Brownian motion: absolute negative particle mobility

Noise effects in technological applications, far from being a nuisance, can be exploited with advantage - for example, unavoidable thermal fluctuations have found application in the transport and sorting of colloidal particles and biomolecules. Here we use a microfluidic system to demonstrate a paradoxical migration mechanism in which particles always move in a direction opposite to the net acting force ('absolute negative mobility') as a result of an interplay between thermal noise, a periodic and symmetric microstructure, and a biased alternating-current electric field. This counterintuitive phenomenon could be used for bioanalytical purposes, for example in the separation and fractionation of colloids, biological molecules and cells.     

毛管内荷电粒子的移动规律与流体电阻率

当前的流体导电理论用于多孔介质时矛盾较多,不适用于研究流体电阻率和多孔介质的其他电学问题。本文认为毛管内的荷电粒子的移动过程服从水力学原则,为一层流运动,并假设毛管内液相中荷电粒子的体密度是均匀的,由力电平衡可导出毛管内荷电粒子移动速度抛物线分布规律,毛管内荷电粒子的平均速度是其最大速度的一半。毛管内的流体电流强度与毛管孔径的4次方程成反比,毛管内的流体电阻率是按抛物线的倒数分布的,其平均电阻率是最小电阻率的2倍,毛管内的电阻率是毛管孔径、离子迁移率和普通流体电阻率的综合函数,比普通流体电阻率复杂得多。     

均匀电磁场中带电粒子的运动学特性

通过对不同电磁场环境中的带电粒子受力情况研究带电粒子的运动规律。带电粒子只受电场或只受磁场作用时,其运动轨迹分别是抛物线或螺旋线。当带电粒子受电磁场作用时(带电粒子初速度、电场与磁场两两相互垂直或磁场垂直于初速度与电场构成的平面且初速度与电场成任意角度),得出带电粒子的运动方程。     

非匀强电场对颗粒污垢的抑垢特性

目前,电场抑垢研究兴趣集中于电场对碳酸钙晶体形貌的改变,电场对已成垢的颗粒是否有抑垢作用鲜有研究。针对电场颗粒污垢抑垢机理不明的问题,提出非匀强电场颗粒物污垢抑垢模型,解释了电场对流体管道内污垢颗粒存在抑垢作用。电场60V至180V、频率50Hz、颗粒质量100条件下研究结果表明:匀强电场并不会使颗粒产生抑垢作用,颗粒污垢在非匀强电场受到介电泳力（）,改变原本运动轨迹,达到抑垢的效果。随着电场强度升高,碳酸钙颗粒受到的从,升至。硫酸钙受到的从升至。碳酸镁颗粒受到的升至。电场强度对多种污垢颗粒产生的,是影响抑垢效果的关键因素。     

电磁式静电除尘空气净化机理的研究

提出了电磁式静电除尘空气净化法。从带电粒子的电漂移理论出发，阐明了加平行磁场较加垂直磁场能得到更高的净化效率的理论依据。在更深一步的试验研究基础上，提出粒子初速存在着一个提高净化效率的最佳值，并应用带电粒子电漂移理论解释了一定的集尘板间距内加垂直磁场后，净化效率低于不加磁场情况下的净化效率这一特殊现象，完善了这一新技术。     

A laboratory test of the electrification phenomenon in wind-blown sand flux

The measured data in the wind-tunnel tests show that the wind-blown sand particles acquired a negative charge when their diameters are smaller than 250 μm and positive charge when their diameters are larger than 500 μm, which confirms Latham’s assumption that the large particles in wind-blown sand flux acquired positive charge while negative charge developed on small ones. In the meanwhile, the measured data also show that the average charge-to-mass ratio for wind-blown sand particles decreases with the increase of the particle diameter and the wind velocity, and increases with the rise of height. The electric field in wind-blown sand flux is mainly formed by the moving charged sand particles. Its direction is vertical to the Earth’s surface and upward, which is opposite to that of the fair-weather field. The electric field increases with wind velocity and height increasing. These experimental results will lay the foundation for developing the theoretical analysis of the electrification phenomenon in wind-blown sand flux.     

