Simulation and analysis of particle trajectory caused by the optical-induced dielectrophoresis force

Optical-induced dielectrophoresis (ODEP) is a novel technology used in the field of micro-/nanoparticles manipulation. The finite element method was applied for ODEP to research the particles motion in this paper. The potential distribution in the optoelectronic chip, which was induced by the incident light spot, was attained through electric current module in the COMSOL 4.3a. The particles motion was studied by coupling the module of electric current and particle tracing for fluid flow. Compared with molecular dynamics, the method proposed in the paper could effectively simplify the tedious programming. The polystyrene sphere (PS) particles with the radius of 2, 5, 10, and 15 μm were, respectively, used as the objects. The kinetic energy of the PS created by the dielectrophoresis (DEP) forces, the Stokes drag forces, the gravity forces, and the Brownian motion forces was calculated during the whole manipulation process. The simulation results indicated that with the decreasing in the particle size, the time on enrichment of the smaller PS would become longer. It was because that for the smaller PS, the effect of DEP forces would play less important role in the system. The conclusions in this paper could be used as a theoretical guidance in the further research.     

Study on the assembly and separation of biological cell by optically induced dielectrophoretic technology

Optically induced dielectrophoretic (ODEP) chip is to combine their own advantages of optical tweezers and electrodynamics manipulation technologies, which can trap single particles in high resolution as well as enrich much of micro-/nanoparticles in high throughput. The paper analyzed the structure of optoelectronic tweezers (OET) chip, moreover, the frequency response of multi-membrane eukaryotic cells about 10³–10⁹ Hz. The Clausius–Mositti (CM) frequency factor in terms of cell membrane, cell cytoplasm, nuclear envelope thickness changes, and volume ratio was illustrated. In the end, the paper presented 3D numeric model of cells in OET chip. The dielectrophoresis force acting on the dipole of 11.8-μm cells subjected to a non-uniform electric field under 60-μm Gaussian-distributed beam spot could be simulated in the enrichment process. The separation of cells that were two different types of CM values was calculated. Furthermore, it was proved to be feasible to achieve the efficient separation of cells using ODEP technology in the biological numerical model. Comparing with the literature of experiment, the results in cell dielectric spectroscopy and numeric model findings were in general agreement. The simplified structure and numeric model of nucleated cell provide a theoretical basis for research of biosensor and complex life.     

Lagrangian particle model for multiphase flows

A Lagrangian particle model for multiphase multicomponent fluid flow, based on smoothed particle hydrodynamics (SPH), was developed and used to simulate the flow of an emulsion consisting of bubbles of a non-wetting liquid surrounded by a wetting liquid. In SPH simulations, fluids are represented by sets of particles that are used as discretization points to solve the Navier-Stokes fluid dynamics equations. In the multiphase multicomponent SPH model, a modified van der Waals equation of state is used to close the system of flow equations. The combination of the momentum conservation equation with the van der Waals equation of state results in a particle equation of motion in which the total force acting on each particle consists of many-body repulsive and viscous forces, two-body (particle-particle) attractive forces, and body forces such as gravitational forces. Similar to molecular dynamics, for a given fluid component the combination of repulsive and attractive forces causes phase separation. The surface tension at liquid-liquid interfaces is imposed through component dependent attractive forces. The wetting behavior of the fluids is controlled by phase dependent attractive interactions between the fluid particles and stationary particles that represent the solid phase. The dynamics of fluids away from the interface is governed by purely hydrodynamic forces. Comparison with analytical solutions for static conditions and relatively simple flows demonstrates the accuracy of the SPH model.     

Separation of particles using acoustic streaming and radiation forces in an open microfluidic channel

In this study, a method to separate particles, within a small sample, based on size is demonstrated using ultrasonic actuation. This is achieved in a fluid, which has been deposited on a flat surface and is contained by a channel, such that it has a rectangular wetted area. The system utilises acoustic radiation forces (ARFs) and acoustic streaming. The force field generates two types of stable collection locations, a lower one within the liquid suspension medium and an upper one at the liquid–air interface. Acoustic streaming selectively delivers smaller particles from the lower locations to the upper ones. Experimental data demonstrate the ability to separate two sets of polystyrene microparticles, with diameters of 3 and 10 μm, into different stable locations. Methods to reduce migration of larger particles to the free surface are also investigated, thereby maximising the efficiency of the separation. Extraction of one set of 99 % pure particles at the liquid–air interface from the initial particle mixture using a manual pipette is demonstrated here. In addition, computational modelling performed suggests the critical separation size can be tuned by scaling the size of the system to alter which of ARFs and acoustic streaming-induced drag forces is dominant for given particle sizes, therefore presenting an approach to tunable particle separation system based on size.     

