Laser-induced Bubble Formation Research Articles

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.

Bubble Dynamics during Laser Irradiated Thermo-Mechanical Response of Pigmented Skin Phantom

During the laser treatment of pigmented dermatosis such as Nevus of Ota, vapor bubbles will be generated by the laser with short pulse width and high energy density. Laser irradiation is efficacious for the clinical treatment of Ota’s Nevus caused by hyperplasia of melanosomes in dermis. Since the mechanism of the laser–melanosome interaction is not yet clear, the clearance rate is generally low and bleeding of irradiated skin frequently occurs. This work conducted a flow visualization experiment to investigate the laser–melanosome interaction mechanism by using high-speed imaging. Pigmented phantom was prepared to simulate the diseased dermis tissue, where agar acted as substrate and synthetic melanin particles was infused as hyperplastic melanosomes. Putting the phantom into water, its thermo-mechanical responses to single-pulse 1064-nm Nd:YAG laser irradiation with energy density of 4–7 J/cm2 and pulse duration of 6 ns were recorded. The results indicated that laser-induced bubble formation caused by the gasification of tissue moisture is the key mechanism of laser–melanosome interaction, and an optimal energy density of 6 J/cm2 is recommended.

Effect of size and interparticle distance of nanoparticles on the formation of bubbles induced by nanosecond laser

When a laser beam is irradiated on nanoparticles (optical absorbers) in a liquid, massive heat is generated, which causes vaporization of a nearby liquid, followed by the formation of bubbles. Despite the potential application of these bubbles for cancer diagnosis and therapy, solar energy harvesting, and local chemical reaction enhancement, little is known on how the size and interparticle distance of optical absorbers affect laser-induced bubble formation. Additionally, observing the bubble dynamics near nanomaterials is challenging due to their fast growth and immediate collapse. In this study, we trapped bubbles using hydrogels and investigated how their diameter changed with the size and interparticle distance of nanoparticles or their agglomerates and the energy level of laser irradiation. We found that the size of trapped bubbles increased with softer hydrogels, higher laser energy level, larger agglomerate size, and smaller interparticle distance among agglomerates. Based on these observations, we suggest that nanobubbles initially formed on nanoparticles merged to become microbubbles.

Multi-pulse laser-induced bubble formation and nanoparticle aggregation using MoS2 nanoparticles

Understanding of how particles and light interact in a liquid environment is vital for optical and biological applications. MoS2 has been shown to enhance nonlinear optical phenomena due to the presence of a direct excitonic resonance. Its use in biological applications is predicated on knowledge of how MoS2 interacts with ultrafast (< 1 ps) pulses. In this experiment, the interaction between two femtosecond pulses and MoS2 nanoparticles suspended in liquid is studied. We found that the laser pulses induce bubble formation on the surface of a nanoparticle and a nanoparticle aggregate then forms on the surface of the trapped bubble. The processes of formation of the bubble and the nanoparticle aggregation are intertwined.

Influence of Laser-Induced Bubble Formation on Laser Chemical Machining

Laser Chemical Machining (LCM) is a non-conventional processing method, which enables very accurate and precise ablation of metallic surfaces. Material ablation results from laser-induced thermal activation of heterogeneous chemical reactions between electrolytes and a metallic surface. However, when processing metallic surfaces with LCM, large fluctuations in ablation quality can occur due to rising bubbles. The for-mation of bubbles during laser chemical machining and their influence on the ablation quality has not been investigated. For a more detailed investigation of the bubbles, ablation experiments on Titanium and Ce-ramic under different thermal process conditions were performed. The experiments were recorded by a high-speed camera. The evaluation of the video sequences was performed using Matlab. The resulting bubbles were analyzed regarding their size and frequency. The results show that boil-ing bubbles formed on both materials during processing. Titanium also produces smaller bubbles, which can be identified as process bubbles ac-cording to their size. Furthermore, it was found that undisturbed laser chemical ablation can be achieved in the presence of a boiling process, since both boiling bubbles and process bubbles were detected during machining within the process window.

