Piezoelectric effect in liquid crystals

A piezoelectric effect has been observed in a cholesteric liquid crystal layer (an electric field effect type, a mixture of cholesteryl oleyl carbonate, cholesteryl chloride and cholesteryl nonanoate) [1]. The layer, subjected to shear vibration, generates an alternating electric potential of the same frequency as the exciting vibration. In the present paper an extension of these investigations is reported to cases with various mixing ratios and also to one of smectic liquid crystals (lecithin with water). For a cholesteric liquid crystal of a particular mixing ratio, the strength of the electric generation is found to reach as high as 450 mV(p-p) for a vibratory shear displacement of 1 μm(p-p) under a certain molecular orientation preparation. Small electric generation is also observed for the above smectic liquid crystal but not for the nematic liquid crystals (MBBA and EBBA). A simple continuum elasticity model can qualitatively explain the behaviour of this electric potential generation.     

水平管外降膜蒸发的熵产分析

从热力学第一、第二定律的角度出发,结合水平管外降膜蒸发的特点,推导出了降膜蒸发过程单位熵产数Ns的公式,并在所建立的管外液膜厚度模型的基础上进行了求解。结果表明:沸腾蒸发时的单位熵产数Ns要小于表面蒸发时的值;采用强化表面可以有效地降低单位熵产数Ns;单位熵产数Ns随液膜雷诺数Re的变化比较小。通过分析,降膜蒸发的单位熵产分析法对于最大限度地减少不可逆损失、提高能量利用率等方面具有指导意义。     

Multifrequency stimulated Raman scattering of light in liquid nitrogen infiltrated into 3D photonic crystals

Spectral characteristics of stimulated Raman scattering at least in synthetic opal matrices infiltrated with liquid nitrogen upon excitation by nanosecond laser pulses are studied. The effect of the stop band position on the generation efficiency of anti-Stokes components of stimulated Raman scattering up to the third order is demonstrated. The excitation threshold of stimulated Raman scattering in liquid nitrogen infiltrated into opals is significantly lower in comparison with the generation in bulk samples.     

Molecular QED of coherent and incoherent sum-frequency and second-harmonic generation in chiral liquids in the presence of a static electric field

Coherent second-order nonlinear optical processes are symmetry forbidden in centrosymmetric environments in the electric-dipole approximation. In liquids that contain chiral molecules, however, and which therefore lack mirror image symmetry, coherent sum-frequency generation is possible, whereas second-harmonic generation remains forbidden. Here we apply the theory of molecular quantum electrodynamics to the calculation of the matrix element, transition rate, and integrated signal intensity for sum-frequency and second-harmonic generation taking place in a chiral liquid in the presence and absence of a static electric field, to examine which coherent and incoherent processes exist in the electric-dipole approximation in liquids. Third- and fourth-order time-dependent perturbation theory is employed in combination with single-sided Feynman diagrams to evaluate two contributions arising from static field-free and field-induced processes. It is found that, in addition to the coherent term, an incoherent process exists for sum-frequency generation in liquids. Surprisingly, in the case of dc-field-induced second-harmonic generation, the incoherent contribution is found to always vanish for isotropic chiral liquids even though hyper-Rayleigh second-harmonic generation and electric-field-induced second-harmonic generation are both independently symmetry allowed in any liquid.     

基于GPU的液晶大气湍流模拟器波面生成的并行实现

为了使液晶大气湍流模拟器具有实时大气模拟能力,在GPU通用计算架构下提出了基于GPU的液晶大气湍流模拟器实时波面生成计算方法。针对液晶湍流模拟器高分辨率、高精度的特点介绍了湍流波面生成计算方法,论述了CUDA通用计算架构。建立基于GPU的波面生成模型,并对该模型进行了并行化优化和共享存储器优化。给出了采用CPU与GPU进行波面生成的实验对比结果。结果表明:采用GPU生成分辨率为256×256,192项Zernike多项式进行波面生成的平均时间为2.5ms,生成速度比CPU少两个量级,满足实时波面生成的要求。     

Nanoparticles by Laser Ablation

This review concerns nanoparticles collected in the form of nanopowder or a colloidal solution by laser ablating a solid target that lies in a gaseous or a liquid environment. The paper discusses the advantages of the method as compared with other methods for nanoparticle synthesis, outlines the factors on which the properties of the produced nanoparticles depend, explains the mechanisms and models involved in the generation of nanoparticles by laser ablation, clarifies the differences between nanoparticle generation in gaseous and liquid environments, presents some experimental desigins and equipment used by the several groups for nanoparticle generation by laser ablation, describes the techniques used for “tuning” the width of the nanoparticles size distribution, and finally presents a few interesting examples of nanoparticles generated by laser ablation.     

