Engineered nanoparticles are considered potential sources for facilitating proton induced reactions, releasing energy as heat and avoiding generation of nuclear byproducts such as neutrons Considering heat collection and confinement conditions, liquids are regarded as interesting options for collecting excess energy in a nuclear environment.
This motivates therefore the study of cavitation effects in liquids containing nanoparticles, notably the interaction of the nanoparticles with laser-induced bubbles. Here an important aspect concerns the hydrodynamic motion of the liquid containing the particles.
Daniel Fuster Webpage | Cavitation
It is of a significant importance for the field of energy harvesting to investigate the role of shock waves and bubble formation, and to check if particles will stay localized or will move with the liquid after laser excitation. The hydrodynamical processes involved in laser-induced cavitation and bubble formation in liquids are fairly known, from the bubble expansion to its collapse and the formation of inner and outer jets.
Water is the medium of choice in a large majority of studies. The typical scenario involves the creation of a charge carrier plasma, usually assumed at the critical density for the corresponding irradiation wavelength, initiating an optical runaway, or at sub-critical densities at the cavitation threshold.
Electronic relaxation is followed by electron vibrational interaction and by the rise of the liquid matrix temperature and pressure on a ps timescale. The mechanical relaxation via pressure waves triggers tensile stresses and initiates the cavitation of the liquid and the onset of a bubble. Several studies focused on the excitation parameters in relation to cavitation threshold 5 , 26 or followed nonlinear propagation of ultrashort laser pulses in water 27 , with specific works dedicated to dynamic techniques for monitoring cavity development 28 , More recent, the interaction of non-standard beams received increased attention due to particular field distributions.
New technique captures diverse effects of cavitation bubbles on individual cells
This is the case of the non-diffractive zero-order Bessel beams The cavitation geometry acquires a one-dimensional character 31 , 32 , with the subsequent distribution of radial pressure. Applications in biology were indicated 33 , increasing the flexibility in dissecting subcellular domains. In this paper, we evaluate the possible influence of the presence of gold NPs on the bubble hydrodynamics, for a cavitation geometry of particular form having a cylindrical evolution symmetry.
Then high-resolution optical imaging synchronized with the characteristic times of the event is used to visualize the phenomenon.
Some of my work on cavitation
In order to have benchmarks, we first discuss the multiscale dynamics of laser-induced cavitation in water, water-solvent mixtures and solvent ethylene-glycol in non-diffractive irradiation modes following excitation and relaxation ranges. Using time-resolved imaging with amplitude and phase sensitivity on multiple fs to ms timescales, we reconstitute with high fidelity the hydrodynamic phenomena leading to bubble evolution as well as the instabilities that occur inside bubbles generated in one-dimensional geometries around the point of collapse.
Information related to excitation degree and the evolution of the cavity interfaces are obtained from analyzing both transmission and associated optical retardance.
The second question we tried to answer is related to irradiation of liquids containing nanoparticles. Following the interest in the interaction of ultrashort ultra-intense pulses with aggregates in particular environments, we interrogate the ability of the nanoparticles to survive primary excitation and resist within the bubble. In particular, we will analyze the interface capability to sweep and remove nanoparticles during the bubble expansion. The evolving bubble border in nanoparticles-containing liquids creates a snow-plough effect that traps and sweeps the nanoparticles at the gas liquid interface.
This observation indicates that during the macroscopic cavity development, the nanoparticles were removed from the interaction region and dragged by the hydrodynamic movement. Trapping of nanoparticles in a liquid by laser-induced microbubbles has already been reported However, another technique was used to capture and manipulate nanoparticles, based on optically-induced thermophoresis and convection phenomena For this purpose, the evolution of the irradiated liquid domain with and without GNPs is compared in the expansion and collapse phase.
The manuscript is organized in several parts.
The experimental method is first presented showing the irradiation geometry and the two time-resolved approaches on multiple timescales. The ultrafast sub-ps and ps dynamics is first discussed.
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Then the slow thermomechanical evolution is indicated over the whole evolution cycle. Considerations on the cavitation thresholds are given.
Thirdly, the presence of gold nanoparticles is analyzed for a possible influence on the bubble hydrodynamics and for interrogating their capacity of surviving inside the bubble after the passage of the laser pulse. The sweep-out role of the bubble expanding interface is outlined. The section includes a description of the excitation geometry, of the sample preparation and of the observation techniques.
The laser source consists of an amplified ultrafast Ti:Sapphire system. The laser pulse duration can be tuned in the fs-ps range using additional second-order dispersion by adjusting the compressor inter-grating distance. High contrast single pulses can be extracted using opto-electronic and mechanical shutters. Several methods exists for generating non-diffractive beams by conical intersection see ref.
