Flow behavior in microchannel made of different materials with wall slip velocity and electro-viscous effects

In a microfluidic system, flow slip velocity on a solid wall can be the same order of magnitude as the average velocity in a microchannel. The flow-electricity interaction in a complex microfluidic system subjected to joint action of wall slip and electro-viscous effect is an important topic. This paper presents an analytic solution of pressuredriven liquid flow velocity and flow-induced electric field in a two-dimensional microchannel made of different materials with wall slip and electro-viscous effects. The Poisson- Boltzmann equation and the Navier-Stokes equation are solved for the analytic solutions. The analytic solutions agree well with the numerical solutions. It was found that the wall slip amplifies the fow-induced electric field and enhances the electro-viscous effect on flow. Thus the electro-viscous effect can be significant in a relatively wide microchannel with relatively large kh, the ratio of channel width to thickness of electric double layer, in comparison with the channel without wall slip.

Periodical streaming potential and electro-viscous effects in microchannel flow

This paper presents an analytical solution to periodical streaming potential, flow-induced electric field and velocity of periodical pressure-driven flows in twodimensional uniform microchannel based on the Poisson-Boltzmann equations for electric double layer and Navier-Stokes equation for liquid flow. Dimensional analysis indicates that electric-viscous force depends on three factors： （1） Electric-viscous number representing a ratio between maximum of electric-viscous force and pressure gradient in a steady state, （2） profile function describing the distribution profile of electro-viscous force in channel section, and （3） coupling coefficient reflecting behavior of arnplitude damping and phase offset of electro-viscous force. Analytical results indicate that flow-induced electric field and flow velocity depend on frequency Reynolds number （Re = wh^2/v）. Flow-induced electric field varies very slowly with Re when Re 〈 1, and rapidly decreases when Re 〉 1. Electro-viscous effect on flow-induced electric field and flow velocity are very significant when the rate of the channel width to the thickness of electric double layer is small.

Fluid flow driven along microchannel by its upper stretching wall with electrokinetic effects

We develop a mathematical model to describe the flow in a microchannel driven by the upper stretching wall of the channel in the presence of electrokinetic effects. In this model, we avoid imposing any unphysical boundary condition, for instance, the zero electrostatic potential in the middle of the channel. Using the similarity transformation, we employ the homotopy analysis method （HAM） to get the analytical solution of the model. In our approach, the unknown pressure constant and the integral constant related to the electric potential are solved spontaneously by using the proper boundary conditions on the channel walls, which makes our model consistent with the commonly accepted models in the field of fluid mechanics. It is expected that our model can offer a general and proper way to study the flow phenomena in microchannels.

PERIODICAL PRESSURE-DRIVEN FLOWS IN MICROCHANNEL WITH WALL SLIP VELOCITY AND ELECTRO-VISCOUS EFFECTS

In a microfluidic system, the flow slip velocity on a solid wall can be the same order of magnitude as the average velocity in the microchannel. The flow-electricity interaction in a complex microfluidic system subjected to a joint action of wall slip and electro-viscosity is an important topic. An analytical solution for the periodical pressure-driven flow in a two-dimensional uniform microchannel, with consideration of wall slip and electro-viscous effect is obtained based on the Poisson-Boltzmann equation for the Electric Double Layer （EDL） and the Navier-Stokes equations for the liquid flow. The analytic solutions agree well with the numerical solutions. The analytical results indicate that the periodical flow velocity and the Flow-Induced Electric Field （FIEF） strongly depend on the frequency Reynolds number （Re = （wh2/v ）, that is a function of the frequency, the channel size and the kinetic viscosity of fluids. For Re 〈 1, the flow velocity and the FIEF behave similarly to those in a steady flow, whereas they decrease rapidly with Re as Re 〉 1. In addition, the electro-viscous effect greatly influences the periodical flow velocity and the FIEF, particularly, when the electrokinetic radius kH is small. Furthermore, the wall slip velocity amplifies the FIEF and enhances the electro-viscous effect on the flow.