Laser-induced carbon nanotube microcapsules formation through depletion enhanced deposition

We report a method to fabricate carbon nanotube (CNT) microcapsule through laser-induced bubble formation in a polyethylene glycol-modified CNT (PEG-CNT) solution in the presence of PEG. A low-power laser is focused on a heat-absorbing surface to create a bubble. While PEG-CNTs in the solution are deposited on the bubble surface to form a microcapsule. The shape stability of microcapsule during swelling process is measured in different concentrations of PEG solutions. The stable CNTs microcapsules form with uniform spherical shape within 8 seconds in a 2% PEG solution, while PEG-CNTs only randomly aggregate without the presence of PEG molecules. Thus, we propose that the deposition of PEG-CNTs is enhanced by PEG molecules due to depletion interaction. Comparing with other methods, assembling PEG-CNTs microcapsules at liquid-air interface of polymer solution by optical heating greatly reduce the time and provide good quality of CNTs microcapsules. This study may enable applications of PEG-CNTs microcapsules.

Laser-induced bubble formation on a micro gold particle levitated in water under ultrasonic field

Laser-induced bubble formation on a micro gold particle with a radius of 5 µm, levitated at the center of a spherical flask filled with water under ultrasound, was investigated experimentally and theoretically. Bubble formation and subsequent growth and collapse were visualized by a high-speed camera. The strength of shock emitted during the expansion of the bubble formed on the gold particle was measured by a needle hydrophone at various positions from the center of the flask. In the theoretical study, the time-dependent radius of the laser-induced bubble with/without gold particle was obtained using solutions of the Navier-Stokes equations for the vapor inside the bubble and the liquid adjacent to the bubble wall. The shock strengths at various points were also obtained using the Kirkwood-Bethe hypothesis with the obtained time-dependent bubble radius and the pressure at the bubble wall. Calculation results provided the correct homologous behavior of bubble motion and shock strengths at various points from the bubble center. The mass increase due to evaporation near the bubble collapse crucially affected the bouncing motion after the first bubble collapse.

Alternativen zur Femtosekundentechnologie

In refractive corneal surgery femtosecond (fs) lasers are used for creating LASIK flaps, dissecting lenticules and for astigmatism correction by limbal incisions. Femtosecond laser systems are complex and expensive and cutting precision is compromised by the large focal length associated with the commonly used infrared (IR) wavelengths. Based on investigations of the cutting dynamics, novel approaches for corneal dissection using ultraviolet A (UVA) picosecond (ps) pulses and ring foci from vortex beams are presented. Laser-induced bubble formation in corneal stroma was investigated by high-speed photography at 1-50 million frames/s. Using Gaussian and vortex beams of UVA pulses with durations between 200 and 850 ps the laser energy needed for easy removal of flaps created in porcine corneas was determined and the quality of the cuts by scanning electron microscopy was documented. Cutting parameters for 850 ps are reported also for rabbit eyes. The UV-induced and mechanical stress were evaluated for Gaussian and vortex beams. The results show that UVA picosecond lasers provide better cutting precision than IR femtosecond lasers, with similar processing times. Cutting energy decreases by >50 % when the laser pulse duration is reduced to 200 ps. Vortex beams produce a short, donut-shaped focus allowing efficient and precise dissection along the corneal lamellae which results in a dramatic reduction of the absorbed energy needed for cutting and of mechanical side effects as well as in less bubble formation in the cutting plane. A combination of novel approaches for corneal dissection provides the option to replace femtosecond lasers by compact UVA microchip laser technology. Ring foci are also of interest for femtosecond laser surgery, especially for improved lenticule excision.

Breast cancer therapy by laser-induced Coulomb explosion of gold nanoparticles.

Laser-induced Coulomb explosion of gold nanoparticles for breast cancer has been studied by nanophotolysis technique. This study aimed to investigate whether laser-induced bubble formation due to Coulomb explosion can provide an effective approach for selective damage of breast cancer with gold nanoparticles. Numerical method involves laser-induced Coulomb explosion of gold nanoparticles. Different parameters related to nanophotolysis such as laser fluence, tumor depth, cluster radius, laser pulse duration, and bubble formation is studied numerically. Numerical simulation was performed using Mat lab. The gold nanoparticles of 10, 20, 30, 40, and 50 nm in radius could penetrate into tumor 1.14, 1.155, 1.189, 1.20 and 1.22 cm in depth respectively. The maximum penetration depth in tumor could be obtained with nanoparticles of 50 nm radius. Short laser pulse of 40 ns with nanoparticles of 10 nm radius could penetrate into tumor 1.14 cm in depth. Bubbles with a radius of 9 µm could effectively kill breast cancer cells without damaging healthy ones. The bubble radius increased from 4 to 9 µm with an increase in pulse duration in the range of 10 to 30 ns. Gold nanoparticles with increasing radius and bubble formation for selective damage of breast cancer cells are successfully probed. The present calculated results are compared with other experimental findings, and good correlation is found between the present work and previous experimental values. It was demonstrated that bubble formation in tumor may further increase the efficacy of breast cancer treatment.