We opted here for axicon generated Bessel-Gauss beams referred hereafter as Bessel beams. The conical phase pattern of the axicon bends the incoming light to the axis. It thus determines a transverse to axial light transformation, where the incoming wavefronts create a stationary field encompassing a narrow intense central core axially elongated over a large distance as compared to the Rayleigh range of an equivalent Gauss beam and surrounded by low intensity rings following the Bessel transverse profile.
The central core and the rings are the consequence of successive constructive interference relations of the incoming conical wavefront. Bessel beams with different cone angles can be obtained by employing objectives with various optical powers. The aspect ratio lies around For observing the laser-excited regions in liquids, in-situ and in real-time, dynamic optical transmission OTM and positive phase-contrast PCM microscopy are employed. Several water distilled and water-ethylene glycol mixtures were used and in particular cases they were doped with gold nanoparticles.
This choice is equally motivated by the possibility of detection by optical means while avoiding sedimentation issues at larger dimensions. The complete relaxation dynamics following ultrashort pulse excitation of liquids in non-diffractive tight focusing conditions is observed using multiscale time-resolved imaging in a microscopy arrangement. The laser-excited liquid evolution is followed for time domains starting with the ultrafast excitation and extending to relatively slow cavity hydrodynamics down to final relaxation.
The method allows for the observation of the laser-affected region in phase and amplitude delivering information on absorbance and phase shifts for a timescale ranging from fs to ms and with sub-micron spatial resolution. It consists of two experimental arrangements, one allowing for inspecting the ultrafast dynamics UFD below ns with sub-ps resolution, the second multiscale MSD one being optimized for long-range mechanical relaxation of the liquid, up to the ms. In OTM, absorbing or scattering regions appear dark on a bright background. In PCM, phase shifts corresponding to negative and positive refractive index changes appear bright, respectively dark on a grey background, provided that the phase shift stays comparable to the observation wavelength.
It has to be noted that the relative index and phase changes observed in PCM represent qualitative variations of the initial quantities. For each time domain sources with various degrees of spatial coherence were used. The first domain of interest relates to fs-ps-ns evolution.
The primary excitation dynamics and relaxation channels in ultrafast domains associated with laser-liquid interaction are observed with time and space resolution using a standard two-color pump-probe time-resolved microscopy technique.
This delivers instantaneous excitation charts at various time moments during and after excitation and covers a time domain from fs to ns. This allows to avoid coherent artifacts between pump and probe pulses and enables better separation. The probe pulses were formed by sampling a part of the main beam and injecting it in an optical delay line, ensuring thus precise optical synchronization. Pulse duration can be varied in fs and ps ranges. The pump and the probe have the same temporal duration, however, since fs and ps pulse will respectively probe dynamics much longer than the probe duration, no loss on measurement precision is identified at passing from fs to ps ranges.
This geometry provides information on the amplitude and the phase of the illuminated object in ultrafast OTM UFD and PCM UFD modes with a view sectioning the image in planes parallel to the propagation axis along an orthogonal path. The probe enters the illumination path of the microscope via the microscope condenser and the optical transmission and phase-contrast microscope images are recorded in the image plane with a back-illuminated electron multiplier EMCCD ANDOR iXon Ultra camera. The temporal delay of the probe pulse with respect to the pump is varied using a motorized translation stage.
The ultrashort illumination source is a spatially coherent source and source of speckles. In view of its short duration, the degree of coherence cannot be easily altered during the pulse. Rotating diffuser plates are used to reduce the average spatial coherence of the probe for multipulse exposures, increasing the uniformity of illumination.
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Snapshots are averaged over 40 images where the sample is moved to a new position between each image. A detailed description of the setup is given in ref. The long range relaxation dynamics following ultrashort pulse excitation of the liquid in non-diffractive tight focusing conditions is observed using multiscale time-resolved imaging.
This corresponds to a comparatively slow hydrodynamic evolution of the bubble. The method allows for the observation of the laser-induced evolving bubble in phase and amplitude for a timescale ranging from ns to ms, and with sub-micron spatial resolution, covering the whole bubble dynamics. A similar two-color pump-probe microscopy technique with respect to the ultrafast dynamics part is used, equally responsive to relative differences in the object optical phase and amplitude characteristics, and having this time ns temporal resolution.
This collects corresponding charts of material transformation at various illumination time moments after excitation where amplitude and phase contrast allows to enhance the observation accuracy. The stroboscopic illumination method is used as mentioned above, where the probe pulse cross-illuminates the interaction zone in an orthogonal geometry. The probe pulse is generated this time from an independent light source. To obtain high-resolution single-shot images with uniform background and low speckle noise, a low spatial coherence pulsed source is developed based on a random lasing effect 39 , as described in ref.
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