Numerical Simulation of the Flow Field and Solute Segregation in Edge‐Defined Film‐Fed Growth

The purpose of this study is to clarify the effect of hydrodynamics and mass transport on the bubble generation in sapphire shaped crystals. The basic idea is that nucleation of bubbles can be anticipated in places were the gas chemical composition is maximum. Finite elements numerical simulation (FIDAP^TM software) is used in order to solve the momentum, mass and chemical species conservation equations in the liquid enclosed in the capillary channel and in the liquid meniscus. A parametric study is performed and the effect of the die geometry and of the physical parameters are clarified. It is shown that the main effect is related to the rejection of the gas at the solid‐liquid interface and that forced and thermo‐capillary convection can explain the experimental observation of the concentration of microbubbles very close to the outer crystal surface in certain cases.     

LHPG法生长单晶光纤中熔区生长界面对消除气泡的影响

采用激光加热基座法(LHPG法)生长的单晶光纤一般具有较好的表面光学质量,但单晶光纤在生长过程中常产生包裹气泡.该类气泡的存在将严重劣化光纤的性能.由于其产生原因复杂,多年来一直没有有效的消除方法.本文采用LHPG法生长Nd3+:YAG单晶光纤,深入分析了熔区生长界面对消除气泡的影响.实验结果表明,相比于其它熔区,凹形熔区稳定性能较好,是最佳的熔区形状,采用此熔区生长出的单晶光纤没有气泡,具有极好的光学品质.     

源自软磁畴段的布洛赫线链行为研究

在从1967—1992年连续25年研制固态磁存储器的故事中,中国没有缺席。1970年代初,中国科学院物理研究所磁学室磁泡组11个人从生长基片单晶,制备单晶磁泡薄膜和进行磁泡测量三方面开始磁泡材料的研制。当时文章作者负责测量,设计制作了磁泡测量装置。为了表征磁泡薄膜,发现了含有“一盘”软磁畴段的H图形,并找到了“脉冲偏磁场作用下硬磁泡的形成”的研究课题。这使作者在1980年代初经受住了磁泡下马的冲击,迎来了1983年布洛赫线存储器方案的提出,发觉已具有研究其机理的条件,最终成为该存储器的终结者,并在1991年国际&;amp;lt;em&;amp;lt;J. Mag. Mag. Mater.&;amp;lt;/em&;amp;lt; 杂志第100纪念卷中荣幸地为“China”占了一席之地。从1992年以来,时间又过去了25年,但活生生的磁泡总在,当年报废的磁泡测量装置已经更新。盼望大学生和刚入门的研究者喜欢这台能显示赏心悦目的磁泡畴运动,能生动阐述铁磁学物理基础的研究导向性的物理实验设备。     

激光空泡溃灭及其产生的声波

强激光脉冲在水中的自聚焦传输、成丝、击穿及击穿后产生的声波,近年来成为强激光脉冲传输应用研究领域的一个热点。强激光脉冲电离产生的空泡在溃灭过程中,对周围水体产生压缩,从而在水下产生声波。在现有理论基础上,考虑空泡含气量、水的粘滞系数和水的表面张力系数对空泡运动及其产生声波的影响。给出双空泡运动方程,分析空泡间距对空泡溃灭和其产生声压关系。根据实际情况,计算不同温度情况下单空泡和双空泡运动过程及其产生的声压,并进行相互比较。计算结果表明： 水温越高,空泡运动时达到的最小半径越小,空泡振荡周期和溃灭时间延长,产生声压越高。随双空泡间距减小,空泡溃灭时达到的最小半径线性减小,而其产生的最大声压则单调增大。     

Medium-high frequency sonication dominates spherical-SiO2 nanoparticle size

Spherical SiO₂ nanoparticles (SSNs) have been inventively synthesized using the Stöber method with sonication at medium–high frequencies (80, 120, and 500 kHz), aiming to control SSN size and shorten reaction time. Compared to the conventional method, such sonication allowed the Stöber reaction complete in 20–60 min with a low molar ratio of NH₄OH/tetraethyl orthosilicate (0.84). The hydrodynamic diameters of 63–117 nm of SSNs were obtained under sonication with 80, 120, and 500 kHz of ultrasonic frequencies. Moreover, the SSNs obtained were smaller at 120 kHz than at 80 kHz in a multi-frequencies ultrasonic reactor, and the SSN size decreased with increasing ultrasonic power at 20 °C, designating the sonochemical unique character, namely, the SSN-size control is associated with the number of microbubbles originated by sonication. With another 500 kHz ultrasonic bath, the optimal system temperature for producing smaller SSNs was proven to be 20 °C. Also, the SSN size decreased with increasing ultrasonic power. The smallest SSNs (63 nm, hydrodynamic diameter by QELS, or 21 nm by FESEM) were obtained by sonication at 207 W for 20 min at 20 °C. Furthermore, the SSN size increased slightly with increasing sonication time and volume, favoring the scale-up of SSNs preparation. The mechanisms of controlling the SSN size were further discussed by the radical’s role and effects of ammonia and ethanol concentration.     

