**6. Formation of filaments**

110 Hydrodynamics – Advanced Topics

The more detailed studies show that for both *in vivo* and *in vitro* cases laser-induced generation of short-pulse intense quasi-periodic acoustic signals. The fragment of spectrogram of acoustic response given at Fig. 17 clearly demonstrates temporal change of spectral components for acoustic signal generated from laser irradiated nucleus pulposus *in vitro* when optical fiber

was moved forward in the intervertebral disc (similar to shown at Fig. 1).

Fig. 17. The fragment of spectrogram (a) ant temporal structure of single pulse (b) of

maxima in the following spectral intervals: 600 – 700 Hz, 1 - 2 kHz and nearby 10 kHz.

strong visual vibrations of needle with laser fiber.

(Buschmann et al., 1995; Bagratashvili et al., 2006).

As one can see, the acoustic response in this case has the form of short, intense and broadband (from 0 to 10 kHz) pulses of about 10 ms in duration combined into the series of pulses generated with frequency of 40 Hz. Fig. 17b shows that the amplitude of single pulse is an order of amplitude higher than the background acoustic noise. The most of acoustic power is concentrated in such pulses. The broad spectrum of acoustic pulses and their low duration indicate to shock-type of generated acoustic waves. The acoustic noise has broad spectral

Appearance of these bands are caused by the dynamics of vapor-gas mixture and are associated with acoustic resonances of the system. Notice that laser-induced formation of channels in degenerated spinal discs *in vitro* has been accompanied by 4 Hz in frequency

Generation of such a strong acoustic vibrations is caused in our opinion by contact of overheated (up to >1000 ºС (Yusupov et al., 2011a)) fiber tip with water and water-saturated tissue of spinal disc. Such contact can result in explosive boiling of water solution nearby the fiber tip and, also, in burning of collagen in cartilage tissues. Intense hydrodynamic processes can take place nearby optical fiber tip, which are caused by fast heating of water and tissue, by generation and collapse of vapor-gas bubbles (Chudnovskii et al., 2010a, 2010b; Leighton, 1994). As a result, the free space of disc or bone is filled by liquid saturated by vapor-gas bubbles. Resonance vibrations are excited, since both disc and bone are quite good acoustic resonators. These vibrations give rise to low-frequency modulation of acoustic noise (Fig. 16) and to quasi-periodic generation of short intense pulses (Fig. 17) (Chudnovskii et al., 2010a). The acousto- mechanic shock-type processes in resonance conditions results in mixing and transport of gas-saturated degenerated tissue in the space of defect (Chudnovskii et al., 2010b). These processes destroy hernia and decrease its density (Fig. 2b), thus lowering the pressure to nervous roots. Another important impact of such processes is the regeneration of disc tissues through the effects of mechanobiology

acoustic response generated from laser irradiated nucleus pulposus *in vitro*.

 In this division we will show that existence of strongly absorbed agents (in a form of Ag nanoparticles, in particular) in laser irradiated water nearby optical fiber tip can result in appearance of filamentary structures of these agents (Yusupov et al., 2011b). Medium power (0.3 – 8.0 W) 0.97 µm in wavelength laser irradiation of water with added Ag nanoparticles (in the form of Ag-albumin complexes) through 400 µm optical fiber stimulates selforganization of filaments of Ag nanoparticles for a few minutes. These filaments represent themselves long (up to 14 cm) liquid gradient fibers with unexpectedly thin (10 – 80 μm) core diameter. They 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 and by formation of liquid fibers.

Fiber laser radiation (LS-0,97 IRE-Polus, Russia) 0-10 W in output and 0.97 µm in wavelength was delivered into water-filled plastic cell through 400 µm transport silica optical fiber, which was placed horizontally in the cell. Low intensity (up to 1 mW) green pilot beam from the built in diode laser was used to highlight the 0.97 µm laser irradiated zone in the cell. The cell was placed at the sample compartment of optical microscope (MC300, MICROS, Austria) equipped with color digital video-camera (Vision). Spectroscopic studies were performed with fiberoptic spectrum analyzer (USB4000, Ocean Optics) and UV/vis absorption spectrometer (Cary 50, Varian). To measure the refraction index of collargol we have applied the fiber-optic reflectometer FOR-11 (LaserChem, Russia), which provides 10-4 precision of refraction index measurements at 1256 nm wavelength. Cleavage of transport optical fiber has been always produced just before each experiment. Ten minutes later (to provide reasonable attenuation of hydrodynamic motions in the cell) the drop (0.01–1 ml in volume) of brown colored collargol (complex of 25 nm in size Ag nanoparticles with albumin) has been smoothly introduced into the water cell 0.5-10 mm aside from the optical fiber tip.

