for the same amount of cavitation events. Up to 800,000 laser-induced cavitation bubbles were generated at a fixed standoff distance of 0.5 to test the mitigation effect of the scribed surface structures on the eroded volume. The eroded volume was computed by means of confocal microscopy. The results show that additional roughness imposed by the hierarchical surface structure acts as an energy dissipator as observed by comparing the sizes of the higher order rebound cavitation bubbles. These findings provide a promising solution for protecting equipment from cavitation erosion.

Laser direct overall water splitting for H2 and H2O2 production

Hydrogen (H2) and hydrogen peroxide (H2O2) play crucial roles as energy carriers and raw materials for industrial production. However, the current techniques for H2 and H2O2 production rely on complex catalysts and involve multiple intermediate steps. In this study, we present a straightforward, environmentally friendly, and highly efficient laser-induced conversion method for overall water splitting to simultaneously generate H2 and H2O2 at ambient conditions without any catalysts. The laser direct overall water splitting approach achieves an impressive light-to-hydrogen energy conversion efficiency of 2.1%, with H2 production rates of 2.2 mmol/h and H2O2 production rates of 65 µM/h in a limited reaction area (1 mm2) within a short real reaction time (0.36 ms/h). Furthermore, we elucidate the underlying physics and chemistry behind the laser-induced water splitting to produce H2 and H2O2. The laser-induced cavitation bubbles create an optimal microenvironment for water-splitting reactions because of the transient high temperatures (104 K) surpassing the chemical barrier required. Additionally, their rapid cooling rate (1010 K/s) hinders reverse reactions and facilitates H2O2 retention. Finally, upon bubble collapse, H2 is released while H2O2 remains dissolved in the water. Moreover, a preliminary amplification experiment demonstrates the potential industrial applications of this laser chemistry. These findings highlight that laser-based production of H2 and H2O2 from water holds promise as a straightforward, environmentally friendly, and efficient approach on an industrial scale beyond conventional chemical catalysis.

Numerical study on laser-induced cavitation bubble dynamics inside a millimetric droplet

In this study, complicated nonlinear interactions of a single laser-induced cavitation bubble inside a millimetric water droplet were numerically investigated using a fully compressible three-phase homogeneous model. A general condensation phase-change model and high-resolution interface-capturing schemes were adopted to accurately predict the bubble collapsing and rebound stages as well as strongly deformable droplet interface evolutions. The numerical model was validated using experimental data in terms of the equivalent bubble radius until the second collapse stage, and good quantitative agreement was achieved. The variation in the droplet surface velocity was detected and could better reveal the mechanism underlying the complicated bubbles and droplet interactions, particularly in droplet surface splash dynamics. Subsequently, the complex bubble–droplet interaction phenomena were studied by investigating the ratio of the maximum bubble radius to the initial droplet radius. The numerical results show that the bubble collapsing time decreases monotonically with an increase in the bubble–droplet radius ratio. The droplet surface instabilities became more dominant as the radius ratio increased. In addition, four distinct patterns of droplet motion, namely, stable, multi-spike, ventilating jet, and splashing phenomena, were captured. Finally, the specific mechanisms leading to droplet surface jetting were identified.

Laser-Induced Plasma and Cavitation Bubble Dynamics at Manganese–Water Interface Under External Electric Fields

Laser-Induced Plasma and Cavitation Bubble Dynamics at Manganese–Water Interface Under External Electric Fields

Investigation of the influence of different liquid temperatures on the dynamics of long-pulse laser-induced cavitation bubbles

Long-Pulse Laser-induced cavitation bubbles have been identified to hold significant applications in various fields, such as industry and medicine. In this study, the dynamics of cavitation bubbles, induced by a 2.94 μm wavelength Er:YAG laser with an energy of 20 ±1 mJ and transmitted via sapphire fiber optics into distilled water at different temperatures, were investigated. The adiabatic expansion theory was utilized to predict the maximum length (Lmax) and the maximum width (Wmax) of the cavitation bubbles at different liquid temperatures. Excellent alignment was observed between the theoretical predictions and the experimental data. Using this theory, the local overheated liquid temperature was deduced to be ∼180 °C during the formation of cavitation bubbles. It was found that as the liquid temperature increased, the collapse position of the cavitation bubbles, relative to the normalized distance (γ) from the fiber end face, decreased. This study reveals the importance of considering the potential risk of damage to fiber optics from the shockwaves and microjet streams generated by cavitation bubbles in high-temperature conditions.