两种气泡混合的声空化

将非线性声波方程和改进的Rayleigh-Plesset方程联立可以描述空化环境中的声场及相应的气泡动力学特征. 用时域有限差分方法模拟了圆柱形容器内两种气泡相互混合时的空化情况. 在烧杯内的稳态背景声场形成过程中, 瓶壁耗散吸收扮演了重要的角色. 在稳态背景声场的基础上, 分析了混合气泡与声场的相互作用、气泡之间的相互作用、混合情况下的频谱特性. 结果表明: 两种气泡平衡半径都不太大时, 气泡与声场的相互作用不强, 声场及气泡的行为也比较规律; 相反, 当其中一种气泡平衡半径相对比较大时, 声场与气泡具有较强的非线性相互作用, 声场及气泡的行为表现出复杂的特性.     

舰船尾流气泡层散射相位函数及其后向散射信号特性分析

根据实测舰船的尾流气泡分布模型,利用米氏理论计算得到尾流气泡层的散射相位函数,并将其应用于尾流气泡层后向散射光接收信号的Monte Carlo仿真分析中,以便为实际海况下的舰船尾流气泡层后向散射光探测的实验设置与信号预测、分析提供可靠的依据.通过对不同距离处尾流气泡层的散射相位函数及后向散射光接收信号特性的分析,可以得出:尾流中气泡层散射相位函数的变化小明显,且对接收信号的影响较小;气泡数密度的变化导致的多次散射效应和气泡层衰减系数的变化是引起后向散射光强度变化的主要原因,数密度越大,散射光信号中的多次散射光成分越大;当气泡层厚度达到一定倍数的衰减长度时,继续增加气泡层厚度对后向散射接收信号的影响较小.     

Theory of non-equilibrium force measurements involving deformable drops and bubbles

Over the past decade, direct force measurements using the Atomic Force Microscope (AFM) have been extended to study non-equilibrium interactions. Perhaps the more scientifically interesting and technically challenging of such studies involved deformable drops and bubbles in relative motion. The scientific interest stems from the rich complexity that arises from the combination of separation dependent surface forces such as Van der Waals, electrical double layer and steric interactions with velocity dependent forces from hydrodynamic interactions. Moreover the effects of these forces also depend on the deformations of the surfaces of the drops and bubbles that alter local conditions on the nanometer scale, with deformations that can extend over micrometers. Because of incompressibility, effects of such deformations are strongly influenced by small changes of the sizes of the drops and bubbles that may be in the millimeter range. Our focus is on interactions between emulsion drops and bubbles at around 100 μm size range. At the typical velocities in dynamic force measurements with the AFM which span the range of Brownian velocities of such emulsions, the ratio of hydrodynamic force to surface tension force, as characterized by the capillary number, is ~ 10^{− 6} or smaller, which poses challenges to modeling using direct numerical simulations. However, the qualitative and quantitative features of the dynamic forces between interacting drops and bubbles are sensitive to the detailed space and time-dependent deformations. It is this dynamic coupling between forces and deformations that requires a detailed quantitative theoretical framework to help interpret experimental measurements. Theories that do not treat forces and deformations in a consistent way simply will not have much predictive power. The technical challenges of undertaking force measurements are substantial. These range from generating drop and bubble of the appropriate size range to controlling the physicochemical environment to finding the optimal and quantifiable way to place and secure the drops and bubbles in the AFM to make reproducible measurements. It is perhaps no surprise that it is only recently that direct measurements of non-equilibrium forces between two drops or two bubbles colliding in a controlled manner have been possible. This review covers the development of a consistent theory to describe non-equilibrium force measurements involving deformable drops and bubbles. Predictions of this model are also tested on dynamic film drainage experiments involving deformable drops and bubbles that use very different techniques to the AFM to demonstrate that it is capable of providing accurate quantitative predictions of both dynamic forces and dynamic deformations. In the low capillary number regime of interest, we observe that the dynamic behavior of all experimental results reviewed here are consistent with the tangentially immobile hydrodynamic boundary condition at liquid–liquid or liquid–gas interfaces. The most likely explanation for this observation is the presence of trace amounts of surface-active species that are responsible for arresting interfacial flow.     

基于数字全息术的水中气泡场获取方法的研究

水中气泡场是海洋和大气热交换的重要媒介,随着对海洋研究的深入,水中气泡场的研究越来越受到人们的重视。数字全息术由于具有可获取三维空间信息、可进行实时实地测量、不干扰被测对象等优点,所以可成为获取水中气泡场的有效手段。研究了数字全息技术在水中气泡场探测中的应用,并进行了数字全息拍摄的实验和再现,探讨了获取水中环境对获取气泡场的影响,讨论了实验中出现的问题,并对所得到的再现图像数据进行了分析和讨论。     

Detonation waves in bubble-drop media

Wave processes in chemically active multicomponent media: liquid — gas bubbles — liquid drops have been studied experimentally. Existence of detonation waves in multicomponent (bubble-drop) media has been proved. Structure of detonation waves in bubble-drop and bubble media is qualitatively identical: detonation waves are solitary waves with pulsation profile the pressure behind which is close in value to the one in unperturbed medium. Propagation velocity of detonation waves in bubble and bubble-drop media drops with the increase in medium gas phase concentration and with the decrease in carrier liquid viscosity. Presence of liquid drops decreases detonation wave velocity compared with bubble medium that does not contain liquid drops. Detonation wave propagation in multicomponent media causes gas bubbles fragmentation as well as fragmentation of individual liquid drops. The work was financially supported by the Russian Foundation for Basic Research (Grant No. 04-03-33106).