Dielectric electrorheological fluids: Theory and experiment

Electrorheological (ER) fluids are a class of materials whose rheological properties are controllable by the application of an electric field. A dielectric electrorheological (DER) fluid is the simplest type of ER fluid, in which the material components follow a linear electrostatic response. We review and discuss the progress of the studies on physics of this type of material. A first-principles theory of DER fluids, along with relevant experimental verifications, are presented in some detail. In particular, the properties presented include static equilibrium structure, shear modulus, static yield stress and its variation with applied electric field frequency, and structure-induced dielectric nonlinearity.     

微晶纤维素电流变液在挤压流中的粘弹性

测量了微晶纤维素/蓖麻油悬浮体构成的电流变液在挤压流中的动态粘弹性.给出了平行于电 场方向的储能模量G′和损耗模量G″对电场和应变幅度的依赖关系.实验显示,当电场超过 临界电场后,随着应变幅度增加, 电流变液的粘弹性从线性变到非线性,电流变液出现了应 变导致的“类固体”和流体之间的转变过程. 还研究了应变幅度和振荡频率对电流变液粘弹性非线性的影响. 关键词： 电流变液 挤压流 粘弹性     

极性分子型电流变液导电机理研究

极性分子型电流变液是一种新型的电流变材料.其介电颗粒上吸附极性分子,极性分子在颗粒间强局域电场作用下发生取向是产生巨电流变效应的关键.通过对Ca—Ti—O(CTO)体系极性分子型电流变液电流密度的测量发现,其导电行为遵从Poole-Frenkel效应的规律,这是极性分子型电流变液的重要特征之一.而500 ℃处理过的CTO粉体不含极性分子,所配制的电流变液无巨电流变效应,其电流密度随外电场强度近似地呈线性关系,显示出传统电流变液特性.     

电流变液的旋光与椭偏光行为

本文研究了电流变液(ER流体)的旋光行为及椭偏光行为.激光束垂直于电场方向,利用HeNe激光器对SiO₂、Si₃N₄和硅油组成的ER流体进行测量.发现电流变液样品存在旋光现象,且对于同一种浓度的ER流体,随着电场强度的增大,旋光角度增大.在同一电场强度下,旋光角度随着浓度的增大而增大.当浓度大于某一值c₀时,旋光角度开始减小,在不同电场强度下,透过电流变液的椭圆偏振光性质不发生改变,且随着电场强度的增大,透射光强度随之增大.电流变液的旋光行为及椭偏光行为起因于在电场作用下ER流体由各向同性的液体转变为各向异性的液体.     

Polar molecule dominated electrorheological effect

The yield stress of our newly developed electrorheological (ER) fluids consisting of dielectric nano-particles suspended in silicone oil reaches hundreds of kPa, which is orders of magnitude higher than that of conventional ones. We found that the polar molecules adsorbed on the particles play a decisive role in such new ER fluids. To explain this polar molecule dominated ER (PM-ER) effect a model is proposed based on the interaction of polar molecule-charge between the particles, where the local electric field is significantly enhanced and results in the polar molecules aligning in the direction of the electric field. The model can well explain the giant ER effect and a near-linear dependence of the yield stress on the electric field. The main effective factors for achieving high-performance PM-ER fluids are discussed. The PM-ER fluids with the yield stress higher than one MPa can be expected.     

固液双相电流变系统流动过程的相转变特性

考虑电流变液多粒子近程相互作用，利用等效平板电导模型，研究了电流变液流动过程相转变点的特性，并设计实验观察了电流变液中的这种相转变现象.研究结果表明，电流变液在一定压力梯度作用下发生流动，此时为双相流；当外加电场达到某一值时，电流变液中颗粒不流动，由固液双相流转变为液体单相流动，发生场控相转变，理论模拟结果与实验观察结果基本相符.阈值电场随外加压力梯度的增加而加大，随颗粒浓度的增加而减小. 关键词： 电流变液 颗粒流 相转变     

Dynamic stability of a sandwich plate with a constraining layer and electrorheological fluid core

The dynamic stability problems of a sandwich plate with a constraining layer and an electrorheological (ER) fluid core subjected to an axial dynamic force are investigated. The rectangular plate is covered in an ER fluid core and a constraining layer to improve the stability of the system. Effects of the natural frequencies, static buckling loads, and loss factors on the dynamic stability behavior of the sandwich plate are studied in the paper. Rheological property of an ER material, such as viscosity, plasticity, and elasticity may be changed when applying an electric field. The modal damper and the natural frequencies for the sandwich plate are calculated for various electric fields. When an electric field is applied, the damping of the system is more effective. In this study, finite element method and the harmonic balance method are used to calculate the instability regions of the sandwich plate. The ER fluid core is found to have a significant effect on the dynamic stability regions.     

