Electrokinetic motion of particles and cells in microchannels

This paper provides an overview of the electrokinetic phenomena associated with particles and cells in microchannel systems. The most important phenomena covered include electrophoresis, dielectrophoresis, and induced-charge electrokinetics. The latest development of these electrokinetic techniques for particle or cell manipulations in microfluidic systems is reviewed, in terms of the basic theories, mathematical models, numerical and experimental methods, and the key results/findings from the published literatures in the most recent decades. Some of the limitations associated with the negative field effects are discussed and the perspectives for the future investigations are summarized.     

Electrokinetic motion of an electrically induced Janus droplet in microchannels

The electrokinetic motion of an electrically induced Janus oil droplet with one side covered with an aluminum oxide nanoparticle film in a circular microchannel was numerically simulated in this paper. The Janus oil droplet is electrically anisotropic as the nanoparticle-covered area carries positive charges and the rest oil–water surface area carries negative charges. A theoretical model was constructed to calculate the electrokinetic velocity of the Janus droplet by considering the force balance on the surface of the Janus droplet at steady state. In the model, the effects of the electric double layer and surface charges on the motion at the oil–water interface are considered. The effects of five parameters on the electrokinetic motion of the Janus droplets were studied: the electric field, the zeta potential ratio of the positively charged side to the negatively charged side of the Janus droplet, the viscosity ratio of the oil phase to the water phase, the nanoparticle coverage of the Janus droplet, and the size ratio of the diameter of the Janus droplet to the diameter of the cylindrical microchannel. The simulation results indicate that the increase in the electrical field, the zeta potential ratio, the viscosity ratio or the nanoparticle coverage leads to faster electrokinetic motion of the Janus droplet. On the other hand, with the increase in size ratio, the electrokinetic velocity of Janus droplet first decreases gradually then increases sharply. The simulated results were compared with the experimental results and good agreement was found.     

Eccentric electrophoretic motion of a sphere in circular cylindrical microchannels

The eccentric electrophoretic motion of a spherical particle in an aqueous electrolyte solution in circular cylindrical microchannels is studied in this paper. The objective is to investigate the influences of separation distance and channel size on particle motion. A theoretical model is developed to describe the electric field, the flow field and the particle motion. A finite element based direct numerical simulation method is employed to solve the model. Numerical results show that, when the particle is eccentrically positioned in the channel, the electric field and the flow field are not symmetric, and the strongest electric field and the highest flow velocity occur in the small gap region. It is shown that the rotational velocity of the particle increases with the decrease of the separation distance. With the decrease of the separation distance, the translational velocity increases in a smaller channel; while it decreases first and then increases in a relatively large channel. When a particle moves eccentrically at a smaller separation distance from the channel wall, both the translational velocity and the rotational velocity increase with the decrease of the channel size.     

Microfluidic motion for a direct investigation of solvent interactions with PDMS microchannels

Solid surface/liquid interactions play an important role in microfluidics and particularly in manipulation of films, drops and bubbles, a basic requirement for a number of lab-on-chip applications. The behavior of solvents in coated microchannels is difficult to be predicted considering theories; therefore, experimental methods able to estimate the properties at the interface in real time and during the operational regime are amenable. Here, we propose to use an experimental setup to evaluate the effective dynamics of solvents inside PDMS microchannels. The influence of the solvent properties as well as of the channel wall’s wettability on the fluid movements was evaluated. Modification of the channel properties was achieved by introducing Teflon coatings that allow producing stable hydrophobic microchannel walls. The results were fitted according to Washburn-type power-law and compared with theoretical calculations of the parameter β that expresses the dependence of capillary dynamics on surface tension γ, liquid viscosity η, contact angles θ and the hydraulic radius R _H. A comparison between the calculated and the experimental values reveled that parameters other than the contemplated ones influenced the measurements. The main parameter that affects the flow of solvents such as water, methanol ethanol, dimethylformamide, acetonitrile and acetone was found to be the γ/η ratio. Considering these results, the investigation tool described here is believed to be promising to predict the dynamics of common organic solvents inside integrated functional fluidic devices and to accurately control solvent flow, particularly in capillary-driven pumpless systems, a basic requirement for widening the application range of PDMS lab-on-chip devices.     

Design of motion along parameterized curves using B-splines

The paper presents a method for design of motion along parameterized curves using B-splines. The presented approach is based on local parameterizations of orientations with respect to the Frenet frame moving along the parameterized curve. Conditions, which ensure C^p continuity of the designed motion, are established. It is shown that the designed motion is invariant under transition between orthonormal reference frames of the Euclidean affine space E³.     

