Iskander S. Akhatov

Collapse and rebound of a laser-induced cavitation bubble

A strong laser pulse that is focused into a liquid produces a vapor cavity, which first expands and then collapses with subsequent rebounds. In this paper a mathematical model of the spherically symmetric motion of a laser-induced bubble is proposed. It describes gas and liquid dynamics including compressibility, heat, and mass transfer effects and nonequilibrium processes of evaporation and condensation on the bubble wall. It accounts also for the occurrence of supercritical conditions at collapse. Numerical investigations of the collapse and first rebound have been carried out for different bubble sizes. The results show a fairly good agreement with experimental measurements of the bubble radius evolution and the intensity of the outgoing shock wave emitted at collapse. Calculations with a small amount of noncondensable gas inside the bubble show its strong influence on the dynamics.

Theory of supercompression of vapor bubbles and nanoscale thermonuclear fusion

HYDRO code model of the spherically symmetric motion for a vapor bubble in an acoustically forced liquid is presented. This model describes cavitation bubble cluster growth during the expansion period, followed by a violent implosion during the compression period of the acoustic cycle. There are two stages of the bubble dynamics process. The first, low Mach number stage, comprises almost all the time of the acoustic cycle. During this stage, the radial velocities are much less than the sound speeds in the vapor and liquid, the vapor pressure is very close to uniform, and the liquid is practically incompressible. This process is characterized by the inertia of the liquid, heat conduction, and the evaporation or condensation of the vapor. The second, very short, high Mach number stage is when the radial velocities are the same order, or higher, than the sound speeds in the vapor and liquid. In this stage high temperatures, pressures, and densities of the vapor and liquid take place. The model presented herein has realistic equations of state for the compressible liquid and vapor phases, and accounts for nonequilibrium evaporation/condensation kinetics at the liquid/ vapor interface. There are interacting shock waves in both phases, which converge toward and reflect from the center of the bubble, causing dissociation, ionization, and other related plasma physics phenomena during the final stage of bubble collapse. For a vapor bubble in a deuterated organic liquid e.g., acetone, during the final stage of collapse there is a nanoscale region diameter 100 nm near the center of the bubble in which, for a fraction of a picosecond, the temperatures and densities are extremely high 10 8 K and 10 g/cm 3 , respectively such that thermonuclear fusion may take place. To quantify this, the kinetics of the local deuterium/deuterium D/D nuclear fusion reactions was used in the HYDRO code to determine the intensity of the fusion reactions. Numerical HYDRO code simulations of the bubble implosion process have been carried out for the experimental conditions used by Taleyarkhan et al. Science 295, 1868 2002; Phys. Rev. E 69, 036109 2004 at Oak Ridge National Laboratory. The results show good agreement with the experimental data on bubble fusion that was measured in chilled deuterated acetone.

Hydrodynamic simulation of air bubble implosion using a level set approach

The hydrodynamics of the implosion and rebound of a small (10@mm diameter) air bubble in water was studied using a three-dimensional direct numerical simulation (DNS). To study this problem, we developed a novel stabilized finite element method (FEM) employing a combination of ghost fluid and level set approaches. This formulation treats both the air and water as compressible fluids. Using this method, a transient three-dimensional (3-D) solution was obtained for the implosion (i.e., collapse) and rebound of an air bubble. These simulation results obtained were qualitatively similar to those observed/predicted in previous experimental/numerical studies. The 3-D simulations show that the conditions within the bubble are nearly uniform until the converging pressure wave is strong enough to create very large temperatures and pressures near the center of the bubble. These dynamics occur on very small spatial (0.1-0.7@mm), and time (ns) scales. The motion of the air/water interface during the initial stages of the implosion was found to be consistent with predictions using a Rayleigh-Plesset model. However, the simulations showed that during the final stage of energetic implosions, the bubble can become asymmetric, which is contrary to the spherical symmetry assumed in many previous numerical studies of bubble dynamics. The direct numerical simulations predicted two different instabilities, namely Rayleigh-Taylor type interfacial/surface and shape instabilities. During the violent collapse stage, the bubble deviates from spherical symmetry and deforms into an ellipsoidal-shaped bubble. A linear stability analysis based on spherical harmonics also indicates that an ellipsoidal bubble shape could be expected. Moreover, interfacial instabilities also appear during the later stage of the implosion process. Distinguishing these phenomena with the help of numerical simulations opens new opportunities to understand many features of recent experiments on sonoluminescence and sonofusion.

Pattern formation in acoustic cavitation

A new approach for the theoretical description of structure formation in acoustic cavitation is developed. The model consists of two coupled partial differential equations describing the spatiotemporal evolution of the sound field amplitude and the bubble concentration. Linear stability analysis and numerical simulations of the pattern formation are presented. The relation between this approach and streamer formation is discussed.