电厂空冷岛喷雾冷却控制系统开发

为了解决内蒙准大电厂在夏季不能满负荷发电这一问题,提出利用喷嘴在散热器周围形成喷雾这一方法,促使散热器环境温度降低,提高散热器热交换率,降低煤耗,提高发电量,实现满负荷发电。为更好的控制雾化效果,设计一套喷雾冷却控制系统。系统以PLC为主控制器,以超声波液位传感器为反馈单元,电动调节阀为执行机构,实现大型容器的液位控制。系统能对水泵进行旁路调节,并具有速断保护功能。该系统运行结果表明,良好的雾化效果有效提高了电厂发电效益。     

An Iterative Fourier Transform Algorithm for digital hologram generation using phase-only information and its implementation in a fixed-point digital signal processor

An Iterative Fourier Transform Algorithm (IFTA) for digital hologram generation from a digital image using phase-only information is described in this paper. The algorithm is implemented in a Texas Instruments TMS320C64x fixed point digital signal processor (DSP). The holograms are reconstructed in real time using a liquid crystal display system and a He–Ne laser. The ideas described in this paper can be extended to digital hologram generation of three-dimensional objects and their reconstruction in real time using liquid crystal spatial light modulators, which could be a way for 3D-display systems.     

Stability analysis of the Luttinger liquid fixed-point in metallic nanotubes

We study the stability of a Luttinger liquid in a metallic single-walled nanotube against generic backscattering and Umklapp perturbations. Analizing the renormalization group equations we identify nontrivial particular solutions that lay in the same universality class of the Luttinger liquid. We describe in detail the mechanism of instability generation in that electron liquid and discuss under which conditions it governs the physical properties in the presence of low frequency or temperature cutoffs.     

Analysis of liquid surface films by pulsed laser photoacoustic spectroscopy

The photoacoustic generation of plane acoustic waves in strongly absorbing or opaque liquids by pulsed laser radiation is discussed both experimentally and theoretically. The regimes of a confined and a free surface of the liquid are considered. The model which takes the temporal shape of the laser pulses applied in the experiments into account, implies that spectroscopic studies are feasible with direct photoacoustic generation and detection also for opaque liquids. The experiments are performed with a tunable hybrid CO₂ laser and piezoelectric detection. For the first time liquid/liquid interfaces are studied by this technique. We demonstrate that the presence of an absorbing liquid film with a thickness of >1 m on the surface of another liquid amplifies the acoustic signal which is detected in the bottom liquid. The enhancement depends on the thickness and the optical and thermal properties of the film medium. The surface layer can be analyzed on the basis of the photoacoustic spectrum. It is also shown that this non-contact method is surface-film selective and should thus prove useful for pollution analysis of liquid surfaces.     

About lack of fragmentation for hot droplets at low subcooling of coolant

It was confirmed in experiments that during contact between cool and hot liquids, the lower sub-cooling of the cool liquid below the saturation temperature changes the characteristics of a vapor layer covering the fragments of hot liquid. This factor also decreases the probability of spontaneous direct contact between two kinds of liquid, explosive incipience of the cool liquid, and pressure pulse generation (the latter triggers fine fragmentation of hot coolant and vapor explosion). The mechanism that describes this trend in vapor layer behavior has been described.     

气泡-液体闭式多股射流的大涡模拟研究

采用气泡-液体两相流动的欧拉-拉氏大涡模拟,研究了矩形通道内多股射流形成的气泡-液体两相湍流流动,得到了气液两相湍流瞬态结构,产生和发展过程。研究结果发现气泡和液体都有瞬态大涡结构,气泡脉动比液体的强。大涡模拟统计结果给出了有无气泡两种情况下的液体湍流脉动速度均方根值分布。瞬态和统计结果都表明,气泡增大了液体湍流,液体湍流来源于其自身的剪切产生和气泡的作用。这与二阶矩模型模拟结果定性一致。     

Nanoparticle formation during laser ablation of solids in liquids

The experimental data on the generation of metal and semiconductor nanoparticles during their laser ablation in liquids is reviewed. The dependence of the morphology of noble metal nanoparticles on the liquid type and laser parameters is discussed. The data on the kinetics of the formation of alloyed Au-Ag nanoparticles under laser irradiation of a mixture of colloid solutions of individual nanoparticles are presented. The effect of femtosecond laser beam self-action during metal ablation in liquids via the second harmonic generation at Ag nanoclusters is discussed. The data on the generation of core-shell nanoparticles during laser ablation of alloys and in the presence of the chemical interaction of formed nanoparticles with surrounding liquid are presented. It was shown that laser ablation of CdS and ZnSe crystals leads to the formation of quantum dots of these semiconductors in solution. The parameters controlling the properties of nanoparticles during ablation in liquids and possible applications of the method are discussed.     

PEMFC电极表面液态水生成的可视化研究

本文设计了一个可视化直流道单片电池,通过高倍高速数字摄影装置观察记录了阴极流道壁面和气体扩散层表面的凝结水出现和成长过程,并分析了电池运行温度,阴极气体压力,气体化学当量比以及气体加湿度等参数对阴极流道内液态水的生成和排出的影响。     

Generation of intense directional radiation during the fast motion of a liquid jet through a narrow dielectric channel

The results of theoretical and experimental studies of the cavitation phenomena in the volume of a moving liquid jet after its passing through thin and long oriented channels in dielectrics are considered. It is shown that the stationary generation of intense directional radiation in the optical range occurs in the moving jet volume as a threshold pressure is reached in the liquid (pure spindle oil). The parameters of radiation are close to those of laser radiation. The effective temperature of the generation region was estimated as corresponding to 50–100 eV. In some cases, optical radiation is accompanied by the pulsed generation of directional gamma radiation. These processes are accompanied by a sequence of high-voltage electric discharges of a great length in the liquid bulk and at the surface, corresponding to potential differences of 50–100 kV. One of the causes of the observed phenomena can be energetically favorable nuclear fusion reactions involving light nuclei in the liquid jet volume. It was shown that such processes can be efficiently stimulated by multibubble cavitation.     