Our in situ optical microscopic studies of laser-induced filament formation were accomplished in two different modes: 1) in transmission mode, using illumination with white light from microscope lamp; 2) in scattering mode, using illumination with green light of pilot laser beam through the same transport fiber.

Experiments show that 0.97 µm fiber laser irradiation of water in the cell with introduced collargol drop causes (in some period of time from seconds to minutes) formation of thin and long quite homogenous filaments, growing along the axis of 0.97 µm laser beam in water. These filaments are brown colored (that gives the evidence of enhanced Ag nanoparticles concentration in filament) and can be seen even with unaided eye.

Fig. 18 demonstrates the microscope image (in transmission mode) of one of such filaments. This filament is located along the axis of output laser beam and is about 17 mm in length. The measured profile of optical density of this filament is triangular in its shape with about the same widths along filament (determined at half-maximum) of ~200 μm.

Fig. 18. Micro-image (in transmission mode) of filament of Ag nanoparticles fabricated in water nearby optical fiber tip at 2.5 W of laser power (Yusupov et al, 2011b).

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 113

It is of importance that filaments of Ag nanoparticles have been formed in our experiments only in the case of existence of initial collargol concentration gradient in laser irradiated water (when collargol drop was introduced initially into water aside from fiber tip). When collargol drop was premixed in water cell before laser irradiation, formation of filaments has never been observed (at any collargol concentrations in the cell and at any laser powers and

The initial stage of filament self-organization process can be clearly seen in scattering mode (Fig. 4). Some visible hydrodynamic flows take place nearby the fiber tip when laser power is on. Such flows result in intrusion of collargol from neighboring area into the area in front of the fiber tip. The slanting filament structure is clearly seen at Fig. 4. One can also see here the initial process of new intrusion formation (outlined with dashed line). The rate of rise-up front of a given intrusion (which is about 150 μm in average thickness) is found to be described be exponential low (1): at 1 mm from laser fiber tip *V*= 1.5· 10-2 cm/s, while at 2

We revealed that filaments of Ag nanoparticles self-organized in the course of 0.97 µm laser irradiation can exist in the cell (in the presence of laser beam and with no external mechanical distortions of liquid in the cell) for quite a long period of time. We have supported such filaments for tens of minutes. Notice that both rectilinear and curved

After 0.97 µm laser radiation being off, the filaments of Ag nanoparticles have been completely destroyed for 10 – 30 s period of time. Notice that time Δt of diffusion blooming

> <sup>3</sup>

where *D* – is diffusion coefficient of nanoparticle; *k*= 1.38· 10-23 J/K – Boltzmann constant; *T(K)* – absolute temperature; *μ* = 1,002· 10-3 (N· s/m2) – dynamic viscosity of water; *d*=25

External mechanical distortions of filament of Ag nanoparticles results in its destruction. However after mechanical distortion being off, the filament can be renewed completely in presence of 0.97 µm laser radiation. Fig. 20 shows the dynamic of such filament renovation after the distortion of self-organized filament (produced by its rapid crossing withthin a metal needle). As one can see from Fig. 20, complete renewal took place for quite a short

Our experiments have shown that there is some range of 0.97 µm laser powers for which the effect of laser-induced filament self-organization takes place and is, also, stable and reproducible. At laser powers higher than 8 W we have newer observed filament formation. At 0.2-0.5 W laser power filaments have been formed but have been unstable. The most stable and long-living filaments were observed in 0.5-3 W laser power range. At laser power less than 0.2 W we have never observed such filament formation. The instability of filaments and even their absence at high laser powers is caused by intense laser-induced hydrodynamic processes nearby the fiber tip. Our experiments show that the fiber tip surface is gradually covered by a deposit, which absorbs laser radiation quite well. The wide absorption band of deposit observed at fiber tip can be caused by island film of Ag nanoparticles, and, possibly, by elementary carbon absorption (deposited at fiber tip due to albumin thermo-decomposition). As a result of such deposits, the fiber tip becomes an

