Laser-induced cavitation bubbles and shock waves in water near a concave surface

Experimental study on attenuation effect of liquid viscosity on shockwaves of cavitation bubbles collapse

How to precisely control and efficiently utilize the physical processes such as high temperature, high pressure, and shockwaves during the collapse of cavitation bubbles is a focal concern in the field of cavitation applications. The viscosity change of the liquid will affect the bubble dynamics in turn, and further affect the precise control of intensity of cavitation field. This study used high-speed photography technology and schlieren optical path system to observe the spatiotemporal evolution of shockwaves in liquid with different viscosities. It was found that as the viscosity of the liquid increased, the wave front of the collapse shockwave of the cavitation bubble gradually thickened. Furthermore, a high-frequency pressure testing system was used to quantitatively analyze the influence of viscosity on the intensity of the shockwave. It was found that the pressure peak of the shockwave in different viscous liquid was proportional to Lb (L represented the distance between the center of bubble and the sensor measuring point), and the larger the viscosity was, the smaller the value of b was. Through in-depth analysis, it was found that as the viscosity of the liquid increased, the proportion of the shockwave energy of first bubble collapse to the maximal mechanical energy of bubble gradually decreased. The proportion of the mechanical energy of rebounding bubble to the maximal mechanical energy of bubble gradually increased. These new findings have an important theoretical significance for the efficient utilization of ultrasonic cavitation.

Investigation of mechanisms of shock wave generation by collapse of cavitation bubbles near particles

The mechanism of generation of shock waves by the collapse of a cavitation bubble near a single particle or dual particles is numerically investigated using OpenFOAM. For the single-particle case, shock waves during bubble inception and jet impacting on the particle surface are revealed in detail. The pressure induced on the particle by the inception shock wave of the bubble decreases with increasing bubble-particle distance, and the pressure is proportional to 1/r1.26 (r being the distance from the center of the shock wave). For the dual particles, the evolution of the neck structure is closely related to the generation mechanism of the shock wave. At extremely close particle-bubble distances, two shock waves propagating in opposite directions are emitted outside and inside the bubble after two necks collide. At long particle-bubble distances, a shock wave is emitted after the neck contracts, and simultaneously the bubble splits into two daughter bubbles. The energy of the shock wave generated by the neck constriction (i.e., the pressure at its generation location) first increases and then decreases with increasing bubble-particle distance. For unequal-sized double particles, the size of the daughter bubble depends on the bubble-particle distance and the particle size. These findings provide new perspectives for understanding the damage sustained by hydro-mechanical components operating in sand-laden water flows.

Quantifying the chemical activity of cavitation bubbles in a cluster

Acoustic cavitation bubbles drive chemical processes through their dynamic lifecycle in liquids. These bubbles are abundant within sonoreactors, where their behavior becomes complex within clusters. This study quantifies their chemical effects within well-defined clusters using a new laser-based method. We focus a laser beam into water, inducing a breakdown that generates a single cavitation bubble. This bubble undergoes multiple collapses, releasing several shockwaves. These shockwaves propagate into the surrounding medium, leading to the formation of secondary bubbles near a reflector, separated from the input laser beam. We evaluate the chemical activity of these bubble clusters of various sizes by KI dosimetry, and to gain insights into their dynamics, we employ high-speed imaging. Hydrophone measurements show that conversion from focused shockwave energy to chemical reactions increases to a maximum of 16.5%. Additional increases in shockwave energy result in denser bubble clusters and a slightly decreased conversion rate, falling to 14.9%, highlighting the key role of bubble dynamics in the transformation of mechanical to chemical energy and as a result in the efficiency of the sonoreactors. The size and frequency of bubble collapses influence the cluster's chemical reactivity. We introduce a correlation for predicting the conversion rate of cluster energy to chemical energy, based on the cluster's energy density. The maximum conversion rate occurs at a cluster energy density of 2500 J/L, linked to a cluster with an average bubble diameter of 91 μ m, a bubble density of 3500 bubbles/ml, and a bubble-to-bubble distance ratio of 8.

