Analysis of electrokinetic pumping efficiency in capillary tubes

A mathematical model for predicting the maximum pumping efficiency and pressure difference generation by an electrokinetic-driven fluid pumping system through a capillary tube is presented in this study. Both the maximum pumping efficiency and optimum pressure difference generation are found to depend on a single variable. This single variable is termed as the figure of merit since it determines the performance of electrokinetic pumping. The figure of merit is found to depend on three dimensionless parameters, the normalized Debye length, zeta potential, and Levine number indicating the nominal ratio of convective current to conductive current. All three parameters can be related to the pH value and concentration of aqueous salt solution by the introduction of concentration-dependent electrical conductivity and pH-dependent zeta potential. By presenting the maximum pumping efficiency and optimum pressure difference generation as functions of pH value, salt concentration, and capillary tube radius, it is found that both maximum pumping efficiency and optimum pressure difference generation increase with the decrease in capillary radius and salt concentration. The optimum pH values at which the maximum pumping efficiency and optimum pressure difference generation occur are found to be in the range between 6 and 9. For the salt concentration of 10(-6) M, pH 6.9, and a capillary tube radius value of 0.5 micro m, the predicted maximum pumping efficiency is 5.4% which is close to the experimental measurement reported in the literature.     

Brownian pump in nonlinear diffusive media

A Brownian pump in nonlinear diffusive media is investigated in the presence of an unbiased external force. The pumping system is embedded in a finite region and bounded by two particle reservoirs. In the adiabatic limit, we obtain the analytical expressions of the current and the pumping capacity as a function of temperature for normal diffusion, subdiffusion, and superdiffusion. It is found that important anomalies are detected in comparison with the normal diffusion case. The superdiffusive regime, compared with the normal one, exhibits an opposite current for low temperatures. In the subdiffusive regime, the current may become forbidden for low temperatures and negative for high temperatures.     

Mechanism of hydrodynamic separation of biological objects in microchannel devices

In this study, the separation mechanism employed in hydrodynamic chromatography in microchannel devices is analyzed. The main purpose of this work is to provide a methodology to develop a predictive model for hydrodynamic chromatography for biological macromolecules in microchannels and to assess the importance of various phenomenological coefficients. A theoretical model for the hydrodynamic chromatography of particles in a microchannel is investigated herein. A fully developed concentration profile for non-reactive particles in a microchannel was obtained to elucidate the hydrodynamic chromatography of these particles. The external forces acting on the particles considered in this model include the van der Waals attractive force, double-layer force as well as the gravitational force. The surface forces, such as van der Waals attractive force as well as the double-layer repulsive force, can either enhance or hinder the average velocity of the macromolecular particles. The average velocity of the particles decreases with the molecular radius because the van der Waals attractive force increases the concentration of the particles near the channel surface, which is the low-velocity region. The transport velocity of the particles is dominated by the gravity and the higher density enlarges the effect caused by gravity.     

Unique viscoelastic behaviors of colloidal nanocrystalline cellulose aqueous suspensions

Unique rheological and phase behaviors of rod-like nanocrystalline cellulose (NCC) suspensions in aqueous media are revealed in the present article. Specifically, the NCC aqueous suspension remained isotropic in a wide NCC concentration range in which the suspension underwent transition from dilute solution to gel, and the relative viscosity of the NCC suspension could be well fitted by the Sato-Teramoto theory in the full concentration range tested. Correspondingly, both zero-shear viscosity and complex viscosity increased monotonically with NCC concentration, and no maximum value was observed along the curves of zero-shear viscosity or complex viscosity versus NCC concentration, indicating a deviation from the lyotropic system. However, a shear-induced birefringence phenomenon was observed, indicating the NCC suspension formed a temporary ordered structure in the external force field but was unable to form an anisotropic (liquid crystalline) phase. The Cox-Merz rule was not applicable for the NCC suspension as a result of oriented domains, i.e., rod-like NCC particles. Moreover, time-concentration superposition was successfully applied to both the storage and loss modulus, attributed to the isotropic feature of the NCC suspension in the tested concentration range. The reason why this NCC suspension remained isotropic could be because of the strong electrostatic repulsions between NCC particles and the weak tendency or driving force of anisotropy formation as a result of the small aspect ratio of NCC particles, Na⁺ counterions and large amounts of negative charges along the NCC particles. The results suggested that not all the rod-like particles were able to form an anisotropic phase in aqueous suspension, but dominated by various factors.     

