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

Abstract

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.

Optodynamic energy-conversion efficiency during an Er:YAG-laser-pulse delivery into a liquid through different fiber-tip geometries

“…3(a) and 3(b)]. This finding is also consistent with the pressure measurements performed with a hydrophone presented in the paper by Frenz et al 37 However, their laser had a wavelength of λ ¼ 2.12 μm, so the characteristic time t s ¼ 0.2 μs in their case is comparable to the time of the vapor formation. For this reason, they observed a slow increase in the pressure during the first bubble expansion.…”

“…With these measurements, we can explain how the FT's geometry, the laser-pulse energy, and the pulse duration influence the cavitation bubble size, which further determines the acoustic transient amplitudes. 32,[34][35][36][37] The experiments were performed with a commercially available laser for clinical applications in laser dentistry. Thus, the experimental parameters are very similar to the parameters in clinical practice.…”

Hollow steel tips for reducing distal fiber burn-back during thulium fiber laser lithotripsy

“…18 The cavitation bubbles provide a conduit for the subsequent laser energy to reach the stone with little absorption. Fig.…”

Combined control of morphology and polymorph in spray drying of mannitol for dry powder inhalation

“…During bubble formation and rupture, several processes may trigger crystallization. For instance, the inflation and rupture of the cavitation bubbles may cause transient pressure due to dramatic changes in the pressure, possibly leading to improved crystallization [52]. It is speculated that a similar cavitation phenomenon during hot stage microscopy might occur during spray-drying, but this cannot be validated at present since it is hard to observe experimentally during the spray drying process.…”