Ionic colloidal crystals of oppositely charged particles

Colloidal suspensions are widely used to study processes such as melting, freezing and glass transitions. This is because they display the same phase behaviour as atoms or molecules, with the nano- to micrometre size of the colloidal particles making it possible to observe them directly in real space. Another attractive feature is that different types of colloidal interactions, such as long-range repulsive, short-range attractive, hard-sphere-like and dipolar, can be realized and give rise to equilibrium phases. However, spherically symmetric, long-range attractions (that is, ionic interactions) have so far always resulted in irreversible colloidal aggregation. Here we show that the electrostatic interaction between oppositely charged particles can be tuned such that large ionic colloidal crystals form readily, with our theory and simulations confirming the stability of these structures. We find that in contrast to atomic systems, the stoichiometry of our colloidal crystals is not dictated by charge neutrality; this allows us to obtain a remarkable diversity of new binary structures. An external electric field melts the crystals, confirming that the constituent particles are indeed oppositely charged. Colloidal model systems can thus be used to study the phase behaviour of ionic species. We also expect that our approach to controlling opposite-charge interactions will facilitate the production of binary crystals of micrometre-sized particles, which could find use as advanced materials for photonic applications.     

Evidence for a macroscopic electric field in the sedimentation profiles of charged colloids

The determination of molecular masses from barometric sedimentation profiles, a main topic in ultracentrifugal analysis, is thought to be quantitatively correct for non-interacting particles. Whereas this expectation is justified for uncharged colloids or macromolecules at low volume fractions, early ultracentrifugation studies on charged particles had already indicated that the obtained masses might be much too low. More recently, expanded sedimentation profiles have been observed for charged particles, sometimes inflated by orders of magnitude relative to the barometric prediction, which highlights a shortcoming in our understanding of centrifugation of even very dilute charged species. Theory and simulations, anticipated by various authors, now propose that strongly non-barometric sedimentation profiles might be caused by an internal macroscopic electric field that, even for non-interacting particles, significantly decreases the buoyant particle mass. The existence of this field and its intriguing consequences still lack experimental verification. Here we report ultracentrifugation experiments on charged colloidal silica spheres, showing both the existence of such a macroscopic electric field and its drastic effects on the sedimentation profiles of very dilute dispersions at low ionic strength.     

双极荷电颗粒在外加交变电场中的静电凝聚

为了定量分析外加交变电场对双极荷电颗粒静电凝聚的影响,基于双极荷电颗粒在交变电场中的静电凝聚系数公式,利用数值计算的方法求解了荷电颗粒凝聚的数量平衡方程,得到了不同时刻颗粒数量浓度、颗粒带电量和颗粒粒径的关系曲线,比较了外电场存在与否时颗粒数量浓度的变化。结果表明:对于初始单弥散度带任意电荷量的颗粒,其数量浓度随凝聚时间单调减少;外加交变电场能够有效地促进双极荷电颗粒间的凝聚;颗粒上所带的电荷量越大,凝聚效果越明显。     

外电场对荷电颗粒静电凝聚的影响

为了定量地分析外电场对荷电颗粒静电凝聚的影响,在理论分析和数值模拟的基础上,求解荷电颗粒凝聚的数量平衡方程,分析比较外电场存在与否时,颗粒数量浓度的变化关系。结果表明:外电场促进荷电颗粒间的凝聚;对于初始对称双极荷电颗粒,外电场不改变颗粒初始电荷分布的对称性,并且带任意电荷量的颗粒的数量浓度随凝聚时间单调减少;对于初始非对称双极荷电以及单极荷电颗粒,其数量浓度并非都随凝聚时间单调减少。     

外电场对双极荷电颗粒碰撞及凝聚的影响

为确定外电场对双极凝聚的影响,采用FORTRAN程序,通过计算得到颗粒在一定电流体场条件下的电荷分布统计规律,并考察外电场对双极荷电颗粒间的碰撞凝聚规律.计算结果表明,只有当荷电颗粒所受电场力与阻力的量级之比接近于1时,外电场才能起到增强双极凝聚效果的作用;在相同条件下,颗粒直径越大,外电场增强双极荷电颗粒的碰撞效果也越大.     

DC PJ－CVD等离子体炬通道中的电弧行为

运用电磁学和热流体学的方法研究了直流大阳极喷嘴长电弧通道等离子体炬中电弧的行为，并通过实验证明了温度梯度与电场梯度作用于流体时所产生的特殊现象；结果表明：电弧弧柱的高温效应剧烈排斥冷气流的混入，使ＣＶＤ过程受到抑制。新产生的胶体粒子在热泳现象作用下逐渐沉积于通道内壁上。改变不同气体的进气位置以及减小温度与电场梯度并增设径向旋转磁场，可有效地解决上述问题     

声悬浮条件下扇谐振荡液滴的内部流动规律

采用激光片光源照明和高速摄影技术,并使用聚苯乙烯微球作为示踪颗粒,研究了声悬浮条件下二阶和三阶扇谐振荡液滴的内部流场.耦合Level Set方法和标记网格法数值求解了不可压两相流的Navier-Stokes方程,模拟了液滴的扇谐振荡过程中的形态演化及其内部流动状态.数值计算的流场与实验结果吻合很好.对液滴扇谐振荡过程中的内部流速分布进行定量分析,发现流速在液滴中心区域几乎为零并沿液滴自由表面方向逐渐增大.二阶和三阶扇谐振荡的内部流速沿径向分别呈线性和二次函数分布并与理论预测结果相一致.     

