**7. Conclusion**

114 Hydrodynamics – Advanced Topics

Digits show the period of time from the beginning of filament destruction (Yusupov et al., 2011b).

destroying filament.

stability.

Fig. 20. Renewal of destroyed filament of Ag nanoparticles in water nearby the tip of optical fiber. intense heat source. That causes explosive water boiling, intense formation of microbubbles, moving rapidly away from fiber tip to liquid (see for example Fig. 1,b) and

We rationalize the observed effect of laser-induced self-organization of filaments from Ag nanoparticles by following mechanisms. Initially (Fig. 21a), laser light absorption by water (the absorption coefficient in water at 0.97 µm is about 0.5 cm-1) causes its heating with the 2-10ºС/s rate. Besides, the intense transfer of impulse to water takes place in this case. As a result, the closed axis-symmetric liquid flows are developed being directed from fiber tip. These flows promote Ag nanoparticles intrusion into the laser beam nearby the fiber tip

Another factor dominates at the second stage of filament self-organization. The refractive index for collargol *nc* is higher than that for clean water *nw*. The value of *nc-nw* = 0.0044 at wavelength λ=1256 nm was directly measured in our experiments using fiber-optic densitometer. Due to the effect of total internal reflection laser light is concentrated inside intrusion which work in fact as a liquid optical fiber. Channeling of laser light inside intrusion with Ag nanoparticles results in deeper propagation of laser light into water. Light pressure promotes faster movement of intrusion front giving rise to filament (Fig. 21c). As it was shown in (Brasselet et al., 2008), for example, laser light pressure is also able to force through the boundary between two unmixed liquids and to form thin channel of one liquid inside another one, thus forming liquid optical fiber with gradient core. Thus, the image of filament in transmission mode shows optical density of Ag nanoparticles. At the same time the image of filament in scattering mode clearly demonstrate channeling effect in fabricated filament which in fact is a liquid gradient fiber. Such liquid gradient fiber provides also effective channeling of 970nm laser beam, thus promoting filament elongation and spatial

(Fig. 21b). Such intrusions are clearly seen in scattered green laser light (Fig. 4).

Hydrodynamic effects induced by a medium power (1–5 W) laser radiation in the vicinity of the heated fiber tip surface in water and in water-saturated tissues are considered. A threshold character of the dynamics of liquid is demonstrated. At a relatively low laser power (about 1 W), the slow formation of vapor-gas bubbles with sizes of hundreds of microns are observed at the optical fiber tip surface. The bubbles can be attached to the tip surface in the course of laser radiation. At higher laser power increases, effective hydrodynamic processes related to the explosive boiling in the vicinity of the overheated fiber tip surface take place. The resulting bubbles with sizes ranging from a few microns to several tens of microns provide the motion of liquid. The estimated velocities of bubbles in

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the vicinity of the fiber tip surface can be as high as 100 mm/s. Generation of bubbles in the capillary leads to the circulating water flows with periods ranging from 0.2 to 1 s. Such circulation intensity increases with the laser power. For the laser radiation with a wavelength of 0.97 μm, we observe such effects only for the blackened fiber tip surface, which serves as a local heat source. At a laser power of less than 3 W, stable bubble microjets, which consist of the bubbles (ranging from several to ten microns) can be generated in the vicinity of the blackened tip surface.

Laser-induced hydrodynamic effects in water and bio-tissues can cause the significant degradation of the fiber tip. Cavitation collapse of bubbles in liquid in the vicinity of fiber tip surface gives rise to the high-speed cumulative microjets which can destroy the solid surface. This effect leads to multiple cracks on the film and the formation of the porous structure, formation of supercritical water and even generation of diamonds nano-crystal.

Laser-induced hydrodynamics processes in water and water-saturated bio-tissues are accompanied by generation of intense acoustic waves in resonance conditions, even of shock-type waves. The acousto-mechanic processes results in mixing and transport of gassaturated degenerated tissue in the space of defect.

We found that medium power (0.3- 8 W) 0.97 µm in wavelength laser irradiation of water with added Ag nanoparticles (in the form of Ag-albumin complexes) through 400μm optical fiber stimulates self-organization of unexpectedly thin (10-80 µm) and lengthy (up to 14 cm) filaments of Ag nanoparticles in the form of liquid gradient fibers. These filaments in water are stable in the course of laser irradiation being destroyed after laser radiation off. Such effect of filaments of Ag nanoparticles self-organization is rationalized by the peculiarities of laser-induced hydrodynamic processes developed in water in presence of laser light.