Secondary cavitation bubble dynamics during laser-induced bubble formation in a small container

We investigated secondary cavitation bubble dynamics during laser-induced bubble formation in a small container with a partially confined free surface and elastic thin walls. We employed high-speed photography to record the dynamics of sub-mm-sized laser-induced bubbles and small secondary bubble clouds. Simultaneous light scattering and acoustic measurements were used to detect the oscillation times of laser-induced bubbles. We observed that the appearance of secondary bubbles coincides with a prolonged collapse phase and with re-oscillations of the laser-induced bubble. We observed an asymmetric distribution of secondary bubbles with a preference for the upstream side of the focus, an absence of secondary bubbles in the immediate vicinity of the laser focus, and a migration of laser-induced bubble toward secondary bubbles at large pulse energies. We found that secondary bubbles are created through heating of impurities to form initial nanobubble nuclei, which are further expanded by rarefaction waves. The rarefaction waves originate from the vibration of the elastic thin walls, which are excited either directly by laser-induced bubble or by bubble-excited liquid-mass oscillations. The oscillation period of thin walls and liquid-mass were Twall = 116 µs and Tlm ≈ 160 µs, respectively. While the amplitude of the wall vibrations increases monotonically with the size of laser-induced bubbles, the amplitude of liquid-mass oscillation undulates with increasing bubble size. This can be attributed to a phase shift between the laser-induced bubble oscillation and the liquid-mass oscillator. Mutual interactions between the laser-induced bubble and secondary bubbles reveal a fast-changing pressure gradient in the liquid. Our study provides a better understanding of laser-induced bubble dynamics in a partially confined environment, which is of practical importance for microfluidics and intraluminal laser surgery.

Evidence of laser-induced nanobubble formation mechanism in water

Principles of laser-induced nanobubble formation in water are studied and presented. Nanobubbles were generated by laser light at intensities below threshold for laser-induced breakdown and subsequently expanded by a rarefaction wave to facilitate their observation and analysis. Different methods were used to study nanobubble formation and characteristics. Firstly, probability of nanobubble formation as a function of water sample purity was examined. Secondly, relation between laser fluence at different wavelengths and the number of generated nanobubbles was investigated. Thirdly, measurements of nanobubble lifetime were conducted indicating a contradiction to the Epstein-Plesset equation-based prediction of free bubble dissociation. Accumulated evidence suggests that the presence of physical impurities is a prerequisite for nanobubble formation. Consequently, a lack of impurities results in the absence of nanobubbles in contrast to assumptions by existing studies. The findings presented in this paper provide new insights into the fundamental properties of laser-induced nanobubbles in water.

Numerical investigation of the cavitation bubble near the solid wall with a gas-entrapping hole based on a fully compressible three-phase model

The solid surface with several cavities containing gas strongly influences the bubble's dynamical behaviors. To reveal the underlying physical mechanism of the cavitation bubble near a rigid boundary with a gas-entrapping hole, a fully compressible three-phase model, accounting for the three-phase volume transport equation, was implemented in OpenFOAM. The predicted bubble shape was validated with the corresponding experimental photos, and good agreement was achieved. The bubble's primary physical features (e.g., the expanding shock wave, upward and downward liquid jet, and high-pressure region) are well reproduced, which helps understand the underlying mechanisms. The numerical results show that the solid wall with a gas-entrapping hole could affect the morphology of both the bubble and liquid jet, as well as shortens the bubble's first oscillation period in comparison to an intact rigid wall. The relationship among the prolongation factor, the standoff distance, and the relative size ratio is analyzed. It is found the prolongation factor increases as the relative size ratio decrease. As the standoff distance decreases, the gas entrapping hole plays a significant role in the oscillation period of the bubble. The current model can be further extended to reveal the microscopic mechanism of aeration avoiding cavitation damage and investigate the interaction between air bubbles and cavitation bubbles, which is of great interest to practical applications.