Electrokinetic transport and separation of droplets in a microchannel

This work presents theoretical, numerical and experimental investigations of electrokinetic transport and separation of droplets in a microchannel. A theoretical model is used to predict that, in case of micron-sized droplets transported by electro-osmotic flow, the drag force is dominant as compared to the dielectrophoretic force. Numerical simulations were performed to capture the transient electrokinetic motion of the droplets using a two-dimensional multi-physics model. The numerical model employs Navier–Stokes equations for the fluid flow and Laplace equation for the electric potential in an Arbitrary Lagrangian–Eulerian framework. A microfluidic chip was fabricated using micromilling followed by solvent-assisted bonding. Experiments were performed with oil-in-water droplets produced using a cross-junction structure and applying electric fields using two cylindrical electrodes located at both ends of a straight microchannel. Droplets of different sizes were produced by controlling the relative flow rates of the discrete and continuous phases and separated along the channel due to the competition between the hydrodynamic and electrical forces. The numerical predictions of the particle transport are in quantitative agreement with the experimental results. The work reported here can be useful for separation and probing of individual biological cells for lab-on-chip applications.     

Development of a stochastic wave force function on ocean-structures

The wave forces exerted on structures are of vital importance in the design of marine structures.

Circular piles are frequently used to provide most or all of the support for such structures. The forces exerted by waves of similar properties vary greatly. These variations are the result of fluctuations in fluid particle velocities and accelerations as well as by eddies and turbulence around piles caused by rapidly reversing flow. Consequently, a deterministic approach to wave force prediction appears to be impossible. As an alternative, a probabilistic approach was developed in this paper.

The method applied here utilizes a multiple linear regression analysis to develop a relationship between wave parameters which are relatively easy to observe and the parameters used in the Morison force equation. They include the velocity and acceleration of water particles and the drag and mass coefficients and are considered to be random variables. The assumption of their lognormal distribution was reasonably well verified. Using Monte Carlo simulation these regression relations were then utilized to generate the frequency function of the wave forces. The wave force function can best be described as stationary periodic process. It can then be used as an input function for a probabilistic dynamic analysis of offshore structures.     

Particle separation in alternating-current electro-osmotic micropumps using field-flow fractionation

This work presents a novel method for continuous particle separation on the microscale by means of field-flow fractionation. It is based on the use of asymmetric interdigitated electrode arrays on the channel bottom, which induce an electro-osmotic channel flow when driven harmonically. Suspended particles are influenced by viscous fluid drag, sedimentation as well as by dielectrophoretic repulsion forces from the driving electrodes due to the emerging electric field. The significant dependance of the present forces on particle properties allows for separation with respect to particle density and size. This work analyzes electric and flow field by means of the finite element method and investigates the size and density dependent particle motion as a function of driving voltage and frequency of the electrode array. Matching these driving parameters permits the separation of sedimenting particles by their density independently from their size as well as the separation by size. Finally, channel designs are proposed which enable standard separation by means of selective particle mobility in the channel, separation in terms of opposing motion directions, as well as continuous lateral separation.     

On-chip separation of Lactobacillus bacteria from yeasts using dielectrophoresis

Dielectrophoresis, the induced motion of dielectric particles in non-uniform electric fields, enables the separation of suspended bio-particles based on their dimensions or dielectric properties. This work presents a microfluidic system, which utilises a combination of dielectrophoretic (DEP) and hydrodynamic drag forces to separate Lactobacillus bacteria from a background of yeasts. The performance of the system is demonstrated at two operating frequencies of 10?MHz and 100?kHz. At 10?MHz, we are able to trap the yeasts and bacteria at different locations of the microelectrodes as they experience different magnitudes of DEP force. Alternatively, at 100?kHz we are able to trap the bacteria along the microelectrodes, while repelling the yeasts from the microelectrodes and washing them away by the drag force. These separation mechanisms might be applicable to automated lab-on-a-chip systems for the rapid and label-free separation of target bio-particles.     

