Molecular dynamics simulations of He bubble nucleation at grain boundaries

The nucleation behavior of He bubbles in single-crystal (sc) and nano-grain body-centered-cubic (bcc) Mo is simulated using molecular dynamics (MD) simulations, focusing on the effects of the grain boundary (GB) structure. In sc Mo, the nucleation behavior of He bubbles depends on irradiation conditions. He bubbles nucleate by either clustering of He atoms with pre-existing vacancies or self-interstitial-atom (SIA) punching without initial vacancies. In nano-grain Mo, strong precipitation of He at the GBs is observed, and the density, size and spatial distribution of He bubbles vary with the GB structure. The corresponding He bubble density is higher in nano-grain Mo than that in sc Mo and the average bubble size is smaller. In the GB plane, He bubbles distribute along the dislocation cores for GBs consisting of GB dislocations and randomly for those without distinguishable dislocation structures. The simulation results in nano-grain Mo are in agreement with previous experiments in metal nano-layers, and they are further explained by the effect of excess volume associated with the GBs.     

金属表面下氦泡融合与释放研究

采用分子动力学方法研究了钛金属表面下不同深度处氦泡的行为,分析了氦泡融合与释放的竞争,对比了不同深度处氦泡的释放对金属的影响。结果表明：在接近金属表面处,氦泡很难通过融合无限长大,当达到临界尺寸后,氦泡将会释放而不再与邻近的氦泡发生融合;植入深度对氦泡的融合有一定的影响,深度越大,越有利于形成具有较高氦密度的大氦泡;较深处氦泡的释放会在金属表面形成较大的突起和表面针孔。实验中观察到的不同尺寸的表面孔,其部分原因来自于金属表面下不同深度处氦泡的释放。     

金属表面下氦泡融合与释放研究

采用分子动力学方法研究了钛金属表面下不同深度处氦泡的行为,分析了氦泡融合与释放的竞争,对比了不同深度处氦泡的释放对金属的影响.结果表明:在接近金属表面处,氦泡很难通过融合无限长大,当达到临界尺寸后,氦泡将会释放而不再与邻近的氦泡发生融合;植入深度对氦泡的融合有一定的影响,深度越大,越有利于形成具有较高氦密度的大氦泡;较深处氦泡的释放会在金属表面形成较大的突起和表面针孔.实验中观察到的不同尺寸的表面孔,其部分原因来自于金属表面下不同深度处氦泡的释放.     

Influence of frequency sweep on sonochemiluminescence and sonoluminescence

Bubbles generated by acoustic cavitation may be efficient in light production by direct emission (sonoluminescence) or indirect emission (sonochemiluminescence) depending on operating parameters such as acoustic pressure and surface tension. These conditions are quite difficult to reach at very high frequencies, even by concentrating the acoustic power at a given location via focusing the acoustic field thanks to the transducer shape (High Intensity Focused Ultrasound). The current work aims at probing the cavitation bubble behaviour under short frequency sweeps by monitoring sonochemiluminescence and sonoluminescence activities. When the frequency was swept in reverse (negative sweep), an enhancement in the SCL, relative to the SCL observed under a single frequency irradiation, was observed. Conversely, a positive frequency sweep resulted in the quenching of SCL intensity. The degree of SCL enhancement and quenching was also dependent on the rate at which the frequency was being swept and on the change in the size of cavitation bubbles. The size of cavitation bubbles varied with varying starting sweep frequency (3.4, 3.6 and 4.2 MHz), affecting both SCL and sonoluminescence (SL) emissions. The addition of a surfactant (sodium dodecyl sulphate) affected the observed results, possibly due to its influence on coalescence between cavitation bubbles. The results suggest that the enhancement and quenching are related to the response of bubbles generated by the starting frequency to the direction of the frequency sweep and the influence of the sweep rate on growth and coalescence of bubbles, which affected the population of the active bubbles.     

Influence of mixing and ultrasound frequency on antisolvent crystallisation of sodium chloride

Ultrasound is known to promote nucleation of crystals and produce a narrower size distribution in a controlled and reproducible manner for the crystallisation process. Although there are various theories that suggest cavitation bubbles are responsible for sonocrystallisation, most studies use power ultrasonic horns that generate both intense shear and cavitation and this can mask the role that cavitation bubbles play. High frequency ultrasound from a plate transducer can be used to examine the effect of cavitation bubbles without the intense shear effect. This study reports the crystal size and morphology with various mixing speeds and ultrasound frequencies. The results show high frequency ultrasound produced sodium chloride crystals of similar size distribution as an ultrasonic horn. In addition, ultrasound generated sodium chloride crystals having a more symmetrical cubic structure compared to crystals produced by a high shear mixer.     