The evolution of cavitation in narrow soft-solid wedge geometry mimicking periodontal and peri-implant pockets

In periodontology and implantology, laser-induced cavitation has not yet been used to treat biofilm-related problems. In this study we have checked how soft tissue affects the evolution of cavitation in a wedge model representing periodontal and peri-implant pocket geometry. One side of the wedge model was composed of PDMS mimicking soft periodontal or peri-implant biological tissue, the other side was composed of glass mimicking hard tooth root or implant surface, which allowed observations of the cavitation dynamics with an ultrafast camera. Different laser pulse modalities, PDMS stiffness, and irrigants were tested for their effect on the evolution of cavitation in the narrow wedge geometry. The PDMS stiffness varied in a range that corresponds to severely inflamed, moderately inflamed, or healthy gingival tissue as determined by a panel of dentists. The results imply that deformation of the soft boundary has a major effect on the Er:YAG laser-induced cavitation. The softer the boundary, the less effective the cavitation. We show that in a stiffer gingival tissues model, photoacoustic energy can be guided and focused at the tip of the wedge model, where it enables generation of secondary cavitation and more effective microstreaming. The secondary cavitation was absent in severely inflamed gingival model tissue, but could be induced with a dual-pulse AutoSWEEPS laser modality. This should in principle increase cleaning efficiency in the narrow geometries such as those found in the periodontal and peri-implant pockets and may lead to more predictable treatment outcomes.

Near threshold nucleation and growth of cavitation bubbles generated with a picosecond laser

The nucleation and growth of cavitation bubbles few micrometers in size in water generated by a 60 ps 515 nm fiber laser is observed and visualized near nucleation threshold. The study is performed by monitoring the plasma size, the cavitation bubble size and the emitted shock waves. The latter two aspects are supported by the Gilmore model using a Noble-Abel-stiffened-gas (NASG) equations of state. For the first time, two types of cavitation events are identified and visualized that exhibit a difference of more than two orders of magnitude in the excitation energy converted to mechanical effects with minimal change in excitation laser pulse energy. The result is localized cavitation and reduced mechanical stress on water-based media with potentially positive implications for laser treatments of biological tissue.

Laser-induced shock-wave-expanded nanobubbles in spherical geometry

The secondary cavitation generation following laser-induced breakdown in aqueous media in spherical geometry, mimicking the geometry of the frontal part of the human eye, was studied. A numerical simulation of the shock wave propagation was performed, yielding peak-pressure maps, correctly predicting the location of the secondary cavitation onset for different shock wave source positions. The comparison between the simulation results and the experiments, performed with a high-precision, multiple-illumination technique, supports the suggested description of the nature of the secondary cavitation onset. It is shown that large transient negative pressures are created at the location of the acoustic image of the shock wave source, which is different from the optical focus. After the passage of the shock wave, abundant secondary cavitation is generated there. Additionally, the existence of an important contributing factor to the reduction of the secondary cavitation threshold is supported by the experimental results, namely the pre-illumination of the water by the breakdown-generating laser pulse, playing a crucial role in conditioning the medium. There is strong experimental evidence of the existence of another mechanism of pre-conditioning the water for the secondary cavitation onset, namely in the form of repetitive negative pressure pulse passage through the same volume, an indication of a possible two- or multiple-stage process.

