U Lei - Academia.edu (original) (raw)

Papers by U Lei

A method is proposed for selective separation and isolation of particles/cells of similar sizes b... more A method is proposed for selective separation and isolation of particles/cells of similar sizes based on their different Clausius-Mossotti factors using dielectrophoresis and microfluidics via both calculation and experiment. The device is a straight micro channel (PDMS) with grooves (or cavities) on its ceiling for capturing particles, and electrodes on its bottom wall (glass) for generating electric field. A particle may be carried downstream or pushed into the groove depending on a designed local force balance between dielectrophoretic force, fluid drag and buoyancy. A mixture of polystyrene particles and CL1-5 lung cancer cells can be separated within 10% accuracy.

Micromachines, 2020

A simple and inexpensive method using planar electrodes was proposed for the measurement of the i... more A simple and inexpensive method using planar electrodes was proposed for the measurement of the imaginary part of the Clausius-Mossotti factor, K i , of particle/cell for electrorotation (ER) and travelling wave dielectrophoresis (twDEP). It is based on the balance between the dielectrophoretic and viscous torques on a particle undergoing ER subject to dual frequency operation in an ER chamber. A four-phase ac voltage signal with a given frequency is applied for generating ER for measurement, and another two-phase signal is applied at a selected frequency for generating a negative dielectrophoretic force for confining the particle motion, instead of using laser tweezer or three-dimensional electrodes in the literature. Both frequencies can be applied to the same electrodes in a four-electrode ER system and to alternative different electrodes in an eight-electrode ER system, and both systems are capable for providing accurate measurement. The measurements were validated by comparing ...

ELECTROPHORESIS, 2019

A method is proposed for measuring the real part of the Clausius–Mossotti factor () of dielectrop... more A method is proposed for measuring the real part of the Clausius–Mossotti factor () of dielectrophoresis for Brownian particles based on a solution of the Smoluchowski equation using a designed polydimethysilloxane microchannel with planar hyperbolic electrodes on its glass substrate. An approximate two‐dimensional spring‐like dielectrophoretic force is generated in the device, and the data necessarily measured is the time evolution of the in‐plane particle displacement undergoing confined Brownian motion. Validity of the measurement was checked against the zeta potentials in the literature based on the classical theory of surface conductance using polystyrene particles of size of one micron. As the dielectrophoretic force depends on , which is usually unknown for bio‐particles and some engineered particles, and is seldom measured; this study is important from the academic point of view and could be helpful for the manipulation and characterization of sub‐micron particles using diel...

Physics of Fluids A: Fluid Dynamics, 1993

Rigorous analysis shows that the spin synchronous mode fluid motion inside a nutation fluid dampe... more Rigorous analysis shows that the spin synchronous mode fluid motion inside a nutation fluid damper on board of a spinning satellite can be modeled as an incompressible, laminar pulsatile flow in a circular straight pipe. The pipe rotates with constant angular velocity ω_ about an axis perpendicular to its own axis. The distance between the rotation axis and the pipe axis is much greater than a, the pipe’s radius. The flow is driven by a three-dimensional harmonic oscillation of the pipe wall with frequency Ω and amplitude w′0, and is governed by three-dimensionless parameters: RΩ(=Ωa2/ν), Δ(=ω/Ω), and A( = w′0/Ωa), where ν is the kinematic viscosity of the fluid. Both the asymptotic analysis and the numerical calculation have been carried out for RΩ=0.1–1000 and Δ=0–2 under A≪1. It is found that the rotating effect increases the energy dissipation significantly in comparison with the result of the pulsatile straight pipe flow in an inertia frame (the previous theory for the nutation...