Effect of pulse duration on plasmonic enhanced ultrafast laser-induced bubble generation in water

Bubbles generated in water by focusing femtosecond and picosecond laser pulses in the presence of 100 nm gold nanoparticles have been investigated in the fluence range usually used for efficient cell transfection (100–200 mJ/cm²). Since resulting bubbles are at the nanoscale, direct observation using optical microscopy is not possible. An optical in-situ method has been developed to monitor the time-resolved variation in the extinction cross-section of an irradiated nanoparticle solution sample. This method is used to measure the bubbles lifetime and deduce their average diameter. We show that bubbles generated with femtosecond pulses (40–500 fs) last two times longer and are larger in average than those generated with picosecond pulses (0.5–5 ps). Controlling those bubble properties is necessary for optimizing off-resonance plasmonic enhanced ultrafast laser cell transfection.     

Modeling bubble dynamics and radical kinetics in ultrasound induced microalgal cell disruption

Microalgal cell disruption induced by acoustic cavitation was simulated through solving the bubble dynamics in an acoustical field and their radial kinetics (chemical kinetics of radical species) occurring in the bubble during its oscillation, as well as calculating the bubble wall pressure at the collapse point. Modeling results indicated that increasing ultrasonic intensity led to a substantial increase in the number of bubbles formed during acoustic cavitation, however, the pressure generated when the bubbles collapsed decreased. Therefore, cumulative collapse pressure (CCP) of bubbles was used to quantify acoustic disruption of a freshwater alga, Scenedesmus dimorphus, and a marine alga, Nannochloropsis oculata and compare with experimental results. The strong correlations between CCP and the intracellular lipid fluorescence density, chlorophyll-a fluorescence density, and cell particle/debris concentration were found, which suggests that the developed models could accurately predict acoustic cell disruption, and can be utilized in the scale up and optimization of the process.     

In-situ TEM observation of the evolution of helium bubbles in Mo during He+ irradiation and post-irradiation annealing

The evolution of helium bubbles in purity Mo was investigated by in-situ transmission electron microscopy (TEM) during 30 keV He⁺ irradiation (at 673 K and 1173 K) and post-irradiation annealing (after 30 keV He⁺ irradiation with the fluence of 5.74×10¹⁶ He⁺/cm² at 673 K). Both He⁺ irradiation and subsequently annealing induced the initiation, aggregation, and growth of helium bubbles. Temperature had a significant effect on the initiation and evolution of helium bubbles. The higher the irradiation temperature was, the larger the bubble size at the same irradiation fluence would be. At 1173 K irradiation, helium bubbles nucleated and grew preferentially at grain boundaries and showed super large size, which would induce the formation of microcracks. At the same time, the geometry of helium bubbles changed from sphericity to polyhedron. The polyhedral bubbles preferred to grow in the shape bounded by {100} planes. After statistical analysis of the characteristic parameters of helium bubbles, the functions between the average size, number density of helium bubbles, swelling rate and irradiation damage were obtained. Meanwhile, an empirical formula for calculating the size of helium bubbles during the annealing was also provided.     

Numerical simulations of acoustic cavitation noise with the temporal fluctuation in the number of bubbles

Numerical simulations of cavitation noise have been performed under the experimental conditions reported by Ashokkumar et al. (2007) [26]. The results of numerical simulations have indicated that the temporal fluctuation in the number of bubbles results in the broad-band noise. “Transient” cavitation bubbles, which disintegrate into daughter bubbles mostly in a few acoustic cycles, generate the broad-band noise as their short lifetimes cause the temporal fluctuation in the number of bubbles. Not only active bubbles in light emission (sonoluminescence) and chemical reactions but also inactive bubbles generate the broad-band noise. On the other hand, “stable” cavitation bubbles do not generate the broad-band noise. The weaker broad-band noise from a low-concentration surfactant solution compared to that from pure water observed experimentally by Ashokkumar et al. is caused by the fact that most bubbles are shape stable in a low-concentration surfactant solution due to the smaller ambient radii than those in pure water. For a relatively high number density of bubbles, the bubble–bubble interaction intensifies the broad-band noise. Harmonics in cavitation noise are generated by both “stable” and “transient” cavitation bubbles which pulsate nonlinearly with the period of ultrasound.     