Amplification of high-intensity pressure waves and cavitation in water using a multi-pulsed laser excitation and black-TiOx optoacoustic lens

A method for amplification of high-intensity pressure waves generated with a multi-pulsed Nd:YAG laser coupled with a black-TiOx optoacoustic lens in the water is presented and characterized. The investigation was focused on determining how the multi-pulsed laser excitation with delays between 50 µs and 400 µs influences the dynamics of the bubbles formed by a laser-induced breakdown on the upper surface of the lens, the acoustic cavitation in the focal region of the lens, and the high-intensity pressure waves generation. A needle hydrophone and a high-speed camera were used to analyze the spatial distribution and time-dependent development of the above-mentioned phenomena. Our results show how different delays (t_d ) of the laser pulses influence optoacoustic dynamics. When t_d is equal to or greater than the bubble oscillation time, acoustic cavitation cloud size increases 10-fold after the fourth laser pulse, while the pressure amplitude increases by more than 75%. A quasi-deterministic creation of cavitation due to consecutive transient pressure waves is also discussed. This is relevant for localized ablative laser therapy.

Microbubble dynamics and jetting near tissue-phantom biointerfaces

Precise excitation of cavitation is a promising mechanism for microsurgery procedures and targeted drug delivery enhancement. The underlying phenomenon of interest, jetting behaviour of oscillating cavitation bubbles, occurs due to near-surface interactions between the boundary, liquid, and bubble. Within this study we measured boundary effects on the cavitation bubble dynamics and morphology, with an emphasis on observation and measurement of jetting behaviour near tissue-phantom biointerfaces. An important mechanism of boundary poration has been observed using time-resolved optical microscopy and explained for different tissue-phantom surface densities and Young's modulus. Below a critical distance to the boundary, around γ = 1.0, the resulting jets penetrated the tissue-phantom, resulting in highly localized few micrometer diameter jets.

The hydroxyl radical yields prediction of cavitation bubble clouds during hydrodynamic cavitation process for chitosan degradation

In order to measure the influence of chemical effects in the process of hydrodynamic cavitation (HC) degradation of chitosan, a prediction model for the hydroxyl radical (·OH) yields of cavitation...

Dynamics of a Laser-Induced Bubble above the Flat Top of a Solid Cylinder—Mushroom-Shaped Bubbles and the Fast Jet

The dynamics of a laser-induced bubble on top of a solid cylinder is studied both experimentally and numerically. When the bubble is generated close to the flat top along the axis of the cylinder and its maximum radius exceeds the one of the flat top surface, it collapses in the form of a mushroom with a footing on the cylinder, a long stem and a hat-like cap typical for a mushroom head. The head may collapse forming a thin, fast liquid jet into the stem, depending on bubble size and bubble distance to the top of the cylinder. Several experimental and numerical examples are given. The results represent a contribution to understand the behavior of bubbles collapsing close to structured surfaces and in particular, how thin, fast jets are generated.

New Approach toward Laser-Assisted Modification of Biocompatible Polymers Relevant to Neural Interfacing Technologies

We report on a new approach toward a laser-assisted modification of biocompatible polydimethylsiloxane (PDMS) elastomers relevant to the fabrication of stretchable multielectrode arrays (MEAs) devices for neural interfacing technologies. These applications require high-density electrode packaging to provide a high-resolution integrating system for neural stimulation and/or recording. Medical grade PDMS elastomers are highly flexible with low Young’s modulus < 1 MPa, which are similar to soft tissue (nerve, brain, muscles) among the other known biopolymers, and can easily adjust to the soft tissue curvatures. This property ensures tight contact between the electrodes and tissue and promotes intensive development of PDMS-based MEAs interfacing devices in the basic neuroscience, neural prosthetics, and hybrid bionic systems, connecting the human nervous system with electronic or robotic prostheses for restoring and treating neurological diseases. By using the UV harmonics 266 and 355 nm of Nd:YAG laser medical grade PDMS elastomer is modified by ns-laser ablation in water. A new approach of processing is proposed to (i) activate the surface and to obtain tracks with (ii) symmetric U-shaped profiles and (iii) homogeneous microstructure This technology provides miniaturization of the device and successful functionalization by electroless metallization of the tracks with platinum (Pt) without preliminary sensitization by tin (Sn) and chemical activation by palladium (Pd). As a result, platinum black layers with a cauliflower-like structure with low values of sheet resistance between 1 and 8 Ω/sq are obtained.