*<sup>d</sup>* , (6)

*kT x Dt t*

2

mm from laser fiber tip *V* falls down to 3· 10-3cm/s.

filaments were self-organized in our experiments.

nm Ag nanoparticle diameter) gives *Δt* =25 s for =100 μm.

of filament by value, estimated by formula

period of time (~ 20 s).

dozes).

Fig. 19a demonstrates the micro-image of another laser fabricated filament in scattering mode. Intensity of light scattered from this filament decreases gradually with the distance from fiber tip. Attenuation of green light in this case is caused by absorption and scattering of green light in the course of its propagation through the filament. To reveal the peculiarities of filament (given at Fig. 19a) we have performed the following processing of its microscope image: all vertical profiles of image were normalized to local maximum (Fig. 19c); the microscope image was represented in shades of gray (Fig. 19b). As it follows from figures 19b and 19c the length of given filament is about 6 mm, its average width is about 40 μm, and scattering intensity decreases rapidly with the distance from filament axis. Notice that vertical profiles of all fabricated filaments (in both transmission and scattering modes) are almost triangular with a sharp top. It was also established that the end of filament has always a needle-like shape and, also, the width of filament obtained in transmission mode measurements exceeds 3-5 times that obtained in scattering mode.

Fig. 19. a - Microscopic picture of filament (in scattering mode) of Ag nanoparticles fabricated in water nearby optical fiber tip at 0.4 W of laser power. b - Image of this filament represented in shades of gray after processing of (see text) of Fig. 19a. c - Normalized vertical profiles of image given at Fig. 19b. (Yusupov et al, 2011b).

Fig. 19a demonstrates the micro-image of another laser fabricated filament in scattering mode. Intensity of light scattered from this filament decreases gradually with the distance from fiber tip. Attenuation of green light in this case is caused by absorption and scattering of green light in the course of its propagation through the filament. To reveal the peculiarities of filament (given at Fig. 19a) we have performed the following processing of its microscope image: all vertical profiles of image were normalized to local maximum (Fig. 19c); the microscope image was represented in shades of gray (Fig. 19b). As it follows from figures 19b and 19c the length of given filament is about 6 mm, its average width is about 40 μm, and scattering intensity decreases rapidly with the distance from filament axis. Notice that vertical profiles of all fabricated filaments (in both transmission and scattering modes) are almost triangular with a sharp top. It was also established that the end of filament has always a needle-like shape and, also, the width of filament obtained in transmission mode

measurements exceeds 3-5 times that obtained in scattering mode.

Fig. 19. a - Microscopic picture of filament (in scattering mode) of Ag nanoparticles

vertical profiles of image given at Fig. 19b. (Yusupov et al, 2011b).

fabricated in water nearby optical fiber tip at 0.4 W of laser power. b - Image of this filament represented in shades of gray after processing of (see text) of Fig. 19a. c - Normalized

It is of importance that filaments of Ag nanoparticles have been formed in our experiments only in the case of existence of initial collargol concentration gradient in laser irradiated water (when collargol drop was introduced initially into water aside from fiber tip). When collargol drop was premixed in water cell before laser irradiation, formation of filaments has never been observed (at any collargol concentrations in the cell and at any laser powers and dozes).

The initial stage of filament self-organization process can be clearly seen in scattering mode (Fig. 4). Some visible hydrodynamic flows take place nearby the fiber tip when laser power is on. Such flows result in intrusion of collargol from neighboring area into the area in front of the fiber tip. The slanting filament structure is clearly seen at Fig. 4. One can also see here the initial process of new intrusion formation (outlined with dashed line). The rate of rise-up front of a given intrusion (which is about 150 μm in average thickness) is found to be described be exponential low (1): at 1 mm from laser fiber tip *V*= 1.5· 10-2 cm/s, while at 2 mm from laser fiber tip *V* falls down to 3· 10-3cm/s.