Oscillating bubble concentration and its size distribution using acoustic emission spectra

New method has been proposed for the estimation of size and number density distribution of oscillating bubbles in a sonochemical reactor using acoustic emission spectra measurements. Bubble size distribution has been determined using Minnaert's equation [M. Minnaert, On musical air bubbles and sound of running water, Philanthr. Mag. 16 (1933) 235], i.e., size of oscillating bubble is inversely related to the frequency of its volume oscillations. Decomposition of the pressure signal measured by the hydrophone in frequency domain of FFT spectrum and then inverse FFT reconstruction of the signal at each frequency level has been carried out to get the information about each of the bubble/cavity oscillation event. The number mean radius of the bubble size is calculated to be in the range of 50-80mum and it was not found to vary much with the spatial distribution of acoustic field strength of the ultrasound processor used in the work. However, the number density of the oscillating bubbles and the nature of the distribution were found to vary in different horizontal planes away from the driving transducer surface in the ultrasonic bath. A separate set of experiments on erosion assessment studies were carried out using a thin aluminium foil, revealing a phenomena of active region of oscillating bubbles at antinodal points of the stationary waves, identical to the information provided by the acoustic emission spectra at the same location in the ultrasonic bath.     

Effect of ultrasonic frequency on size distributions of nanosized mist generated by ultrasonic atomization

Ultrasonic atomization is used to produce fine liquid mists with diameter ranges below 100 nm. We investigated the effect of the frequency on the size distribution of ultrasonic mist. A bimodal distribution was obtained for the mist generated by ultrasonic atomization with a wide-range particle spectrometer. The peak diameter decreased with increasing frequency, and the number concentration of the mist increased in the smaller range. We determined the relation between the size distribution of the mist and the ultrasonic frequency, and we proposed a generation mechanism for the ultrasonic nanosized mist based on the amount of water vapor around the liquid column. Increasing the power intensity and density by changing the surface diameter of the ultrasonic oscillator affected the number concentration and size distribution of the nanosized mist. Using this technique, the diameter of the mist can be controlled by changing the frequency of the ultrasonic transducer.     

Finite-size effects on nucleation in a first-order phase transition

We discuss finite-size effects on homogeneous nucleation in first-order phase transitions. We study their implications for cosmological phase transitions and to the hadronization of a quark–gluon plasma generated in high-energy heavy ion collisions. Very general arguments allow us to show that the finite size of the early universe has virtually no relevance in the process of nucleation and in the growth of cosmological bubbles during the primordial quark–hadron and the electroweak phase transitions. In the case of high-energy heavy ion collisions, finite-size effects play an important role in the late-stage growth of hadronic bubbles.     

Study on shock wave-induced cavitation bubbles dissolution process

This study investigated dissolution processes of cavitation bubbles generated during in vivo shock wave(SW)-induced treatments. Both active cavitation detection(ACD) and the B-mode imaging technique were applied to measure the dissolution procedure of bi Spheres contrast agent bubbles by in vitro experiments. Besides, the simulation of SW-induced cavitation bubbles dissolution behaviors detected by the B-mode imaging system during in vivo SW treatments, including extracorporeal shock wave lithotripsy(ESWL) and extracorporeal shock wave therapy(ESWT), were carried out based on calculating the integrated scattering cross-section of dissolving gas bubbles with employing gas bubble dissolution equations and Gaussian bubble size distribution. The results showed that(i) B-mode imaging technology is an effective tool to monitor the temporal evolution of cavitation bubbles dissolution procedures after the SW pulses ceased, which is important for evaluation and controlling the cavitation activity generated during subsequent SW treatments within a treatment period;(ii) the characteristics of the bubbles, such as the bubble size distribution and gas diffusion, can be estimated by simulating the experimental data properly.     

Modeling variations in near-infrared spectra caused by the coherent backscattering effect

We study a new optical effect, a spectral manifestation of coherent backscattering, which reveals itself as systematic variations in the depth of absorption bands with changing phase angle. We used Cassini VIMS near-infrared spectra of Saturn's icy satellite Rhea in order to identify and characterize the spectral change with phase angle, focusing on the change in the depth of water-ice absorption bands. To model realistic characteristics of the surfaces of icy satellites, which are most likely covered by micron-sized densely packed particles, we perform simulations using a theoretical approach based on direct computer solutions of the macroscopic Maxwell equations. Our results show that this approach can reproduce the observed phase-angle variations in the depth of the absorption bands. The modeled changes in the absorption bands are strongly affected by physical properties of the regolith, especially by the size and packing density of the ice particles. Thus, the phase-angle spectral variations demonstrate a promising remote-sensing capability for studying properties of the surfaces of icy bodies and other objects that exhibit a strong coherent backscattering effect.     

The problems of the upward monodispersed microbubble flow in a vertical tube

The paper presents the first experimental results concerning the wall shear stress in an upward monodispersed microbubble flow in vertical tube. A bubble generator using a microfluid focusing technique was designed to produce monodispersed submillimeter bubbles. The experimental results allow to think that there is an optimal size of the bubbles and the optimal gas fraction in the bubble sublayer that provide the maximal mass transfer coefficient beween the flow and the tube wall.     