Physics of Fluids A: Fluid Dynamics, 1991

The motion of small spherical solid particles are simulated numerically in a decaying homogeneous... more The motion of small spherical solid particles are simulated numerically in a decaying homogeneous isotropic turbulent gas flow field generated by the large eddy simulation. By comparing with the previous experimental and theoretical studies, the present method is found to be a successful tool to generate the properties of the particle motion involving the second-order statistics, such as the mean-square displacement, the dispersion coefficient, and the root-mean-square velocity fluctuation. The present results are complementary to the experimental data and include a detailed study of the effects of the flow turbulence, the particle’s inertia, and the particle’s free-fall velocity in a still fluid on the particle dispersion and turbulence intensity. By performing particle simulation in the flow fields generated with different values of the coefficient in the subgrid model and with different sizes of the calculation domain, it is found that the particle motion is indeed controlled mai...

2011 6th IEEE International Conference on Nano/Micro Engineered and Molecular Systems, 2011

The mixer in the present study is essentially a Y- type micro channel with an electrode array bui... more The mixer in the present study is essentially a Y- type micro channel with an electrode array built on the bottom wall of its straight main channel. Electro-thermally driven vortices are generated in the electrolyte above the corners of electrodes by applying ac signals at 6 peak-to-peak volts and 1 MHz with o 180 phase shift to each electrode pair of the arrays. The vortices stir the liquid and thus enhance mixing. Experiments were carried out by feeding two equal streams of saline water with different dyes into the mixer for various parameters of electrode array configuration and background flow rates. For a mixing channel with height 30 μm, width 400 μm and length 13 mm, the mixing is enhanced substantially by the vortices even when the background average flow speed is increased up to 972 μm/s. Numerical simulation was carried out for understanding the structure of the vortices and the behind physics. Keywords-mixer; electro-thermally driven vortices; conductive medium

Physics of Fluids A: Fluid Dynamics, 1990

Detailed numerical simulation has been carried out for fully developed laminar flow through a cir... more Detailed numerical simulation has been carried out for fully developed laminar flow through a circular straight pipe with radius a, which is rotating with constant speed Ω about an axis perpendicular to its own axis. The flow is symmetric about a plane containing the pipe axis with its normal parallel to the rotation axis. There are four types of flow regime that result from the various effects of the secondary flow on the main stream via the convection and Coriolis term. When RΩ≤10 and RΩG≤100, the axial velocity profile is essentially axisymmetric and parabolic. Here RΩ=Ωa2/ν and G=G*a3/(ρν2), where G* is the reduced axial pressure gradient driving the flow, ρ is the fluid density, and ν is the kinematic viscosity. When RΩ<0.85(RΩG)1/3 and RΩG>100, the axial velocity profile is skewed toward the pressure side with one maximum occurring on the symmetric plane. When RΩ>1.26(RΩG)2/5 and RΩ>10, the axial velocity shows a dumbell-like profile with the ‘‘dumbell’’ center coi...

IET nanobiotechnology / IET, 2011

Generalised dielectrophoresis (gDEP), including conventional dielectrophoresis (cDEP), electrorot... more Generalised dielectrophoresis (gDEP), including conventional dielectrophoresis (cDEP), electrorotation (ER) and travelling wave dielectrophoresis (twDEP), is an effective tool for particle (cell) manipulation and characterisation, even down to the level of nano-sized objects such as DNA, proteins and viruses. All the disciplines of gDEP are originated from the interaction of an applied electric field with its polarisation effect on the particle and can be studied systematically in a unified approach under electrostatics. In this review, the authors discuss both the quasi-static and transient theory of gDEP in an unbounded medium for both spherical and ellipsoidal particles. Then the quasi-static theory of wall effect is discussed on gDEP for a spherical particle. The wall effect is minor for ER, twDEP and cDEP parallel to wall(s), but could be significant for cDEP normal to wall(s). Force and torque expressions in terms of electric potential and its derivatives are provided and sugg...