Multibody interactions of actuated magnetic particles used as fluid drivers in microchannels

The forced motion of superparamagnetic particles and their multibody interactions are studied in view of the application as integrated fluid drivers in microchannel systems. Previous studies on particle manipulation in open fluid volumes serve as our starting point for the analysis of particle dynamics and interplay effects in confined fluid volumes. An experimental setup is designed that offers a constant force field on all individual particles dispersed in a microchannel. Distinguishable multi-particle configurations are observed and analyzed on the basis of magnetic and hydrodynamic particle interaction mechanisms. The fluid driving performance and the efficiency of the particles are evaluated on system level by means of numerical simulation models.     

The effect of sinusoidal rolling ground motion on lifting biomechanics

The objective of this study was to quantify the effects of ground surface motion on the biomechanical responses of a person performing a lifting task. A boat motion simulator (BMS) was built to provide a sinusoidal ground motion (simultaneous vertical linear translation and a roll angular displacement) that simulates the deck motion on a small fishing boat. Sixteen participants performed lifting, lowering and static holding tasks under conditions of two levels of mass (5 and 10 kg) and five ground moving conditions. Each ground moving condition was specified by its ground angular displacement and instantaneous vertical acceleration: A): +6°, −0.54 m/s²; B): +3°, −0.27 m/s²; C): 0°, 0 m/s²; D): −3°, 0.27 m/s²; and E): −6°, 0.54 m/s². As they performed these tasks, trunk kinematics were captured using the lumbar motion monitor and trunk muscle activities were evaluated through surface electromyography. The results showed that peak sagittal plane angular acceleration was significantly higher in Condition A than in Conditions C, D and E (698°/s² vs. 612-617°/s²) while peak sagittal plane angular deceleration during lowering was significantly higher in moving conditions (conditions A and E) than in the stationary condition C (538-542°/s² vs. 487°/s²). The EMG results indicate that the boat motions tend to amplify the effects of the slant of the lifting surface and the external oblique musculature plays an important role in stabilizing the torso during these dynamic lifting tasks.     

The effect of sinusoidal rolling ground motion on lifting biomechanics

The objective of this study was to quantify the effects of ground surface motion on the biomechanical responses of a person performing a lifting task. A boat motion simulator (BMS) was built to provide a sinusoidal ground motion (simultaneous vertical linear translation and a roll angular displacement) that simulates the deck motion on a small fishing boat. Sixteen participants performed lifting, lowering and static holding tasks under conditions of two levels of mass (5 and 10 kg) and five ground moving conditions. Each ground moving condition was specified by its ground angular displacement and instantaneous vertical acceleration: A): +6°, −0.54 m/s²; B): +3°, −0.27 m/s²; C): 0°, 0 m/s²; D): −3°, 0.27 m/s²; and E): −6°, 0.54 m/s². As they performed these tasks, trunk kinematics were captured using the lumbar motion monitor and trunk muscle activities were evaluated through surface electromyography. The results showed that peak sagittal plane angular acceleration was significantly higher in Condition A than in Conditions C, D and E (698°/s² vs. 612–617°/s²) while peak sagittal plane angular deceleration during lowering was significantly higher in moving conditions (conditions A and E) than in the stationary condition C (538–542°/s² vs. 487°/s²). The EMG results indicate that the boat motions tend to amplify the effects of the slant of the lifting surface and the external oblique musculature plays an important role in stabilizing the torso during these dynamic lifting tasks.     

Control of the motion of a disk rolling on a plane curve

This work deals with the control of the motion of a disk rolling without slipping on a curve located in the horizontal plane. The disk’s motion is driven by a pedalling torque and by using two overhead rotors. In addition, the case where the disk rolls on a plane curve with its plane vertical to the (X,Y)-plane, is discussed.     

Theoretical analysis of a new, efficient microfluidic magnetic bead separator based on magnetic structures on multiple length scales

We present a theoretical analysis of a new design for microfluidic magnetic bead separation. It combines an external array of mm-sized permanent magnets with magnetization directions alternating between up and down with μm-sized soft magnetic structures integrated in the bottom of the separation channel. The concept is studied analytically for simple representative geometries and by numerical simulation of an experimentally realistic system geometry. The array of permanent magnets provides long-range magnetic forces that attract the beads to the channel bottom, while the soft magnetic elements provide strong local retaining forces that prevent captured beads from being torn loose by the fluid drag. The addition of the soft magnetic elements increases the maximum retaining force by two orders of magnitude. The design is scalable and provides an efficient and simple solution to the capture of large amounts of magnetic beads on a microsystem platform.     

Stabilization of translational-rotational motion of uniaxial wheeled platform along a linear trajectory

The problem of stabilization of a uniaxial wheeled platform with the property of being an upper inverted pendulum along a program trajectory is considered. Control problems for the center of mass of the platform and its stabilization about the center of mass in the horizontal and vertical planes are solved. The specific feature of constructed controls is their continuous correction based on application of analytical relations of wheel interaction with a ground surface for providing nonholonomic constraints for the wheels and the surface.     