多台阶圆形衍射微透镜聚焦性能的矢量分析

给出了基于旋转体时域有限差分法的严格的矢量分析模型,应用该模型分析了多个多台阶圆形衍射微透镜的设计实例.研究了设计参量和制作误差对其聚焦性能的影响.结果表明增加环带数,但保持其他设计参量例如焦距、入射波长不变时,焦点场强增大、焦斑变小、焦深变短;只增加台阶数时,聚光作用加强,但台阶数达到8～10后,聚焦效果趋向稳定,因此建议在实际加工中,台阶数选择8～10.用二元掩模板套刻制作多台阶时,容易产生制作误差.设计了在第三次套刻制成8个台阶过程中产生了对准误差和系统刻蚀误差的两类多台阶微透镜.分析结果表明对准误差对多台阶微透镜聚焦效果的影响较大,加工时要尽量避免,而系统刻蚀误差对多台阶微透镜聚焦效果的影响较小.     

Thermal-exergetic behavior of a vertical double-tube heat exchanger with bubble injection

The use of air bubbles as a heat transfer improvement technique for heat exchangers has been proposed by some researchers. The vertical motion of tiny bubbles because of density difference with liquid provides extra vibration, eddies, turbulences, and consequently further heat transfer rate. The variety of affected parameters, such as injection method, air mass flow rate, bubbles size, number of perforations that forms bubbles, etc., has added to the complexity of this phenomena so that any change in the said parameters significantly influences the thermal-exergetic behavior of the heat exchanger. The quality and quantity of the impact of bubbles on the thermal performance of heat exchangers are different for any type of them. Moreover, each type of heat exchanger requires a specific injection method depending on the heat exchanger structure. In the present research, an injection way is proposed for vertical double-tube heat exchangers, and the effect of bubbles on thermal-exergetic characteristics is experimentally studied and discussed, which have not been performed before. Nondimensional exergy destruction, number of heat transfer units, and effectiveness are the main evaluated parameters in the present paper. Results showed a significant thermal improvement of the heat exchanger under the bubble injection.     

Characterization of continuously inhomogeneous media with arbitrary thermophysical profiles using photothermal technique

A numerical method of determining depth profile of thermo-physical parameters of an inhomogeneous sample is presented. By dividing the inhomogeneous material into number of sublayers in which each single layer can be treated as homogeneous, the thermal-wave field of the material is obtained based on recurrence relations of thermophysical parameters of each divided layer. A numerical algorithm of determining the depth profile of thermophysical parameters in the continuously inhomogeneous material under excitation of arbitrary beam size has been demonstrated by a polynomial fitting process for three arbitrary generated thermophysical profiles.     

Analysis of closed-boundary cylindrical microlenses with long focal depth designed by the general focal length function

In this paper, the general focal length function is used to design two-dimensional closed-boundary cylindrical microlenses (CBCMs) with long focal depth. The focusing characteristics of the designed microlenses is investigated by rigorous electromagnetic theory and the boundary element method. A number of focusing performance measures of the designed microlenses, such as the real focal depth, the focal depth range, the focal spot size, and the diffraction efficiency, are presented in detailed. As comparison, the focusing performance of the conventional lenses with the same parameters are investigated simultaneously. Our analysis indicates that the general focal length function is valid in designing CBCMs with larger extended focal depth. Comparing with the open-boundary cylindrical microlenses (OBCMs) designed using the same focal length function, we also find that the designed CBCMs with low f-number exhibit superiority of long focal depth.     

Influence of dissolved-air concentration on spatial distribution of bubbles for sonochemistry

The pulsation of ultrasonic cavitation bubbles at various dissolved-air concentration in a sonochemical reaction field of standing-wave type is investigated experimentally by laser-light scattering. When a thin light sheet, finer than half the wavelength of sound, is introduced into the cavitation bubbles at an antinode of sound pressure, the scattered light intensity oscillates. The peak-to-trough light intensity is correlated with the number of bubbles that contribute to the sonochemical reaction. It is shown that as the dissolved air concentration becomes higher, the weighted center of the spatial distribution of the peak-to-trough intensity tends to shift towards the liquid surface. At higher concentration of the dissolved air, a great deal of bubbles with size distribution generated due to coalescence between bubbles disturbs sound propagation to change the sound phase easily. A standing wave to trap tiny oscillating bubbles is established only at the side which is nearer to the liquid surface. Also at higher concentration, liquid flow induced by drag motion of bubbles by the action of radiation force becomes apparent and position-unstable region of bubble is enlarged from the side of sound source towards the liquid surface. Therefore, the position of oscillating bubbles active for sonochemical reaction is limited at the side which is nearer to the liquid surface at higher concentration of the dissolved air.