Adsorption and order formation of colloidal nanoparticles on a substrate: a Brownian dynamics study

Adsorption process and order formation of electrostatically stabilized colloidal particles with a radius of 50 nm onto a planar surface with countercharge are examined. We perform Brownian dynamics simulations with a new three-dimensional cell model, in which the particle-particle and particle-substrate interactions are modeled based on the DLVO theory. The simulations yield the following results: (1) a larger bulk concentration would be required for larger kappaa to reach order formation to compensate for the decrease in the bulk potential; (2) the phase transition from a disordered to an ordered structure of the adsorbed particles on the substrate is considered to be of the Kirkwood-Alder type of transition through the examination of the two-dimensional pressure of the adsorbed particles; (3) the adsorbed particles are found to form a hexagonally ordered array, only if what we call "one-directional average force" acting on an adsorbed particle exceeds a critical value, which is independent of the ionic strength, or the interaction potentials. The critical value of the one-directional average force is interpreted as the force needed to keep an ordered structure by localizing adsorbed particles at fixed positions. In addition, the critical force is used to develop a new model to estimate the surface coverage at the order-disorder transition and it is demonstrated that the new model gives better estimation than other models previously reported.     

Effect of Surfactants on the Particle Capture Mechanism in an Electromagnetic Filter

In this article, effects of the parameters of the filtration process and surfactants on the effectiveness of the separation of particles have been investigated. Water samples containing corrosion particles (rust) of low concentrations were mixed with a typical domestic type of detergent passed through an electromagnetic filter. It was found that the capturing capacity of the particles in the filter increased as the external magnetic field, filter length and the detergent concentration of the suspension increase, but the increase in diameter of the filter elements gave an opposite effect. Furthermore, it was recorded that the pH value of the suspension changes the efficiency of magnetic filter.     

Steady flow generation in microcirculatory systems

In this paper we explore the mechanical generation of steady-non pulsatile-flow in microfluidic systems. The rationale of the paper is inspired in the example of cardiovascular systems where at the microscale (i.e. capillaries) the flow is steady rather than pulsatile to optimize performance. We present a solution to the generation of steady flow in engineered microfluidic systems either in open or closed loop configurations via the use of disc pumps. The disc pump consists of a flat rotating disc and utilizes both viscous drag and centrifugal force to achieve pumping. Experiments using single loop and double loop microfluidic systems are presented to characterize the disc pump. Continuous flow generated by the disc pumps can be used to separate particles based on size using recirculating loops and for extraction of small particles without disturbing the concentration of bigger particles. The potential impact of this technology includes sample separation and extraction techniques into portable microfluidic labs-on-a-chip, and long term culture systems for cells in suspension.     

石墨烯负载零价纳米铁材料的合成及去除水中Cr(VI)的研究

以石墨粉为原料,采用改良Hummers方法合成石墨烯,然后通过液相还原法制备出石墨烯负载纳米铁材料(Graphene-supported nanoscale zero-valent iron,G-nZVI),借助扫描电子显微镜(SEM)、红外光谱分析仪(FTIR)进行表征,并以G-nZVI为反应材料,研究其对水体中Cr(VI)的去除效率,结果显示:室温下,当G-nZVI投加量为0.4g/L,Cr(VI)的初始浓度为20 mg/L,初始pH值为3.0时,Cr(VI)的去除率在2h内可以达到95%以上。G-nZVI具有磁性,使用后可通过外加磁力除去,以防对水体的二次污染,具有较好的应用前景。     