Nonlinear microscopy techniques for assessing the UV laser polymer interactions

A new diagnostic approach for assessing the in-depth laser induced modifications upon ultraviolet polymer irradiation is presented. The methodology relies on the observation of morphological alterations in the bulk material (Paraloid B72) by using third harmonic generation. This non destructive methodology allows the detailed and accurate imaging of the structurally laser modified zone extent in the vicinity of the irradiated area. Additionally, for the first time, the visualization and quantitative determination of the contour of the laser-induced swelling/bulk material interface is reported. The observed polymer surface swelling following single-pulse KrF laser irradiation at sub-ablation fluences is interpreted in the context of a model for laser-induced bubble formation due to droplet explosion mechanism.

Plasma Membrane Integrity and Survival of Melanoma Cells After Nanosecond Laser Pulses

Circulating tumor cells (CTCs) photoacoustic detection systems can aid clinical decision-making in the treatment of cancer. Interaction of melanin within melanoma cells with nanosecond laser pulses generates photoacoustic waves that make its detection possible. This study aims at: (1) determining melanoma cell survival after laser pulses of 6 ns at λ = 355 and 532 nm; (2) comparing the potential enhancement in the photoacoustic signal using λ = 355 nm in contrast with λ = 532 nm; (3) determining the critical laser fluence at which melanin begins to leak out from melanoma cells; and (4) developing a time-resolved imaging (TRI) system to study the intracellular interactions and their effect on the plasma membrane integrity. Monolayers of melanoma cells were grown on tissue culture-treated clusters and irradiated with up to 1.0 J/cm2. Surviving cells were stained with trypan blue and counted using a hemacytometer. The phosphate buffered saline absorbance was measured with a nanodrop spectrophotometer to detect melanin leakage from the melanoma cells post-laser irradiation. Photoacoustic signal magnitude was studied at both wavelengths using piezoelectric sensors. TRI with 6 ns resolution was used to image plasma membrane damage. Cell survival decreased proportionally with increasing laser fluence for both wavelengths, although the decrease is more pronounced for 355 nm radiation than for 532 nm. It was found that melanin leaks from cells equally for both wavelengths. No significant difference in photoacoustic signal was found between wavelengths. TRI showed clear damage to plasma membrane due to laser-induced bubble formation.

Femtosecond Pulsed Laser as a Microscalpel for Microdissection and Isolation of Specific Sections from Biological Samples

The femtosecond laser is a candidate tool for dissecting and obtaining specific tissues or cells from biological samples. In this paper, we report on the dissection of flammable (heat-sensitive) plant tissues (Eucommia ulmoides stem) and that of a transparent zebrafish embryo placed in water. During laser dissection in the plant tissues, we observed that a low repetition rate of the irradiations is crucial for reducing the risk of thermal damage. During laser dissection of the zebrafish embryo, the laser-induced bubble formation at the tissue surface disturbed the laser-induced dissection. We concluded that the “interior laser dissection method”, in which the dissection is performed by moving the laser spot inside a biological sample, is an efficient way to dissect a sample that is placed in water. In view of our findings, the advantages of fs laser dissection are discussed on the basis of the physical characteristics of fs laser machining.

Anthracene Crystallization Induced by Single-Shot Femtosecond Laser Irradiation: Experimental Evidence for the Important Role of Bubbles

A mechanism of femtosecond laser-induced crystallization was investigated using a supersaturated solution of anthracene. When a single-shot femtosecond laser pulse with a pulse energy above 3.1 μJ/pulse was shot into a sufficiently supersaturated solution, crystallization of anthracene was induced immediately after irradiation at the vicinity of the laser focal point. The threshold energy of the crystallization (3.1 μJ/pulse) was in agreement with that of laser-induced bubble formation, which was a sequential process after shockwave emission, cavitation bubble formation, and collapse. Sufficient supersaturation for crystallization decreased with an increase in the pulse energy. These results suggest that crystallization is triggered in the processes resulting in bubble formation. Furthermore, crystallization was enhanced at the surface of the bubble. The crystallization mechanism was completely different from that reported previously based on photochemical reactions or molecular alignment due to a strong optical field.