高占空比颗粒电场荷电的计算

采用包含颗粒间相互作用的等效场强代替原外场,利用点偶极子模型计算出高占空比颗粒表面的最大场强,并与有限元法的计算结果进行比较,结果表明,对于具有不同介电常数的颗粒来说,在颗粒占空比较高时,这种方法仍具有足够的精度.根据颗粒场致荷电的机理,进一步推导出计算高占空比颗粒电场饱和荷电量和随时间变化的荷电量的近似公式.结果表明,颗粒电场荷电量不仅随场强、颗粒粒径和介电常数的增大而增大,还随颗粒占空比的增大而增大;颗粒电场荷电的速率与颗粒占空比相关,占空比越大,荷电速率越大.     

静电除尘器内的流场分布

对静电除尘器中电晕放电所产生的二次流和诱导紊流进行了理论分析。在不考虑粒子电荷密度情况下,导出电场场强分布函数。并在非均匀电场强度下,把电晕放电所产生的电体积力加入紊流流场的各流体力学力中,导出紊流流场的流速分布函数。经计算机模拟计算,所得到的电晕放电对流场分布的影响与 Leonard 的实测结果一致。     

交变电场下向列相液晶的非线性光学增强效应

本文研究了向列相液晶在交变电场作用下引起的Fredericks转变,测量了交变电场和直流电场作用下5CB和MBBA的阈值电场。文中还论述了MBBA在交变电场(450Hz)作用下产生的非线性衍射束的增强效应。     

浅论平行板电场对酒精灯火焰的影响

实验研究了各种极板距离以及电压下平板电场对酒精灯火焰的运动取向特性。结果表明,平板电场放电前,火焰中带电粒子受电场力作用,其偏移随电场增大而趋于明显;放电后,板间形成离子风,诱导周围气流流动,火焰被很大程度拉伸。实验结果为掌握电场对液体燃料扩散火焰的影响规律,用适当的电场来控制燃烧走向提供了有益的参考。     

室内实验规模的出砂数学模拟

为了研究疏松砂岩的出砂特征,根据连续性孔隙介质渗流、微粒释放和微粒运移理论,建立了室内实验规模的出砂数学模型,并采用有限差分法对该模型进行了求解。结果表明,实验条件下数学模拟与物理实验模拟的结果吻合较好。用理论出砂模型模拟了胶体力、水动力导致的出砂和砂粒运移过程中滞留引起的渗透率变化。模拟结果表明,渗流速度高于临界流速才引起冲刷出砂;胶体力和冲刷力是出砂的诱发因素,砂微粒在运移过程中在孔隙表面再沉积和孔喉处被捕集将降低出砂程度;渗透率比值与注水孔隙体积倍数或渗流速度的关系曲线均呈"S"形;在运移过程中,砂微粒滞留导致渗透率比值随岩心长度的变化呈线性降低趋势。     

正脉冲电晕放电的数值研究

根据等离子体的流体方程提出了脉冲电晕放电中流注形成和传播的数学模型，导出了流注空间电荷电场的普遍积分表达式．用Ｆｌｕｘ－ＣｏｒｒｅｃｔｅｄＴｒａｎｓｐｏｒｔ（ＦＣＴ）算法，就实验中所用的超窄脉冲电压波形对该模型进行了数值求解，得到了带电粒子密度、电场和电子温度等参量沿流注发展轴线上的时空分布     

地球外太空的奥秘:一个巨大的动力辐射区域

2010年,作者在地球外太空发现一个以磁口(cusp)为中心的巨大的动力辐射区域.这个新辐射区域纵深可达10.5Re;在7-8Re高度上,其尺度在纬线和经线方向上可分别达到6Re和＞10Re;当人造卫星穿越该区域时,测得的电磁涨落强度与高能带电粒子强度都有数量级的增加.本文对此进行了综述分析,认为这是太空时代最关键和最...