The measurement of subcritical electroconvection

Under idealized conditions, electroconvection in a nematic liquid crystal sets in at a well-defined threshold of the driving ac voltage. Fluctuating convection rolls of a small amplitude below that threshold have been observed recently. The measurement of the amplitude of these fluctuations is described in detail in this paper. It is based on averaging the light intensity signals using the structure function and a quantitative analysis of the light deflection.     

Alternating Current Electrohydrodynamics Induced Nanoshearing and Fluid Micromixing for Specific Capture of Cancer Cells

We report a new tuneable alternating current (ac) electrohydrodynamics (ac‐EHD) force referred to as “nanoshearing” which involves fluid flow generated within a few nanometers of an electrode surface. This force can be externally tuned via manipulating the applied ac‐EHD field strength. The ability to manipulate ac‐EHD induced forces and concomitant fluid micromixing can enhance fluid transport within the capture domain of the channel (e.g., transport of analytes and hence increase target–sensor interactions). This also provides a new capability to preferentially select strongly bound analytes over nonspecifically bound cells and molecules. To demonstrate the utility and versatility of nanoshearing phenomenon to specifically capture cancer cells, we present proof‐of‐concept data in lysed blood using two microfluidic devices containing a long array of asymmetric planar electrode pairs. Under the optimal experimental conditions, we achieved high capture efficiency (e.g., approximately 90 %; % RSD=2, n=3) with a 10‐fold reduction in nonspecific adsorption of non‐target cells for the detection of whole cells expressing Human Epidermal Growth Factor Receptor 2 (HER2). We believe that our ac‐EHD devices and the use of tuneable nanoshearing phenomenon may find relevance in a wide variety of biological and medical applications.     

Electrically induced flows in the vicinity of a dielectric stripe on a conducting plane

We report a theoretical and experimental study of the hydrodynamic flow induced by an a.c. electric field in the vicinity of a dielectric stripe deposited on a conducting plate. In the theoretical part, we model the stripe as a small change of the surface capacitance of the plate, and a perturbative approach is used to perform the calculations. This approach predicts an outwards rectified electro-osmotic slip along the surface that generates two steady counter-rotating rolls, the size of which decreases with the frequency. In the experimental section, we use tracers to determine the structure of the flow and investigate its dependence on the frequency and the amplitude of the applied voltage. The structure and amplitude of the observed flow compares satisfactorily with the theoretical analysis. This could guide the design of surface-controlled flows and help to understand the collective behavior of colloids near electrodes. Received 20 June 2002 RID="a" ID="a"e-mail: nadal@crpp.u-bordeaux.fr     

Numerical study of homogeneous dynamo based on experimental von Kármán type flows

A numerical study of the magnetic induction equation has been performed on von Kármán type flows. These flows are generated by two co-axial counter-rotating propellers in cylindrical containers. Such devices are currently used in the von Kármán sodium (VKS) experiment designed to study dynamo action in an unconstrained flow. The mean velocity fields have been measured for different configurations and are introduced in a periodic cylindrical kinematic dynamo code. Depending on the driving configuration, on the poloidal to toroidal flow ratio and on the conductivity of boundaries, some flows are observed to sustain growing magnetic fields for magnetic Reynolds numbers accessible to a sodium experiment. The response of the flow to an external magnetic field has also been studied: The results are in excellent agreement with experimental results in the single propeller case but can differ in the two propellers case.     

Flexoelectric origin of oblique rolls with helical flow in electroconvective nematics under DC excitation

We present a 3-dimensional, linear analysis of electrohydrodynamic (EHD) roll instability in a nematic liquid crystal under DC excitation. It is shown that the flexoelectric effect leads to a new symmetry of the flow pattern, viz. ahelical flow inoblique rolls. Our experimental observations agree with the theoretical predictions.     

电场作用下离散相行为研究

为明晰电场对两相系统中离散相的作用,本文针对均匀电场作用下两相系统中的单个离散相的行为进行了可视化试验研究及数值模拟。通过建立电场数学模型。得到了均匀电场作用下两相系统的电场分布;运用电应力表面积分的方法求得了离散相所受的电场力。并于试验中观察了不同粒径的离散相在电场作用下的运动情况,其试验结果与数值计算结果基本吻合。     

EHD两相系统中的电场数值模拟

针对均匀电场作用下两相系统中的单个离散相周围电场分布情况,建立了数学模型.在建模过程中,考虑了离散相的存在对电场分布的影响.通过求解电场控制方程,得到了均匀电场作用下离散相周围及其内部的电势及电场分布的数值解,并理论验证了该数值解的准确性.     

阿姆斯特朗电解液液桥的实验现象及断裂机理研究

阿姆斯特朗液桥是电流体力学领域中的一种有趣现象,它悬在空中抵抗重力流动的神奇状态引起了人们的广泛关注.近十几年来,去离子水液桥已经通过理论和实验得到深入研究,但是对于电解液液桥的研究依然十分有限.本文以Na2SO4电解液作为形成液桥的实验液体,利用高速摄相机和红外热像仪研究了电解液液桥的形成过程、焦耳效应、流动中的热气泡产生及其破碎导致的液桥断裂等现象,提出了不同于去离子水液桥的断裂机理,有助于进一步加深人们对液桥这一复杂现象的理解.     

Long-wavelength limit of the EMHD tearing mode

Summary The frequency spectrum of the modes that lead to fast reconnection of magnetic-field lines in a plasma configuration with sharp current density gradients is obtained and compared to that of the more familiar magnetohydrodynamic tearing modes.