一种织物造型的迭代算法*

给出一种织物造型的迭代算法,它属于弹性质点模型。此模型把织物用质点网络来表示后,主要考虑质点间的三种作用力:质点间的推拉力即网线纤维的张力;使同一网线上的连续三点保持在一条直线上的力即网线的弹力;在网线的交点处使相交网线保持垂直的力,即网线的编织力。网络中的质点在这三种力的作用下产生位移,从而表现出织物的质地、特征和动态效果。     

Stepwise gray-scale light-induced electric field gradient for passive and continuous separation of microparticles

This article presents a gray-scale light-induced dielectrophoresis (GS-LIDEP) method that induces the lateral displacements normal to the through-flow for continuous and passive separation of microparticles. In general, DEP force only can affect the particles within very local areas due to the electric field is exponentially decayed by the distance away from the electrodes. Unlike with conventional LIDEP, a broad-ranged electrical field gradient can easily be created by GS pattern illumination, which induces DEP forces with two directions for continuous separation of particles to their specific sub-channels. Candia albicans were effectively guided to the specific outlet with the efficiency of 90% to increase the concentration of the sample below the flow rate of 0.6?μl/min. 2 and 10?μm polystyrene particles can also be passively and well separated using the multi-step GS pattern through positive and negative DEP forces, respectively, under an applied voltage of 36?V_p–p at the frequency of 10?kHz. GS-LIDEP generated a wide-ranged DEP force that is capable of working on the entire area of the microchannel, and thus the mix of particles can be passively and continuously separated toward the opposite directions by the both positive and negative GS-LIDEP forces. This simple, low cost, and flexible separation/manipulation platform could be very promising for many applications, such as in-field detections/pretreatments.     

对称区域边界处理方法及基于表面粒子提取的表面张力计算

流固边界处理一直是流体模拟的研究重点,边界力法和虚粒子法是研究流固边界 的常用方法。边界力法通过对铺设在边界上的粒子施加排斥力防止粒子穿透,但边界力的计算 限制了模拟速度。虚粒子法在边界处生成虚粒子,随着粒子数的增加所需的虚粒子数也随之增 加,导致计算速度下降,且会出现流体与边界分离的现象。为此,提出一种对称区域边界处理 方法,在保证逼真度的前提下满足实时性要求,随着粒子数的增加,其耗时增长也明显比其他 传统方法慢,更适合对复杂场景的模拟,同时避免了边界处流体与边界分离的现象。CSF 方法 是处理表面张力常用的方法,可将表面张力看作体积力进行计算,大大减弱了表面形状对曲率 计算的影响,而事实上曲率的计算只与表面的形状有关。为此,对CSF 方法进行了改进,提出 了一种基于表面粒子提取的表面张力计算方法,减小了传统CSF 方法计算曲率的误差,提高了 计算速度。模拟仿真的效果验证了该方法的有效性。     

Precisely sized separation of multiple particles based on the dielectrophoresis gradient in the z-direction

We present a new 3D dielectrophoresis-field-flow fraction (DEP-FFF) concept to achieve precise separation of multiple particles by using AC DEP force gradient in the z-direction. The interlaced electrode array was placed at the upstream of the microchannel, which not only focused the particles into a single particle stream to be at the same starting position for further separation, but also increased the spacing between each particle by the retard effect to reduce particle–particle aggregation. An inclined electrode was also designed in back of the focusing component to continuously and precisely separate different sizes of microparticles. Different magnitudes of DEP force are induced at different positions in the z-direction of the DEP gate, which causes different penetration times and positions of particles along the inclined DEP gate. 2, 3, 4, and 6?μm polystyrene beads were precisely sized fractionation to be four particle streams based on their different threshold DEP velocities that were induced by the field gradient in the z-direction when a voltage of 6.5?V_p–p was applied at a flow rate of 0.6?μl/min. Finally, Candida albicans were also sized separated to be three populations for demonstrating the feasibility of this platform in biological applications. The results showed that a high resolution sized fractionation (only 25% size difference) of multiple particles can be achieved in this DEP-based microfluidic device by applying a single AC electrical signal.     