A Review on Aerosol-Based Direct-Write and Its Applications for Microelectronics

Aerosol-based direct-write refers to the additive process of printing CAD/CAM features from an apparatus which creates a liquid or solid aerosol beam. Direct-write technologies are poised to become useful tools in the microelectronics industry for rapid prototyping of components such as interconnects, sensors, and thin film transistors (TFTs), with new applications for aerosol direct-write being rapidly conceived. This paper aims to review direct-write technologies, with an emphasis on aerosol-based systems. The different currently available state-of-the-art systems such as Aerosol Jet CAB-DW, MCS, and aerodynamic lenses are described. A review and analysis of the physics behind the fluid-particle interactions including Stokes and Saffman force, experimental observations, and how a full understanding of theory and experiments can lead to new technology are presented. Finally, the applications of aerosol direct-write for microelectronics are discussed.

Structure formation in cavitation bubble fields

Abstract Two approaches for modelling the formation of filamentary structures in cavitation bubble fields are presented. The first one describes the interaction of the sound field and the distribution of microbubbles in terms of a set of two coupled partial differential equations that determine the evolution of the sound-field amplitude and the bubble density. The second approach consists of a quasideterministic aggregation model, where the bubbles are treated as pulsating particles which experience radiation forces due to the sound-fields radiated from the other pulsating bubbles. Results of numerical simulations are presented for both models. The validity and limitations of both approaches are discussed.

Rapid Prototyping RFID Antennas Using Direct-Write

Optimization of radio-frequency identification (RFID) tags often requires several iterations of antenna design/fabrication/testing to meet cost and performance targets. The use of a rapid prototyping approach for antenna development would allow the designer an inexpensive and fast route to the refinement process. In this study, the performance of a commercial-off-the-shelf ultrahigh frequency (UHF) etched copper antenna was compared to printed silver antennas prepared by the following three direct-write techniques: maskless mesoscale materials deposition; matrix-assisted pulsed laser evaporation direct-write; and, collimated aerosol beam direct-write. The morphologies of the antennas were analyzed using contact and optical profilers with sheet resistance also being measured. Operational characteristics were determined by mounting silicon integrated circuits (IC) to the four different types of antennas. The performance of tags that utilized direct-write silver antennas was comparable to the copper-based commercial tag. To our knowledge, this is the first demonstration where some of the direct-write rapid prototyping attributes (e.g., slight overspray, overlap of written lines, overall thickness less than 500 nm) are shown to not seriously impede RFID tag performance. These results demonstrate the utility of direct-write for rapid prototyping studies for UHF RFID antennas.

Sonoluminescence and the Search for Sonofusion

Publisher Summary This chapter presents a discussion on sonoluminescence (SL) and the search for sonofusion. The field of multiphase flow and heat transfer has many important practical applications. An interesting and important subset of this field has to do with the bubble dynamics associated with sonofusion and/or SL phenomena. The chapter discusses the phenomena of multibubble sonoluminescence (MBSL) in which multiple bubbles formed by cavitation grow and collapse in the induced pressure field, giving off light pulses during bubble implosions. Some interesting hypotheses and models concerning the origin of these light pulses have included mechanisms associated with (1) triboluminescence, (2) electrical microdischarge, (3) mechanochemical models, (4) chemiluminescence model, and (5) Hot Spot model. While research continues to improve our understanding of the interesting phenomena associated with SL, most researchers believe the validity of the extended Hot Spot model. The chapter illustrates through diagrams a typical setup for a double detector technique, bubble radius and photon detection systems, and a laser beam polarized parallel to the paper illuminating a spherical bubble. The chapter concludes with a discussion on neutron-induced bubble nucleation.

Dynamics of laser-induced cavitation bubbles

Abstract Single cavitation bubble luminescence induced by laser in contrast to single bubble sonoluminescence has no need in a sound field for a strong collapse and for light emission. The cavitation bubbles are produced by focused laser light and make the single strong collapse. As shown experimentally, the number of emitted photons from cavitation luminescence is much greater than it was observed in sonoluminescence due to the large bubble size during the final stage of collapse. To describe the process of laser-induced bubble collapse a mathematical model is used, which is based upon the spherically symmetric motion including compressibility, heat and mass transfer effects. The basic results of the numerical solution are presented for the bubbles with maximum radii of about 1 mm. According to the observed results the minimum bubble radius in collapse is about 15 μm, and the mass decreases up to 5% of the initial value. Calculations with a small amounts of noncondensable gas inside the bubble predict its strong influence on the dynamics. As shown numerically the theoretical model gives a good agreement with experimental measurements.

Capillary-based liquid microdroplet deposition

Liquid droplet deposition through a capillary onto a substrate is studied. The application of pressure into the capillary causes a liquid meniscus to form at the outlet. Touching the substrate with the liquid meniscus and subsequent capillary retraction gives liquid deposition on the substrate. Theoretical and experimental studies of the steady liquid bridge structure between the capillary and substrate identified the range of parameters when deposition of small droplets (no blot) can be performed. Experiments revealed that in this range of parameters the size of deposited liquid droplets is less than 10%–15% of inner diameter of the capillary.