Cavitation bubble dynamics inside a droplet suspended in a different host fluid

In this paper, we present a theoretical, experimental and numerical study of the dynamics of cavitation bubbles inside a droplet suspended in another host fluid. On the theoretical side, we provided a modified Rayleigh collapse time and natural frequency for spherical bubbles in our particular context, characterized by the density ratio between the two liquids and the bubble-to-droplet size ratio. Regarding the experimental aspect, experiments were carried out for laser-induced cavitation bubbles inside oil-in-water (O/W) or water-in-oil (W/O) droplets. Two distinct fluid-mixing mechanisms were unveiled in the two systems, respectively. In the case of O/W droplets, a liquid jet emerges around the end of the bubble collapse phase, effectively penetrating the droplet interface. We offer a detailed analysis of the criteria governing jet penetration, involving the standoff parameter and impact velocity of the bubble jet on the droplet surface. Conversely, in the scenario involving W/O droplets, the bubble traverses the droplet interior, inducing global motion and eventually leading to droplet pinch-off when the local Weber number exceeds a critical value. This phenomenon is elucidated through the equilibrium between interfacial and kinetic energies. Lastly, our boundary integral model faithfully reproduces the essential physics of the non-spherical bubble dynamics observed in the experiments. We conduct a parametric study spanning a wide parameter space to investigate bubble–droplet interactions. The insights from this study could serve as a valuable reference for practical applications in the field of ultrasonic emulsification, pharmacy, etc.

Interactions of a collapsing laser-induced cavitation bubble with a hemispherical droplet attached to a rigid boundary

We investigate experimentally and theoretically the interactions between a cavitation bubble and a hemispherical pendant oil droplet immersed in water. In experiments, the cavitation bubble is generated by a focused laser pulse right below the pendant droplet with well-controlled bubble–wall distances and bubble–droplet size ratios. By high-speed imaging, four typical interactions are observed, namely: oil droplet rupture; water droplet entrapment; oil droplet large deformation; and oil droplet mild deformation. The bubble jetting at the end of collapse and the migration of the bubble centroid are particularly different in each bubble–droplet interaction. We propose theoretical models based on the method of images for calculating the Kelvin impulse and the anisotropy parameter which quantitatively reflects the migration of the bubble centroid at the end of the collapse. Finally, we explain that a combination of the Weber number and the anisotropy parameter determines the regimes of the bubble–droplet interactions.

Experiment and Simulation Study of the Laser-Induced Cavitation Bubble Technique for Forming a Microgroove in Aluminum Foil.

The present work introduces a laser-induced cavitation bubble technique for forming an axisymmetric structure (i.e., microgroove) and the dynamics of a cavitation bubble from initial expansion to the collapse stages that were also simulated. Furthermore, the shock wave signals and dynamic properties of the cavitation bubble were recorded using a hydrophone and a high-speed camera. The experiments on microgrooves formed by laser-induced cavitation bubble stamping were carried out, and the effects of laser energy, the initial position of the bubble, and the number of impacts on the microformability of aluminum sheets are discussed. The depth of the microgroove was investigated using experiments, and it was found that the process can serve as a rapid technique for impressing microfeatures on thin-sheet metals. The experimental results showed that as the initial position of the bubble increased, the deformation depth decreased. As the laser energy and number of impacts increased, the deformation depth increased. The results of the response surface experiments showed that a laser energy of 27 mJ, 3 impacts, and a bubble position of 3 mm were optimal for the process. By using the optimal parameters, flat and smooth microgrooves with a forming depth of 102.54 µm were successfully fabricated. Furthermore, the maximum thickness thinning of the microgroove section occurred at the entrance areas, and this area had the greatest hardness. This also indicated that the greatest amount of plastic deformation of the material and grain refinement occurred in this area. On the other hand, the aluminum foil did not undergo oxidation during the plastic deformation process. These results demonstrated that laser-induced bubble stamping is an advanced micromachining method with promising applications.