22nd Fluid Dynamics, Plasma Dynamics and Lasers Conference, 1991

Journal of The Chinese Society of Mechanical Engineers, 1993

This paper studies numerically the steady or quasi-steady flow field of the impingement of a two-... more This paper studies numerically the steady or quasi-steady flow field of the impingement of a two-phase axisymmetric supersonic cold jet suspended with solid spherical particles onto a perpendicular flat plate via a MacCormack-type TVD scheme. It is assumed that the particulate phase can be treated as a compressible continuum and that the particles adhere to the plate once they collide

18th IEEE International Conference on Micro Electro Mechanical Systems, 2005. MEMS 2005.

Sensors and Actuators A: Physical, 2007

Physics of Fluids, 1994

Some measurements have been obtained for the axial velocity of the fully developed laminar flow i... more Some measurements have been obtained for the axial velocity of the fully developed laminar flow in a circular straight pipe with radius a, which is rotating with constant angular speed Ω about an axis perpendicular to its own axis. A diode laser LDA system was mounted together with a circulating pipe flow system on a rotating table for the experiment. According to previous analyses and calculations, there exist four types of axial velocity distributions that result from the various effects of the secondary flow on the main stream via the convection and Coriolis effect for different values of R( = wm′a/ν) and RΩ(=Ωa2/ν), where wm′ is the mean axial velocity and ν is the kinematic viscosity of the fluid. The present study provides experimental validation for the previous theoretical and numerical analyses. Experiments have also been carried out for studying the asymptotic nature of the slow flow in a rapidly rotating pipe (RΩ≫1 and RΩ≫R) and the rapid flow in a slowly rotating pipe (R...

Microfluidics and Nanofluidics, 2007

In this paper, a PZT micropump excited by amplified squarewave signals with various frequencies w... more In this paper, a PZT micropump excited by amplified squarewave signals with various frequencies was used to study the transient flow behaviors in an obstacle-type valveless micropump. A micro-particle-image-velocimetry (micro-PIV) with an external trigger was developed to obtain flow fields at the outlet and around the obstacle with various phases in a cycle. In comparison with previous studies on the pump performance, such as pump pressure and volume flow rate, more detailed information about the pump was obtained. The velocity profiles and periodic sectional mean velocities exhibited the unsteady flow nature. The total net flow generation efficiency per cycle was obtained experimentally by integrating the phase-dependent velocities. The flow recirculation around the obstacle was observed and quantified to investigate the influence on the pump performance. The duration, circulation, and the size of the recirculation regions indicated that this flow behavior could enhance the flow-directing capability. These results are very useful for the design and improvement of obstacle-type valveless micropumps.

Journal of Fluid Mechanics, 1998

The average settling velocity of heavy particles under a body force field is studied numerically ... more The average settling velocity of heavy particles under a body force field is studied numerically in stationary homogeneous isotropic turbulent flows generated by the direct numerical simulation and the large eddy simulation of the continuity and Navier–Stokes equations. The flow fields corresponding to different selected ranges of turbulent scales are obtained by filtering the results of the direct numerical simulation, and employed for calculating the particle motion. Wang & Maxey (1993) showed that as a consequence of the particle accumulation in the low vorticity region and the preferential sweeping phenomenon, the average settling rate in turbulence is greater than that in still fluid. In the present study, the phenomenon of particle accumulation in the low vorticity region is found to be controlled mainly by the small scales with wavenumber kω corresponding to the maximum of the dissipation (vorticity) spectrum. However, the increase of the average settling rate, 〈ΔvS〉, also de...

Journal of Applied Physics, 2007

Analytic expressions for the quasistatic force and torque on ellipsoidal particles covered with a... more Analytic expressions for the quasistatic force and torque on ellipsoidal particles covered with a shell under the action of generalized dielectrophoresis have been derived using the effective moment method. Here the generalized dielectrophoresis includes traditional dielectrophoresis generated by nonuniform electric field without phase shift, electrorotation, and traveling wave dielectrophoresis. Characteristics of the results of ellipsoids are contrasted with those of spherical particles and illustrated through various examples using erythrocytes as model particles. In particular, we derived the force and torque equations in terms of electric potential, which are useful for simulating various phenomena associated with the characterization and manipulation of particles and cells in microfluidic devices.