极性分子型电流变液

电流变液在电场作用下软硬连续可调的奇特性质具有广泛和重要的应用价值。传统电流变液是基于颗粒极化产生的相互作用,根据介电理论预测,其剪切屈服强度的上限约为10kPa。电流变液被发明50年来,阻碍其应用的主要原因是剪切强度低。近年来发明的"极性分子型电流变液"是一类新型电流变液,其屈服强度比传统电流变液大一个数量级以上,且与电场强度呈线性关系,这一点和传统电流变液中的平方关系也明显不同。文章作者提出了极性分子在颗粒间强局域电场中的取向并与极化电荷作用的模型,成功地解释了观察到的实验现象。根据这一原理,有可能制备出屈服强度高达MPa的电流变液。     

极板形貌修饰对电流变液/极板界面滑移抑制实验研究

被外电场极化而固化的电流变液容易在极板处产生剪切滑移而降低其力学性能.使用抛光、激光打坑、覆盖尼龙网和光刻腐蚀四种方法对极板形貌进行了修饰,并对电流变液的压缩力学性能进行了测试.研究表明,光滑极板和光刻腐蚀柱阵列极板易产生界面滑移而压缩强度较低,粗糙坑阵列和覆盖尼龙网可抑制界面滑移而压缩强度高.极板形貌增强极板附近局部电场强度,强化了链末端与极板间的作用,迫使链结构屈服位置远离链末端,从而有效抑制了滑移.研究结果对进一步认识电流变液的屈服强度,提高电流变器件的力学性能有重要参考价值. 关键词： 电流变液 滑移 极板形貌修饰 压缩应力     

纳米TiO2颗粒对电流变悬浮液中硅油的挥发增强效应

物理化学性能稳定的二甲基硅油常作为电流变液分散相, 当与纳米量级的介电颗粒混合组成电流变悬浮液时, 在非密闭环境下极易挥发, 时间足够长时, 可完全挥发. 本文通过实验研究了纳米二氧化钛颗粒对二氧化钛和硅油组成的悬浮液中硅油挥发增强现象, 分析表明, 纳米颗粒在电流变悬浮液的硅油气-液界面上形成纳米尺度的凸型曲面, 使液面上蒸气压大大提高, 导致挥发增强. 本文还对颗粒浓度, 环境温度和硅油黏度等对硅油挥发增强效应的影响进行了系统的研究和分析.     

电流变液微波反射可调控性

研究了纳米二氧化钛包覆高岭土和钇掺杂钛酸钡两种电流变液的微波反射行为.实验发现，在垂直于微波传播方向电场作用下，两种电流变液微波反射系数均随电场强度的增大而增大；钇掺杂钛酸钡电流变液微波反射系数变化幅度比二氧化钛包覆高岭土电流变液宽，并且电场调节的能力随浓度的增加有减弱的趋势.对钇掺杂钛酸钡电流变液存在一个临界浓度，低于这个浓度，电场的调控能力随电场强度的增加而增大，高于这个浓度，电场的调控能力随电场的增加先增大后减小.另外，二氧化钛包覆高岭土电流变液反射回波相位随电场强度的增大出现最大值后逐渐减小趋于平 关键词： 电流变液 微波反射 相位     

用光学方法研究玻璃微珠电流变液颗粒间的相互作用

运用一套适合研究颗粒间相互作用的双光镊系统,通过对粒聚集时间的测量,得出颗粒聚集时间和电场的平方成反比。这是第一次用电流变液颗粒在动态情况下直接验证电偶极子对间的相互作用。发展了一套使用高速CCD摄像机进行扩散波谱（DWS）测量的方法,首次实时测量具有颗粒结构的非各态历经体系的自相关函数,以研究电流变液机理,得到了玻璃微珠电流变液的结构响应时间和力和响应时间;测量了不同电场下体系相关函数的特征衰减时间随时间的变化。在不同电场下测量扩散系数可以反映出相互作用力与电场的平方成正比。     