We revealed that filaments of Ag nanoparticles self-organized in the course of 0.97 µm laser irradiation can exist in the cell (in the presence of laser beam and with no external mechanical distortions of liquid in the cell) for quite a long period of time. We have supported such filaments for tens of minutes. Notice that both rectilinear and curved filaments were self-organized in our experiments.

After 0.97 µm laser radiation being off, the filaments of Ag nanoparticles have been completely destroyed for 10 – 30 s period of time. Notice that time Δt of diffusion blooming of filament by value, estimated by formula

$$
\overline{\mathbf{x}}^2 = D\Delta t = \frac{kT}{\Im \pi \mu d} \Delta t \tag{6}
$$

where *D* – is diffusion coefficient of nanoparticle; *k*= 1.38· 10-23 J/K – Boltzmann constant; *T(K)* – absolute temperature; *μ* = 1,002· 10-3 (N· s/m2) – dynamic viscosity of water; *d*=25 nm Ag nanoparticle diameter) gives *Δt* =25 s for =100 μm.

External mechanical distortions of filament of Ag nanoparticles results in its destruction. However after mechanical distortion being off, the filament can be renewed completely in presence of 0.97 µm laser radiation. Fig. 20 shows the dynamic of such filament renovation after the distortion of self-organized filament (produced by its rapid crossing withthin a metal needle). As one can see from Fig. 20, complete renewal took place for quite a short period of time (~ 20 s).

Our experiments have shown that there is some range of 0.97 µm laser powers for which the effect of laser-induced filament self-organization takes place and is, also, stable and reproducible. At laser powers higher than 8 W we have newer observed filament formation. At 0.2-0.5 W laser power filaments have been formed but have been unstable. The most stable and long-living filaments were observed in 0.5-3 W laser power range. At laser power less than 0.2 W we have never observed such filament formation. The instability of filaments and even their absence at high laser powers is caused by intense laser-induced hydrodynamic processes nearby the fiber tip. Our experiments show that the fiber tip surface is gradually covered by a deposit, which absorbs laser radiation quite well. The wide absorption band of deposit observed at fiber tip can be caused by island film of Ag nanoparticles, and, possibly, by elementary carbon absorption (deposited at fiber tip due to albumin thermo-decomposition). As a result of such deposits, the fiber tip becomes an

Laser-Induced Hydrodynamics in Water and Biotissues Nearby Optical Fiber Tip 115

d. Intense formation of micro-bubbles, hampering filament formation at high laser power. Fig. 21. To the explanation of the effect of laser-induced formation of filaments of Ag

irradiated medium nearby fiber tip and possibility of liquid fiber formation.

Laser induced formation of 10-50 μm in thickness and up to few millimeters micro-bubble streams (Fig. 11) can also promote the filaments fabrication observed in our experiments. It is clear, however, that too intense chaotic formation of micro-bubble streams observed at

We believe that such filaments of nanoparticles can be developed not only in water media but, also, in other fluids, with other laser wavelength and particles types. The indispensable conditions in this case are the availability of sufficient level of laser light absorption in

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

a. Formation of water flow nearby the fiber tip. b. Formation of Ag nanoparticles intrusions. c. Fabrication of filaments from Ag nanoparticles.

high laser power can hamper filament fabrication (Fig. 21d).

nanoparticles (Yusupov et al., 2011b).

**7. Conclusion** 

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

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 destroying filament.

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

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 stability.


Fig. 21. To the explanation of the effect of laser-induced formation of filaments of Ag nanoparticles (Yusupov et al., 2011b).

Laser induced formation of 10-50 μm in thickness and up to few millimeters micro-bubble streams (Fig. 11) can also promote the filaments fabrication observed in our experiments. It is clear, however, that too intense chaotic formation of micro-bubble streams observed at high laser power can hamper filament fabrication (Fig. 21d).

We believe that such filaments of nanoparticles can be developed not only in water media but, also, in other fluids, with other laser wavelength and particles types. The indispensable conditions in this case are the availability of sufficient level of laser light absorption in irradiated medium nearby fiber tip and possibility of liquid fiber formation.