Electrohydrodynamic Velocity and Pumping Measurements in Water and Alcohols

Bubble and particle velocities in water and alcohols, under the influence of an electric field, were investigated in this work. Air bubbles were injected into the liquids through an electrified metal capillary insulated by glass with its tip left exposed. The end of the capillary from which the bubbles were released was conical in shape. Due to an electric field formed between the noninsulated capillary tip and a ground electrode immersed in the solvent, small bubbles were formed and used as tracers for the electrohydrodynamic (EHD) flow field. The pressure inside the capillary was measured for all liquids used in this study. For water, ethanol, and n-propanol, it was found that, at relatively low applied voltage, the pressure increases with voltage, reaches a maximum (pressure breakpoint), and then sharply decreases. This behavior is a result of the competition between the electric force appearing at the interface and the force due to the EHD flow near the capillary tip. The electric force tends to increase the pressure inside the capillary, while the EHD flow tends to decrease this pressure. For isopropanol and butanol, the pressure breakpoint was not observed in the range of voltage applied in the experiments. The EHD flow velocity was measured by using microbubbles and particles as flow tracers. An adaptive phase-Doppler velocimeter was employed to measure the velocity of bubbles, while the velocity of particles was measured by trajectory visualization of fluorescent particles. A discrepancy was observed between the two methods because of the location at which the measurements were made. It was found that average velocities of both bubbles and particles increase linearly with applied voltage. Experiments were also conducted to investigate pumping of water, which is a result of the EHD velocity near the capillary tip. The pumping flow rate was linearly related to the applied voltage and agreed well with EHD velocity measurements obtained from particle trajectories. Copyright 2000 Academic Press.     

Mesoscale constitutive modeling of magnetic dispersions

A constitutive model for dispersions of acicular magnetic particles has been developed by modeling the particles as rigid dumbbells dispersed in a solvent. The effects of Brownian motion, anisotropic hydrodynamic drag, a steric force in the form of the Maier-Saupe potential, and, most importantly, a mean-field magnetic potential are included in the model. The development is similar to previous models for liquid-crystalline polymers. The model predicts multiple orientational states for the dispersion, and this phase behavior is described in terms of an orientational order parameter S and an average alignment parameter J; the latter is introduced because the magnetic particles have distinguishable direction due to polarity. A transition from isotropic to nematic phases at equilibrium is predicted. Multiple nematic phases-both prolate and oblate-are predicted in the presence of steady shear flow and external magnetic field parallel to the flow. The effect of increasing magnetic interparticle interactions and particle concentration is also presented. Comparisons with experimental data for the steady shear viscosity show very good agreement.     

重力场流分离系统中载液及流速条件对分离的影响

重力场流分离作为最简单的一种场流分离技术,常用于分离微米级颗粒。选择两种不同粒径(20 μ m和6 μ m)的聚苯乙烯(PS)颗粒作为样品,通过改变载液中叠氮化钠浓度、混合表面活性剂的比例及载液流速,利用自行设计生产的重力场流分离(gravitational flow field-flow fractionation, GrFFF)仪器,对颗粒混合样品进行分离,得到了相关谱图与数据,考察了这3种因素对分离效果(保留比(R)、塔板高度(H))的影响。结果表明:20 μ m PS颗粒的R值均大于6 μ m PS颗粒的R值,H值均小于6 μ m颗粒的H值;PS颗粒的R值与H值均随着载液中叠氮化钠浓度的增加而增加;但随着载液流速的增加,R值增加,H值减小。该研究为GrFFF系统的开发及应用提供了重要的参考价值。     

Microflotation Suppression and Enhancement Caused by Particle/Bubble Electrostatic Interaction

The processes of attachment and detachment of small or medium-sized particles to relatively large bubbles during microflotation are considered in terms of the heterocoagulation theory. Calculations are made for the conditions that the surface potentials are of similar sign and constant, that one of the surface potentials is small, that hydrophobic attraction is absent, and that there are no surface deformations. Under these conditions bubble-particle aggregates may form as a result of an electrostatic attraction which exceeds the repulsive van der Waals force at intermediate distances. Next to electrostatic and van der Waals forces, hydrodynamic and gravitational forces are considered. These forces may overcome the electrostatic repulsion at large distances and promote particle bubble attachment. Strong electrostatic attraction at small distances, arising at a large difference of the surface potentials of the bubble and the particle and of low electrolyte concentrations, can prevent subsequent detachment by hydrodynamic and gravitational forces. With increasing electrolyte concentration the electrostatic barrier increases and the attractive electrostatic force diminishes. As a result, a critical electrolyte concentration for microflotation exists. Above this concentration attachment may still occur but it is followed by detachment. At lower electrolyte concentrations the electrostatic attractive force prevents the detachment. The dependence of the critical electrolyte concentration on the values of the bubble and particle potentials and the Hamaker constant is calculated. The critical concentration does not depend on particle or bubble size if the absolute values of the total detachment force and the total pressing force coincide, which is the case for Stokes and potential flow. For every electrolyte concentration lower than the critical value there are two critical particle sizes that limit the flotation possibility. For small particle sizes attachment is impossible because the pressing force is smaller than the electrostatic barrier. For large particle sizes detachment cannot be prevented because the detachment force exceeds the maximum electrostatic attraction. A microflotation domain of intermediate particle sizes exists in which irreversible heterocoagulation occurs. Copyright 2001 Academic Press.     