电场方向对电流变液剪切应力的影响

采用分子动力学模拟的方法,研究了电场方向对电流变液剪切应力的影响结果表明由于流体中自发倾斜链的形成,剪切应力对电场方向有明显的依赖关系,并且在某些特定方向应力会显著增强一种关于电流变液理想结构的理论模型计算证实了该结果,这样人们可通过选择电场的最优方向来增强电流变效应,而不需要额外的能量消耗,从而为电流变液走向实用提供了一条可能的途径 关键词：     

Acoustical scattering by multilayer spherical elastic scatterer containing electrorheological layer

A computational procedure for analyzing acoustical scattering by multilayer concentric spherical scatterers having an arbitrary mixture of acoustic and elastic materials is proposed. The procedure is then used to analyze the scattering by a spherical scatterer consisting of a solid shell and a solid core encasing an electrorheological (ER) fluid layer, and the tunability in the scattering characteristics afforded by the ER layer is explored numerically. Tunable scatterers with two different ER fluids are analyzed. One, corn starch in peanut oil, shows that a significant increase in scattering cross-section is possible in moderate frequencies. Another, fine poly-methyl methacrylate (PMMA) beads in dodecane, shows only slight change in scattering cross-sections overall. But, when the shell is thin, a noticeable local resonance peak can appear near ka=1, and this resonance can be turned on or off by the external electric field.     

电流变液与电-磁流变液的衍射特性

本文把电流变液装置作为反射光栅研究其衍射特性,首次提出由电场强度改变来调节光栅常数的概念.并讨论了同时外加电场与磁场的情况下,电磁流变液的衍射特性.     

First observation of electrorheological plasmas

We report the experimental discovery of "electrorheological (ER) complex plasmas," where the control of the interparticle interaction by an externally applied electric field is due to distortion of the Debye spheres that surround microparticles (dust) in a plasma. We show that interactions in ER plasmas under weak ac fields are mathematically equivalent to those in conventional ER fluids. Microgravity experiments, as well as molecular dynamics simulations, show a phase transition from an isotropic to an anisotropic (string) plasma state as the electric field is increased.     

电流变液的流变学响应与其非线性介电性质的关系

对两类电流变液的介电性质的测量结果表明，强交流电场下电流变液经历介面极化；而且是一种非线性极化．根据海藻酸钠和NaY沸石电流变液的动态剪切应力与强电场介电谱相似这一事实，确定了这两种样品的流变学响应时间下限，它们分别是1ms和0.7ms．还发现NaY电流变液的动态切应力与非线性三次谐波的强度有关；一种高分子半导体电流变液的流变学响应基本不受样品含水量的影响．对上述实验结果进行了解释 关键词：     

电流变液中悬浮颗粒的体积与形状的变化

基于偶极近似 ,运用电极化方法 ,定量研究了电流变液中悬浮颗粒在外电场作用下其体积和形状的变化 ,并计算了其体积和形状的相对变化率 .研究结果表明 :悬浮颗粒的体积和形状的相对变化率均与电场强度的平方成正比 ,并与ER本身的性质有关 .一般情况下 ,颗粒的体积和形状的相对变化率分别为 8.4 %和 12 .5 % ,对总体积变化率的贡献为 1.6 8% .这种变化对于ER系统的电涨和电热都作出了贡献 ,仅当将颗粒当作刚性球时 ,其自由能才与电场强度的平方成正比 .     

Structure and dynamics of a dense dipolar system in an electric field and their relevance to electrorheological fluids

Results of computer simulations of a dense system of dipolar spheres in an electric field are summarized. Dissipative and Hamiltonian dynamics algorithms have been used to find energy minima of the system for varying particle densities. The structures obtained by these simulations are in reasonable agreement with experimentally observed structures in electrorheological (ER) fluids. Qualitative agreement is also obtained with the limited available experimental observations on the dynamics of ER fluids.     

Characteristics and mechanisms of electrorheological fluids

Electrorheological (ER) fluids consist of suspensions of fine polarizable particles in a dielectric medium, which upon application of an electric field take on the characteristics of a solid in times of the order of milliseconds and reversibly return to liquid behavior upon removal of the field. The rheology, electrical characteristics, and structure of typical ER fluids are here reviewed. The proposed mechanisms and their accord with experimental data are discussed. Some directions for future research are mentioned.