The impacts of material acoustic impedance and thickness on single laser-induced bubble dynamics and determining factors in resulting pressure

The objective of this paper is to experimentally identify the primary sources of pressure when a laser-induced cavitation bubble is collapsing to a wall with specific emphases on the material acoustic impedance and thickness. Both high-speed videos and local wall pressure measurements were performed for various standoff ratios γ, bubble diameters, and wall materials. In the case of a rigid wall, in addition to the known high pressure for γ<0.6 where the bubble attaches and collapses on the wall (ring collapse), at γ≈1.12 where the jet is dominant, and low pressure obtained at γ≈0.913, where neither effect is significant, we further captured similar pressure profiles during the collapse after the first rebound at γ≈1.16 for the ring collapse, γ≈1.79 for the jet, and γ≈1.41 for the minimal, respectively. This indicates a strong jet is typically followed by a strong ring collapse. Generally, the pressure from the second collapse increases faster with the bubble size than that of the first collapse. For walls featuring smaller acoustic impedance or thickness, which cannot be approximated as rigid bodies or accessed by pressure sensor, our unique bubble edge analyzing tool shows that the ring collapse and jet effects are moved to smaller values of γ. The maximum pressure exerted on the wall in these cases is smaller than that on the rigid wall. Finally, we summarized the asymptotic evolution curves of each edge which bound the bubble dynamics at different standoff ratios.

Laser-induced cavitation bubble near boundaries

Laser-induced cavitation bubble near boundaries

Dynamics of laser-induced plasma and cavitation bubble at high pressures and the impacts on underwater LIBS signals

Characteristics of laser-induced plasma depends strongly on its ambient pressure, and a deeper understanding of the high pressure impact on the underwater plasma emission is essential for the application of LIBS in the deep-sea. In this work, we investigated the temporal evolution of underwater plasma and cavitation bubble at the high pressures up to 50 MPa, by using fast imaging and shadowgraph techniques. It showed that as the ambient pressure increases, the plasma expansion is much suppressed with a stronger emission intensity and a slower emission decay, but it quenches much earlier and ends with a sharp drop at the higher pressures. From the shadowgraph imaging results, it showed that the increase in pressure has a great impact on the bubble evolution. Both the bubble maximum radius and the bubble lifetime decrease dramatically with the pressure as a consequence of faster dynamics. By comparing between the plasma and bubble results, we demonstrated the critical role of bubble dynamics on the characterization of plasma emission at high pressures. At relatively low pressures, the bubble expansion time is long enough to finish the plasma radiation. While at high pressures, strong bubble-plasma interaction occurs that will guide the plasma evolution behavior at the late stage. The plasma can gain energy from the confinement effect caused by the bubble shrinking process which leads to an increase in the emission intensity, but it will quench immediately after the bubble collapse. Such effects caused by bubble dynamics at high pressures explain well the evolution behaviors of underwater LIBS signals of OH, Ca II, Ca I, and CaOH, considering the different lifetimes of these emitting species in the plasma. This work reveals the complex impacts of high pressure on underwater plasma emissions and underwater LIBS signals.

Plasma-based identification of gases in a laser-induced cavitation bubble

This study presents a general methodology and an experimental approach to identify the gas components within laser-induced cavitation bubbles. A needle electrode inside the cavitation bubble, which introduces low electric energy into the bubble, produces a homogenous plasma discharge inside the vapor cavity. The primary bubble dynamics remain identical while the rebound bubble becomes about twice as large when a discharge is applied. The effect of non-condensable gases and the electric charge on bubble dynamics is explored theoretically, and the role of the electric charge is found to be significant. Optical emission spectroscopy reveals the evolution of emission lines from gases inside bubbles. H lines and OH lines are persistently observed in all cases, providing a dominant presence of water vapor. The results also confirm that the gases, which are initially present in the water rather than transported from the water, contribute to the optical emission characteristics with different dissolved gases.