Electrodipping force acting on solid particles at a fluid interface

We report experimental results which show that the interfacial deformation around glass particles (radius, 200-300 microm) at an oil-water (or air-water) interface is dominated by an electric force, rather than by gravity. It turns out that this force, called for brevity "electrodipping," is independent of the electrolyte concentration in the water phase. The force is greater for oil-water than for air-water interfaces. Under our experimental conditions, it is due to charges at the particle-oil (instead of particle-water) boundary. The derived theoretical expressions, and the experiment, indicate that this electric force pushes the particles into water. To compute exactly the electric stresses, we solved numerically the electrostatic boundary problem, which reduces to a set of differential equations. Convenient analytical expressions are also derived. Both the experimental and the calculated meniscus profile, which are in excellent agreement, exhibit a logarithmic dependence at long distances. This gives rise to a long-range electric-field-induced capillary attraction between the particles, detected by other authors. Deviation from the logarithmic dependence is observed at short distances from the particle surface due to the electric pressure difference across the meniscus. The latter effect gives rise to an additional short-range contribution to the capillary interaction between two floating particles. The above conclusions are valid for either planar or spherical fluid interfaces, including emulsion drops. The electrodipping force, and the related long-range capillary attraction, can engender two-dimensional aggregation and self-assembly of colloidal particles. These effects could have implications for colloid science and the development of new materials.     

Force-dependent mobility and entropic rectification in tubes of periodically varying geometry

We investigate transport of point Brownian particles in a tube formed by identical periodic compartments of varying diameter, focusing on the effects due to the compartment asymmetry. The paper contains two parts. First, we study the force-dependent mobility of the particle. The mobility is a symmetric non-monotonic function of the driving force, F, when the compartment is symmetric. Compartment asymmetry gives rise to an asymmetric force-dependent mobility, which remains non-monotonic when the compartment asymmetry is not too high. The F-dependence of the mobility becomes monotonic in tubes formed by highly asymmetric compartments. The transition of the F-dependence of the mobility from non-monotonic to monotonic behavior results in important consequences for the particle motion under the action of a time-periodic force with zero mean, which are discussed in the second part of the paper: In a tube formed by moderately asymmetric compartments, the particle under the action of such a force moves with an effective drift velocity that vanishes at small and large values of the force amplitude having a maximum in between. In a tube formed by highly asymmetric compartments, the effective drift velocity monotonically increases with the amplitude of the driving force and becomes unboundedly large as the amplitude tends to infinity.     

接枝微粒PMAA/SiO2在水介质中对杀虫剂抗蚜威的吸附机理

以硅胶表面接枝聚甲基丙烯酸(PMAA)的复合型功能微粒PMAA/SiO2为固体吸附剂, 对水介质中的抗蚜威进行了静态吸附实验, 通过考察温度、pH值及盐度(NaCl浓度)对吸附容量的影响, 重点研究了PMAA/SiO2对抗蚜威的吸附机理. 为确认所提出的机理的正确性, 还采用紫外光谱吸收法, 研究了单体甲基丙烯酸与抗蚜威之间的相互作用, 也考察了在非水介质CCl4中PMAA/SiO2对抗蚜威的吸附作用. 研究结果表明, PMAA/SiO2对抗蚜威具有强的吸附作用, 吸附的驱动力是氢键、静电以及疏水相互作用三种作用的协同, 其中主驱动力是静电相互作用. 温度升高, 吸附容量减小; 盐度增大, 吸附容量降低; 当pH<8时, 吸附容量随pH升高而增大; 当pH>8时, 吸附容量随pH升高而减小; 当pH=8 时,吸附容量最大.     