Stabilization and control of the motion of a rolling disk by using two overhead rotors

This work deals with the stabilization and control of a system which is composed of a disk rolling on a plane, a controlled slender rod that is pivoted through its center of mass about the disk's center and two overhead rotors with their axes fixed in the upper part of the rod (see Figures 1 and 2). The rod is controlled in such a manner that it is always aligned along the line passing through the points O and C, where O denotes the center of the disk and C denotes the point of contact between the disk and the plane. The upper rotor rotates in a plane that passes through the disk's axis and the rod, whereas the lower rotor rotates in a plane that is perpendicular to the rod (see Figures 1 and 2). By using a kind of inverse dynamics control a control strategy is proposed under which the disk's inclination is stabilized about its vertical position and the disk's motion is able asymptotically to track any given smooth ground trajectory.     

Modelling and control of the motion of a riderless bicycle rolling on a moving plane

This work deals with the modelling and control of a riderless bicycle rolling on a moving plane. It is assumed here that the bicycle is controlled by a pedalling torque, a directional torque and by a rotor mounted on the crossbar that generates a tilting torque.In particular, a kinematic model of the bicycle’s motion is derived by using its dynamic model. Then, using this kinematic model, the expressions for the applied torques are obtained.     

Dynamic motion analysis of magnetic particles in microfluidic systems under an external gradient magnetic field

To gain an insightful understanding of motion behavior of paramagnetic particles suspended in a nonmagnetic fluid under a gradient magnetic field, a coupled fluid–structure model based on a direct numerical scheme is developed in this work. The governing equations of magnetic field, fluid flow field and particle motion are simultaneously solved using an Arbitrary Lagrangian–Eulerian method, taking into account magnetic and hydrodynamic interactions between particles in a fully coupled manner. The accuracy of the proposed method is validated using the magnetic particulate flows of two particles under a uniform magnetic field as the test problem and is then applied to investigate effects of magnetic and hydrodynamic interactions between particles on the particle motion behavior. Results show that neighboring magnetic particles are easy to form chain-like clusters along field direction due to magnetic interactions between particles and then move together toward the surface of magnetic source under the action of gradient magnetic force. More importantly, it has been found that both magnetic and hydrodynamic interactions between particles are conducive to the acceleration of particles and the chain formation of particles. The present method and results could help in understanding the basic mechanism underlying the low-gradient magnetophoretic separation process and designing magnetic aggregate-based microfluidic devices.     

运动状态下磁场模量修正技术研究

考虑到现有三轴磁通门传感器的修正存在方法复杂、对设备要求较高、效果不佳等问题.根据三轴磁通门传感器是由于三轴不正交、灵敏度不一致和零点偏移所引起的误差进行分析和理论计算,提出了三轴磁通门传感器修正模型并运用奇异值分解和单纯型法进行实验.实验结果表明,该方法能够简单、有效的对磁场模量进行修正,能够广泛的应用于三轴磁通门传感器静态和动态的总模量修正.     

Rotational motion and lateral migration of an elliptical magnetic particle in a microchannel under a uniform magnetic field

We have numerically investigated the motion of an elliptical magnetic particle in a microfluidic channel subjected to an external uniform magnetic field. By using the direct numerical simulation method and an arbitrary Lagrangian–Eulerian technique, the involved particle–fluid-magnetic field problem can be solved in a fully coupled manner. The numerical predictions of the particle trajectory and orientation with and without a uniform magnetic field are in qualitative agreement with the existing experimental results, and numerical results have revealed the impacts of key parameters such as inlet flow velocity, magnetic field direction, and particle shape on the rotational motion and lateral migration of the elliptical particle. Meanwhile, the shape-based particle separation in a low Reynolds number flow with the aid of an applied uniform magnetic field has also been numerically demonstrated.     

Directional control of the motion of a rolling disk by using a rotor fixed in the disk's plane

This work deals with the stabilization and control of a system which is composed of a disk rolling on a plane, and a circular rotor plate fixed in the disk's plane. The disk's motion is controlled by the above-mentioned rotor, a “tillting moment” and a pedalling moment. It is shown here that by applying a kind of inverse dynamics control, the motion of the disk is stabilized about a given angle, while simultaneously controlling its speed and direction in such a manner that the point of contact between the disk and the horizontal plane will be able, during a given time interval [0, t_f], to move from a given point. r_A to a another given point r_B, both of them fixed in the plane.     

载流导线在磁场中混沌运动区域分析

在考虑电磁场对结构变形影响基础上,假设导线变形为小变形,采用弦的模型建立了载流导线在磁场中的周期激励作用下的横向振动控制方程.利用伽辽金原理及Melnikov方法推导出了载流导线发生混沌运动的临界条件,并讨论了导线张力、导线距离、电流等因素对载流导线混沌运动区域的影响.得到了如下的结论:载流导线的混沌运动区域随导线张力、导线距离的增大而变大;电流小于某一值时,载流导线混沌运动区域随电流增大而减小.