Sedimentation pinched-flow fractionation for size- and density-based particle sorting in microchannels

A simple and efficient device for density-based particle sorting is in high demand for the purification of specific cells, bacterium, or environmental particles for medical, biochemical, and industrial applications. Here we present microfluidic systems to achieve size- and density-based particle separation by adopting the sedimentation effect for a size-based particle sorting technique utilizing microscale hydrodynamics, called ??pinched-flow fractionation (PFF).?? Two schemes are presented: (a) the particle inertia scheme, which utilizes the inertial force of particle movement induced by the momentum change in the curved microchannel, and (b) the device rotation scheme, in which rotation of the microdevice exerts centrifugal force on the flowing particles. In the experiments, we successfully demonstrated continuous sorting of microparticles according to size and density by using these two schemes, and showed that the observed particle movements were in good agreement with the theoretical estimations. The presented schemes could potentially become one of the functional components for integrated bioanalysis systems that can manipulate/separate small amount of precious biological samples.     

Particle focusing in microfluidic devices

Focusing particles (both biological and synthetic) into a tight stream is usually a necessary step prior to counting, detecting, and sorting them. The various particle focusing approaches in microfluidic devices may be conveniently classified as sheath flow focusing and sheathless focusing. Sheath flow focusers use one or more sheath fluids to pinch the particle suspension and thus focus the suspended particles. Sheathless focusers typically rely on a force to manipulate particles laterally to their equilibrium positions. This force can be either externally applied or internally induced by channel topology. Therefore, the sheathless particle focusing methods may be further classified as active or passive by the nature of the forces involved. The aim of this article is to introduce and discuss the recent developments in both sheath flow and sheathless particle focusing approaches in microfluidic devices.     

Flow rate-insensitive microparticle separation and filtration using a microchannel with arc-shaped groove arrays

Inertial microfluidics can separate microparticles in a continuous and high-throughput manner, and is very promising for a wide range of industrial, biomedical and clinical applications. However, most of the proposed inertial microfluidic devices only work effectively at a limited and narrow flow rate range because the performance of inertial particle focusing and separation is normally very sensitive to the flow rate (Reynolds number). In this work, an innovative particle separation method is proposed and developed by taking advantage of the secondary flow and particle inertial lift force in a straight channel (AR = 0.2) with arc-shaped groove arrays patterned on the channel top surface. Through the simulation results achieved, it can be found that a secondary flow is induced within the cross section of the microchannel and guides different-size particles to the corresponding equilibrium positions. On the other hand, the effects of the particle size, flow rate and particle concentration on particle focusing and separation quality were experimentally investigated. In the experiments, the performance of particle focusing, however, was found relatively insensitive to the variation of flow rate. According to this, a separation of 4.8 and 13 µm particle suspensions was designed and successfully achieved in the proposed microchannel, and the results show that a qualified particle separation can be achieved at a wide range of flow rate. This flow rate-insensitive microfluidic separation (filtration) method is able to potentially serve as a reliable biosample preparation processing step for downstream bioassays.     

Computational and performance analysis of a continuous magnetophoretic bioseparation chip with alternating magnetic fields

This paper presents the modeling and optimization of a magnetophoretic bioseparation chip for isolating cells, such as circulating tumor cells from the peripheral blood. The chip consists of a continuous-flow microfluidic platform that contains locally engineered magnetic field gradients. The high-gradient magnetic field produced by the magnets is spatially non-uniform and gives rise to an attractive force on magnetic particles flowing through a fluidic channel. Simulations of the particle–fluid transport and the magnetic force are performed to predict the trajectories and capture lengths of the particles within the fluidic channel. The computational model takes into account key forces, such as the magnetic and fluidic forces and their effect on design parameters for an effective separation. The results show that the microfluidic device has the capability of separating various cells from their native environment. An experimental study is also conducted to verify and validate the simulation results. Finally, to improve the performance of the separation device, a parametric study is performed to investigate the effects of the magnetic bead size, cell size, number of beads per cell, and flow rate on the cell separation performance.     