Second harmonic generation by femtosecond pulses in nematic liquid crystal

The second harmonic generation (SHG) by the femtosecond pulses in a nematic liquid crystal has been experimentally studied. It was found that the SHG by a light beam occurs in the bulk of the liquid crystal sample, is accompanied by a luminescence, and is only observed in the presence of the light-induced director reorientation.     

Optically active second-harmonic generation from a uniaxial fluid medium

We have shown that optically active second-harmonic generation is allowed in a uniaxial fluid medium. A homeotropically aligned chiral smectic-A liquid crystal was used as an example. Phase matching was achievable by angle tuning. Chiral nonlinear susceptibility for the liquid crystal was deduced. The signal dropped precipitously as the sample underwent the transition from smectic-A to isotropic.     

Detailed simulation of primary atomization mechanisms in Diesel jet sprays (isolated identification of liquid jet tip effects)

In this study, the atomization characteristics of Diesel jet front tip have been investigated to elucidate the physical mechanisms by detailed numerical simulation. The computations are carried out with the finest grid resolutions ever that can resolve the final droplet generation by surface tension. The numerical methods are based on level-set interface tracking. The methods were validated by test cases and the grid resolution survey shows that the resolutions for the present study are sufficient. The present flow setup excludes nozzle disturbances to investigate how the disturbances from the liquid jet front would lead to atomization where the liquid jet impacts against the quiescent gas. The liquid jet front becomes an umbrella-like shape. From the front umbrella tip edge, ligament breakup first occurs. Ligament breakup is strongly correlated with the gas motion in the vicinity. The gas region behind the front is highly disturbed by atomization. By the gas recirculation motion here, air and some droplets are entrained and mixed. Also, the disturbances are fed back to the front umbrella by this motion and become synchronized with the breakup. Droplet pinch-off is mainly in the short-wave mode, but some ligaments are elongated by local gas stretch to finally have a long-wave mode shape, namely a mode shift occurs. The above findings of liquid jet front umbrella formation, atomization at the umbrella edge, mixing and atomization loop in the recirculation flow region and droplet generation mode give an insight to the modeling of droplet generation in actual sprays.     

Weakly nonlinear focused ultrasound in viscoelastic media containing multiple bubbles

To facilitate practical medical applications such as cancer treatment utilizing focused ultrasound and bubbles, a mathematical model that can describe the soft viscoelasticity of human body, the nonlinear propagation of focused ultrasound, and the nonlinear oscillations of multiple bubbles is theoretically derived and numerically solved. The Zener viscoelastic model and Keller–Miksis bubble equation, which have been used for analyses of single or few bubbles in viscoelastic liquid, are used to model the liquid containing multiple bubbles. From the theoretical analysis based on the perturbation expansion with the multiple-scales method, the Khokhlov–Zabolotskaya–Kuznetsov (KZK) equation, which has been used as a mathematical model of weakly nonlinear propagation in single phase liquid, is extended to viscoelastic liquid containing multiple bubbles. The results show that liquid elasticity decreases the magnitudes of the nonlinearity, dissipation, and dispersion of ultrasound and increases the phase velocity of the ultrasound and linear natural frequency of the bubble oscillation. From the numerical calculation of resultant KZK equation, the spatial distribution of the liquid pressure fluctuation for the focused ultrasound is obtained for cases in which the liquid is water or liver tissue. In addition, frequency analysis is carried out using the fast Fourier transform, and the generation of higher harmonic components is compared for water and liver tissue. The elasticity suppresses the generation of higher harmonic components and promotes the remnant of the fundamental frequency components. This indicates that the elasticity of liquid suppresses shock wave formation in practical applications.     

Flow-driven voltage generation in carbon nanotubes

The flow of various liquids and gases over single-walled carbon nanotube bundles induces an electrical signal (voltage/current) in the sample along the direction of the flow. The electrical response generated by the flow of liquids is found to be logarithmic in the flow speed over a wide range. In contrast, voltage generated by the flow of gas is quadratically dependent on the gas flow velocity. It was found that the underlying physics for the generation of electrical signals by liquids and gases are different. For the liquid, the Coulombic interaction between the ions in the liquid and the charge carriers in the nanotube plays a key role while electrical signal generation due to gas flow is due to an interplay of Bernoulli’s principle and Seebeck effect. Unlike the liquid case which is specific to the nanotubes, the gas flow effect can be seen for a variety of solids ranging from single and multi-walled carbon nanotubes, graphite and doped semiconductors.