An improved hybrid continuum‐atomistic four‐way coupled model for electrokinetics in nanofluidics

In this study, an efficient hybrid continuum‐atomistic method is proposed to study electrokinetic transport of aqueous solutions in nanofluidics. The aqueous phase is considered as a continuous phase containing immersed ion particles. The behavior of the system is then simulated through utilization of an improved hybrid continuum‐atomistic four‐way coupled approach, including the MultiPhase Particle‐In‐Cell method for the short‐ranged interaction between the ion particles, the Brownian force for the collision between the aqueous phase molecules and the ion particles, and a wall force accounting for the short‐ranged interaction of ions and walls. The validation of the proposed model with the results of Molecular Dynamics simulations suggests that this model can be a promising approach for studying the electrokinetic phenomena in more complicated geometries where the Molecular Dynamics approach is computationally prohibitive. Finally, the effects of electrokinetic parameters, such as the height of the channel, the external electric field, and bulk ionic concentration, on the electroosmotic flow in a nanochannel are investigated and discussed.     

Partition-induced vector chromatography in microfluidic devices

We investigate by means of macrotransport theory the transport of Brownian particles in a slit geometry in the presence of an arbitrary two-dimensional periodic energy landscape and driven by an external force or convected by a flow field. We obtained analytical expressions for the probability distribution and the average migration angle of the particles under the Fick-Jacobs approximation. The migration angle is shown to differ from the angle of the driving field and to strongly depend on the physical properties of the suspended species, thus providing the basis for vector chromatography, in which different species move in different directions and can be continuously fractionated. The potential of microfluidic devices as a platform for partition-induced vector chromatography is demonstrated by considering the particular case of a piece-wise constant, periodic potential that, in equilibrium, induces the spontaneous partition of different species into high and low concentration stripes, and which can be easily fabricated by patterning physically or chemically one of the surfaces of a channel. We show the feasibility to fractionate a mixture of particles for systems in which partition is induced via 1-g gravity and Van der Waals interactions in physically or chemically patterned channels.     

α-Al2O3与Co-Ni合金电化学共沉积动力学模型

在经典复合电沉积机理基础上,考虑到粒子与电极表面之间的多种作用力,以吸附强度来表征粒子与电极表面的作用力大小,根据粒子在电极表面的临界吸附强度,把粒子的吸附分为有效吸附和非有效吸附.当吸附强度大于临界吸附强度时,粒子能被有效吸附嵌入到沉积层中,粒子被有效吸附的概率和平均吸附强度有关.建立了相应的复合电沉积动力学模型.该模型在α-Al2O3与Co-Ni合金的复合共沉积体系中,在电流密度为1～20 A•dm2范围内得到了验证.通过数学模型和实验结果研究了电流密度对粒子沉积量φc、吸脱附常数K、有效吸附概率P和平均吸附强度的影响规律.     

Heavy (or large) ions in a fluid in an electric field: The diffusion equation exactly following from the Fokker-Planck equation

The problem of the derivation of the diffusion equation exactly following from the Fokker-Planck (or Klein-Kramers) equation for heavy (or large) particles in a fluid in an external force field is solved in the case in which the particles are ions subject to a uniform (but in general time-varying) electric field. It is found that such a diffusion equation maintains memory of the initial ion velocity distribution, unless sufficiently large values of time are considered. In such temporal asymptotic limit, the diffusion equation exactly becomes (i) the Smoluchowski equation when the electric field is constant in time, and (ii) a new equation generalizing the Smoluchowski equation, when the electric field is arbitrarily time varying. Finally, it is shown that the obtained exact (or asymptotic) results make questionable the procedures and the results of approximate theories developed in the past to get a "corrected" Smoluchowski equation when the external force can also be, in general, position dependent.     

Magnetic particle based electrochemical sensing platform for PCR amplicon detection

This work reports a novel electrochemical PCR detection platform using magnetic particles as a separation tool. A redox-active intercalator, anthraquinonemonosulfonic acid (AQMS), was firstly intercalated into biotin labeled PCR amplicons, and the resulting complex was then captured by streptavidin-coated magnetic particles (MPs) to form AQMS–DNA–MP conjugates. Subsequently, these conjugates were attracted to the bottom of the tube and separated from the solution by applying an external magnetic field, resulting in a significant reduction of the concentration of solution AQMS. The concentration changes of solution AQMS, which reflect the presence and quantity of PCR amplicons, were monitored by differential pulse voltammetry (DPV) on a chip electrode. PCR cycle number-dependent as well as the initial template DNA concentration-dependent performances were investigated. This electrochemistry based PCR detection platform is simple, convenient and inexpensive, and may have potential applications for practical sample monitoring.