Microbubbles-overlapping mode for laser killing of cancer cells with absorbing nanoparticle clusters

Laser-induced bubble formation around nanoparticles may play a crucial role in selective laser nanophotothermolysis of cancer cells targeted with nanoparticles. In this paper, we propose theoretically, and confirm experimentally, a new dynamic mode for selective cancer treatment that involves the overlapping of bubbles inside the cell volume. This bubbles-overlapping mode (BOM) can dramatically increase the efficiency of cancer treatment by laser-heated nanoparticles as a result of the large damage range. On the basis of nanoparticle optics below the diffraction limit and the kinetic model of bubble dynamics, we found the criteria and conditions (interparticle distance and particle size and concentration) for BOM initiation in cancer cells by laser radiation. Using MDA-MB-231 breast cancer cells, we showed that the optimal size range of the gold nanoparticles for effective laser initiation of BOM is 30–40 nm and the lower concentration limit is n ≈ 2.44 × 1011 cm–3 (i.e. the absolute number of particles homogeneously distributed inside a tumour cell is n ≈ 430). It was demonstrated that the formation of nanoclusters on the cell surface with sizes larger than the sizes of individual nanoparticles, may further increase the efficiency of the laser treatment of cancer.

Synergistic enhancement of selective nanophotothermolysis with gold nanoclusters: Potential for cancer therapy

We developed a new approach that enhances selective photothermolysis of tumor through laser activation of synergistic phenomena around nanoclusters, which are self-assembled into cancer cells. In vitro verification of this approach was performed by laser pulse irradiation (420-570 nm and 1064 nm; 8-12 nanosecond; 0.1-10 J/cm2) of MDA-MB-231 breast cancer cells targeted with primary antibodies to which 40-nm gold nanoparticles were selectively attached by means of secondary antibodies. Photothermal (PT) radiometry, thermolens techniques, electron microscopy, atomic force microscopy, silver and gold enhancing kits, and viability test (Annexin V-propidium iodide) were employed to study nanoparticle spatial organization, the dynamics of microbubble formation, and cell damage. The assembly of gold nanoclusters on the cell membrane was accompanied by increased local absorption and red-shifting as compared to cells that did not have nanoclusters. These effects were amplified by a silver-enhancing kit and pre-irradiation of cells with low laser-pulse energy. Finally, a significant increase in laser-induced bubble formation and cancer cell killing was observed using near-IR lasers (1064 nm). A cancer cell antigens was used to provide target specificity for nanoclusters formation making the cancer cells sensitive to laser activation. The described approach uses relatively small and simple gold nanoparticles offering more effective delivery to target. In addition, the further self-assembling of these nanoparticles into nanoclusters on live cells provides significant enhancement of laser-induced cell damage. These nanoclusters (gold "nanobombs") can be activated in cancer cells only by confining near-IR laser pulse energy within the critical mass of the nanoparticles in the nanoclusters.

Zur Entstehung von Kavitationsblasen bei der Erbium:YAG-Laser-Vitrektomie

Clinical and experimental studies demonstrated the potential advantages of Erbium:YAG laser vitrectomy for posterior segment surgery. However, a detailed knowledge on the laser-tissue interaction is needed for an optimization of new ophthalmic applications. The aim of this experimental work was to investigate the cavitation bubble formation during Erbium:YAG laser vitrectomy and to find optimized laser parameters and the best geometry of the aspiration port of the microsurgical probe. We investigated the formation of cavitation bubbles in water by high-speed photography. The output energy at the quartz tip reached up to 50 mJ and the laser pulse duration ranged from 50 to 300 microseconds. Various commercially available microsurgery probes were investigated regarding the extent of the cavitation bubbles. The threshold for laser-induced cavitation bubble formation was found to be 0.32 +/- 0.1 mJ for a laser pulse duration of 130 microseconds and a core diameter of 320 microns of the quartz fiber tip. The length of the cavitation bubbles increases with the laser pulse energy up to a length of 1.6 mm at a pulse energy of 10 mJ. In contrast, the size of the vapor bubbles decreases with an increase of the laser pulse duration. A slit-shaped aspiration port led to a 50% smaller volume of the cavitation bubble exiting the port compared with a circular aspiration port. The optimized laser parameters and microsurgery probe geometry may significantly decrease the risk of intraoperative ocular damages during Erbium:YAG laser vitrectomy.