Dielectrophoresis in aqueous suspension: impact of electrode configuration

Dielectrophoresis (DEP) allows to moving neutral or charged particles in liquids by supplying a non-uniform electric field. When using alternating current and insulated electrodes, this is possible in conducting media such as aqueous solutions. However, relatively high field strength is required that is discussed to induce also an undesired Joule heating effect. In this paper, we demonstrate boundary conditions for avoiding this side effect and suggest a novel design of an interdigitated electrode (IDE) configuration to reduce the power consumption. Numerical simulation using OpenFOAM demonstrated that, when replacing conventional plate IDE by cylindrical micro-IDE in microchannel systems, the dielectrophoretic force field, i.e., the electric field gradient squared, becomes stronger and more homogeneously distributed along the electrodes array. Also the resulting particle DEP velocities were highest for the cylindrical IDE. The simulations were experimentally confirmed by measuring velocity of resin particle located at the subsurface of demineralized water. Surprisingly the fluid flow induced by electrothermal effect turned out to be negligible in microchannels when compared to the DEP effect and becomes dominant only for distances between particle and IDE larger than 6,000 μm. The well-agreed experimental and simulation results allow for predicting particle motion. This can be expected to pave the way for designing DEP microchannel separators with high throughput and low energy consumption.     

Particle Monte Carlo and lattice-Boltzmann methods for simulations of gas-particle flows

A numerical solution concept is presented for simulating the transport and deposition to surfaces of discrete, small (nano-)particles. The motion of single particles is calculated from the Langevin equation by Lagrangian integration under consideration of different forces such as drag force, van der Waals forces, electrical Coulomb forces and not negligible for small particles, under stochastic diffusion (Brownian diffusion). This so-called particle Monte Carlo method enables the computation of macroscopic filter properties as well the detailed resolution of the structure of the deposited particles. The flow force and the external forces depend on solutions of continuum equations, as the Navier-Stokes equations for viscous, incompressible flows or a Laplace equation of the electrical potential. Solutions of the flow and potential fields are computed here using lattice-Boltzmann methods. Essential advantage of these methods are the easy and efficient treatment of three-dimensional complex geometries, given by filter geometries or particle covered surfaces. A number of numerical improvements, as grid refinement or boundary fitting, were developed for lattice-Boltzmann methods in previous studies and applied to the present problem. The interaction between the deposited particle layer and the fluid field or the external forces is included by recomputing of these fields with changed boundaries. A number of simulation results show the influence of different effects on the particle motion and deposition.     

SPH particle boundary forces for arbitrary boundaries

This paper is concerned with approximating arbitrarily shaped boundaries in SPH simulations. We model the boundaries by means of boundary particles which exert forces on a fluid. We show that, when these forces are chosen correctly, and the boundary particle spacing is a factor of 2 (or more) less than the fluid particle spacing, the total boundary force on a fluid SPH particle is perpendicular to boundaries with negligible error. Furthermore, the variation in the force as a fluid particle moves, while keeping a fixed distance from the boundary, is also negligible. The method works equally well for convex or concave boundaries. The new boundary forces simplify SPH algorithms and are superior to other methods for simulating complicated boundaries. We apply the new method to (a) the rise of a cylinder contained in a curved basin, (b) the spin down of a fluid in a cylinder, and (c) the oscillation of a cylinder inside a larger fixed cylinder. The results of the simulations are in good agreement with those obtained using other methods, but with the advantage that they are very simple to implement.     

Microfluidic separation of magnetic particles with soft magnetic microstructures

This paper demonstrates simple and cost-effective microfluidic devices for enhanced separation of magnetic particles by using soft magnetic microstructures. By injecting a mixture of iron powder and polydimethylsiloxane (PDMS) into a prefabricated channel, an iron–PDMS microstructure was fabricated next to a microfluidic channel. Placed between two external permanent magnets, the magnetized iron–PDMS microstructure induces localized and strong forces on the magnetic particles in the direction perpendicular to the fluid flow. Due to the small distance between the microstructure and the fluid channel, the localized large magnetic field gradients result a vertical force on the magnetic particles, leading to enhanced separation of the particles. Numerical simulations were developed to compute the particle trajectories and agreed well with experimental data. Systematic experiments and numerical simulation were conducted to study the effect of relevant factors on the transport of superparamagnetic particles, including the shape of iron–PDMS microstructure, mass ratio of iron–PDMS composite, width of the microfluidic channel, and average flow velocity.