聚乙二醇-十二烷基硫酸钠水溶液的黏度特性研究

采用乌氏黏度计测定了阴离子表面活性剂十二烷基硫酸钠(SDS)与非离子聚合物聚乙二醇(PEG)间不同程度缔合后的黏度行为。以基于SDS的比浓黏度变化获得PEG-SDS的双临界浓度,以基于PEG的比浓黏度变化反映其在较低缔合物浓度时呈现聚电解质的电黏效应以及盐屏蔽效应,并发现了PEG的链扩张现象。与PVP-SDS体系类似,上述现象归结为带电的SDS束缚胶束缔合于PEG分子链上形成了"拟聚电解质"。

微通道周期流动电位势及电粘性效应

求解了双电层的Poisson-Boltzmann方程和流体运动的Navier-Stokes方程,得到在周期压差作用下,二维微通道的周期流动电位势,流动诱导电场和液体流动速度的解析解.量纲分析表明,流体电粘性力与以下3个参数有关:1)电粘性数,它表示定常流动时,通道最大电粘性力与压力梯度的比;2)形状函数,它表示电粘性力在通道横截面的分布形态;3)耦合系数,它表示电粘性力的振幅衰减特征和相位差.分析结果表明,微通道周期流动诱导电场、流动速度与频率Reynolds数有关.在频率Reynolds数小于1时,流动诱导电场随频率Reynolds数变化很慢.在频率Reynolds数大于1时,流动诱导电场随频率Reynolds数的增加快速衰减.在通道宽度与双电层厚度比值较小情况下,电粘性效应对周期流动速度和流动诱导电场有重要影响.

蒙脱土悬浮液体系流变性研究

研究了Na-蒙脱土和Ca-蒙脱土在不同含量下形成悬浮液体系的流变特性。试验结果表明,Na Mt和Ca Mt悬浮液体系的流变模型遵循幂律方程,且Na Mt悬浮液体系比Ca Mt具有良好的结构粘度和触变性结构。电粘作用是带电悬浮液体系具有非牛顿流体特性的一个主要因素,且悬浮体系的流变性与界面电导率和热动力学有关。蒙脱土悬浮液体系的粘性变化、结构触变性以及流变模型的建立,为评价、预测和改变涂料工艺性能提供试验依据。

微扩散通道中的流动电位势和电黏性效应分析

采用电解质溶液离子输运的Nernst-Planck方程、液体运动的Navier-Stokes方程和电场的Poisson方程研究了微扩散管的双电层、电解质流动电位势和电黏性效应。采用有限体积法分析了微扩散管流动电位势、流动电阻力、流量损失和流动速度形态变化。结果表明,与均匀截面微通道不同的是,流动电位势在微扩散管内呈非线性增长,流动电阻力沿微通道截面扩张方向下降。在微扩散管的横截面上也会产生流动电位势和电阻力。在微收缩管流动的电黏性效应和流量损失率比微扩散管流动略大。论文数值解给出流动电位势从瞬态到稳态发展过程的时间尺度特征,并分析了微通道扩散角对电黏性效应产生的微通道流量损失率。

粘度法研究SDS与PEO之间的相互作用

采用粘度法测定了不同温度下 ( 2 98 1 5K ,30 8 1 5K ,31 8 1 5K)阴离子表面活性剂十二烷基硫酸钠 (SDS)—水溶性聚合物聚氧乙烯 (PEO)体系在有无电解质 (NaOH)存在时溶液的宏观粘度 ,研究了SDS—PEO体系分子间相互作用对PEO构型的影响 ,得到了不同SDS—PEO体系的临界聚集浓度 (CAC)和聚合物饱和浓度 (PSP) ,并根据表面活性剂—聚合物体系相互作用模型计算了SDS胶束被吸附到聚合物PEO过程的吉布斯函数变 ,讨论了SDS—PEO体系分子间相互作用强度 结果表明 ,在所研究范围内 ,SDS—PEO体系有无电解质溶液的比浓粘度均随聚合物PEO浓度的减小而急剧升高 ,呈现典型的聚合电解质行为 ,而且SDS的浓度越大 ,电粘效应愈明显 由此可以推断 ,聚合物PEO与表面活性剂SDS之间主要是通过疏水键相互结合 ,PEO分子链处于更伸展的状态 NaOH的存在使聚合物PEO分子与胶束结合位置减少 ,聚集体的体积减小 ,或者介电常数增大 ,因此表现出电粘效应弱于不含碱的表面活性剂—聚合物体系 。