Laser-induced bubble formation and collapse in a nonlinear Rayleigh-Taylor unstable interface in a thin layer of molten metal

Bubble formation and bubble collapse zones in a laser-induced nonlinear Rayleigh-Taylor (R-T) instability were shown to follow the contours of a series of R-T shock-fronts generated on an irregular planar (terrace-like) target surface. Bubble formation occurs in the regime of target planar vaporization. Spatial variation of the bubble density distribution, the bubble size, and the bubble-bubble distance, as a function of distance from the shock front envelope, were determined. Bubble collapse occurs in the regime of planar-to-volume boiling transition and proceeds by the so-called “chain reaction” collapse mechanism inside a 2D bubble array. The contours of bubble generation and of bubble collapse were simulated by using the analytical model of Ott with variable phase and variable amplitude of R-T modes.

Starting mechanisms and dynamics of bubble formation induced by a Ho:Yttrium aluminum garnet laser in water

The starting mechanisms and dynamics of laser-induced bubble formation at a submerged fiber tip in distilled water were experimentally investigated using pressure measurements and fast flash videography. A fiber guided Ho:YAG laser operating in the free running (τ=200 μs) and Q-switched (τ=45 ns) mode at a wavelength of λ=2.12 μm was used as a light source. It is shown that the beam profile at the distal fiber tip (multimode fiber d=300 μm) exhibits hot spots that result in an inhomogeneous temperature distribution in the heated water volume. Depending on the laser irradiance, three different bubble formation processes are distinguished: bubble formation by heating, by rarefraction (cavitation), and by a combination of these two processes. For laser irradiances of less than 0.5 MW/ cm2 bubble formation takes place at temperatures near the critical point of water (T=280 °C). A rapid decrease in the threshold temperature for bubble formation was found for laser irradiances between 0.5 and 1 MW/cm 2. At laser irradiances higher than 3 MW/cm2, microbubbles with radii of up to 20 μm were formed at the front of the laser pulse even though the average water temperature was far below 100 °C. The water temperature distribution during the laser pulse was determined by numerical simulation. Simultaneous pressure measurements revealed that each subablative laser spike induces a bipolar pressure transient. The onset of the bubble expansion was found to be correlated with a characteristic pressure increase that can be used for on-line monitoring of the ablation process. The distortion of the temporal profile of the pressure wave is shown to be an effect of diffraction. The reduction of pressure by the negative part of the bipolar pressure transients leads to a lowering of the evaporation pressure and therefore to the initiation of bubbles by cavitation. With increasing irradiance this mechanism becomes more efficient.

Photographic studies of laser-induced bubble formation in absorbing liquids and on submerged targets: implications for drug delivery with microsecond laser pulses

Pulsed laser ablation of blood clots in a fluid-filled blood ves- sel is accompanied by an explosive evaporation process. The resulting vapor bubble rapidly expands and collapses to disrupt the thrombus (blood clot). The hydrodynamic pressures following the bubble expan- sion and collapse can also be used as a driving force to deliver clot- dissolving agents into thrombus for enhancement of laser thrombolysis. Thus, the laser-induced bubble formation plays an important role in the thrombus removal process. We investigate the effects of boundary con- figurations and materials on bubble formation with time-resolved flash photography and high-speed photography. Potential applications in drug delivery using microsecond laser pulses are also discussed. © 1998 So- ciety of Photo-Optical Instrumentation Engineers. (S0091-3286(98)00108-1)

Absolute pressure measurements on a nanosecond time scale using surface plasmons

Transient acoustic waves generated by laser-induced bubble formation at a liquid–solid interface are sensitively monitored using optically excited surface plasmons. This method enables the detection of both the compressive and tensile waves with high accuracy as demonstrated for the propagation and reflection of acoustic pulses at a quartz–water interface. Unique advantages of this new technique are the high sensitivity of 0.1–0.2 MPa that could be achieved for absolute pressure measurements on a nanosecond time scale and its ability to probe exact pulse profiles due to the localized probe depth of surface plasmons.