ORCID Profile
0000-0003-1191-7642
Current Organisation
Nanyang Technological University
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Publisher: Elsevier BV
Date: 03-2006
Publisher: Springer Science and Business Media LLC
Date: 21-05-2010
Publisher: Elsevier BV
Date: 02-2009
Publisher: IOP Publishing
Date: 04-2006
Publisher: American Chemical Society (ACS)
Date: 18-06-2008
DOI: 10.1021/AC800947E
Abstract: Moving dielectrophoresis has been recently developed by the authors as an alternative method to achieve simultaneous cell fractionation and transportation. With an array of independently excitable microelectrodes, this method generates a moving electric field to sequentially fractionate and transport cells across a microchannel. Due to the peculiarity of this method, the motion of the cells is unsteady and there are interesting and distinct differences between cells experiencing positive or negative dielectrophoresis. For a proper understanding and design of a microdevice utilizing this methodology, this study presents a model for the equation of motion for a polarized cell and its unsteady motion under moving dielectrophoresis. The model considers the basic module to generate a moving electric field, where there is a finite-width top electrode and an infinite-width bottom electrode, in a parallel-plate configuration. The forces considered include dielectrophoretic force, fluid drag, buoyancy, and gravitational force. These forces are modeled as equivalent point forces acting at the center of mass of the cell. A parallel-plate wall correction factor is employed to account for the effect of the large cell size to microchannel height ratio. Various parameters are examined including the initial position of the cell relative to the electrodes, cell's Clausius-Mossotti factor, cell size, applied voltage, electrode width, interelectrode gap, microchannel height, number of energized electrodes, and types of electrode configurations. Reasonable agreements were obtained between simulated and experimental results. As the solution of the unsteady motion is rather tedious, a MATLAB algorithm, with all the associated files, for the prediction of the cell trajectory, is available on request.
Publisher: Wiley
Date: 02-2006
Abstract: Joule heating is inevitable when an electric field is applied across a conducting medium. It would impose limitations on the performance of electrokinetic microfluidic devices. This article presents a 3-D mathematical model for Joule heating and its effects on the EOF and electrophoretic transport of solutes in microfluidic channels. The governing equations were numerically solved using the finite-volume method. Experiments were carried out to investigate the Joule heating associated phenomena and to verify the numerical models. A rhodamine B-based thermometry technique was employed to measure the solution temperature distributions in microfluidic channels. The microparticle image velocimetry technique was used to measure the velocity profiles of EOF under the influence of Joule heating. The numerical solutions were compared with experimental results, and reasonable agreement was found. It is found that with the presence of Joule heating, the EOF velocity deviates from its normal "plug-like" profile. The numerical simulations show that Joule heating not only accelerates the s le transport but also distorts the shape of the s le band.
Publisher: Springer Science and Business Media LLC
Date: 25-10-2008
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 05-2007
Publisher: Elsevier BV
Date: 02-2007
Publisher: Wiley
Date: 15-04-2019
Abstract: Recently, particle concentration and filtration using inertial microfluidics have drawn attention as an alternative to membrane and centrifugal technologies for industrial applications, where the target particle size varies between 1 µm and 500 µm. Inevitably, the bigger particle size (>50 µm) mandates scaling up the channel cross-section or hydraulic diameter (D
Publisher: AIP Publishing
Date: 03-2010
DOI: 10.1063/1.3279790
Publisher: Elsevier BV
Date: 05-2009
Publisher: ASME International
Date: 13-04-2012
DOI: 10.1115/1.4005702
Abstract: Nanofluidics is the science and technology involving a fluid flowing in or around structures with a least one dimension in the nanoscale, which is defined as the range from 1 nm to 100 nm. In this paper, we present the fabrication and characterization of nanochannels in silicon and glass. Since the lateral dimension of the channels is limited by the wavelength of UV light used in photolithography, the channel width can only be fabricated in the micrometer scale. However, the depth of the channel can be controlled precisely by the etching rate of reactive ion etching (RIE). Microchannels and access holes were etched with deep reactive ion etching (DRIE). Both nanochannels and microchannels were sealed by a Pyrex glass wafer using anodic bonding. The fabricated nanochannels were characterized by capillary filling and evaporation experiments. Due to the small channel height and weak fluorescent signal, fluorescent techniques are not suitable for the characterization of the nanochannels. A long exposure time is needed because of the limited amount of fluorescent molecules inhibit the measurement of transient and dynamic processes. However, as the channel height is shorter than all visible wavelengths, the contrast in refractive indices of air and liquid allows clear visualization of nanochannels filled with liquids. Automatic image processing with matlab allows the evaluation of capillary filling in nanochannels. Interesting phenomena and discrepancies with conventional theories were observed.
Publisher: American Chemical Society (ACS)
Date: 17-08-2010
DOI: 10.1021/LA1010902
Abstract: We investigated the interactions between liquid, gas, and solid phases in the capillary filling process of closed-end nanochannels. This paper presents theoretical models without and with absorption and diffusion of gas molecules in the liquid. Capillary filling experiments were carried out in closed-end silicon nanochannels with different lengths. The theoretical and measured characteristics of filling length versus time are compared. The results show that the filling process consists of two stages. The first stage resembles the capillary filling process in an open-end nanochannel. However, a remarkable discrepancy between the experimental results and the theory without gas absorption is observed in the second stage. A closer investigation of the second stage reveals that the dissolution of gas in the liquid is important and can be explained by the model with gas absorption and diffusion.
Publisher: Elsevier BV
Date: 02-2011
Publisher: Springer Science and Business Media LLC
Date: 03-2018
Publisher: IOP Publishing
Date: 08-11-2005
Publisher: Elsevier BV
Date: 10-2016
Publisher: Elsevier BV
Date: 09-2008
Publisher: ASME International
Date: 06-2005
DOI: 10.1115/1.1865216
Abstract: This study presents a numerical analysis of electrokinetic mass transport in a microchannel with Joule heating effects. A nonuniform electric field caused by the presence of the Joule heating is considered in the model development. Numerical computations for electrokinetic mass transport under Joule heating effects are carried out using the Crank-Nicolson scheme of second-order accuracy in space and time for two different cases: (i) the translating interface and (ii) the dispersion of a finite s le plug. The simulations reveal that the presence of Joule heating not only causes the s le species to transport faster, but also causes the s le peak to decrease and the s le band to deviate from its flat interface or pluglike shape.
Publisher: Elsevier BV
Date: 09-2007
Publisher: Elsevier BV
Date: 2016
Publisher: American Chemical Society (ACS)
Date: 19-09-2008
DOI: 10.1021/JE800455A
Publisher: Informa UK Limited
Date: 06-2011
Publisher: Wiley
Date: 02-2006
Abstract: The zeta potentials of channel surfaces and tracer particles are of importance to the design of electrokinetic microfluidic devices, the characterization of channel materials, and the quantification of the microparticle image velocimetry (microPIV) measurement of EOFs. A method is proposed to simultaneously measure the zeta potentials of the channel surface and the tracer particles in aqueous solutions using the microPIV technique. Through the measurement of the steady velocity distributions of the tracer particles in both open- and closed-end rectangular microchannels under the same water chemistry condition, the electrophoretic velocity of the tracer particles and the EOF field of the microchannel are determined using the expressions derived in this study for the velocity distributions of charged tracer particles in the open- and closed-end rectangular microchannels. Thus, the zeta potentials of the tracer particles and the channel surfaces are simultaneously obtained using the least-square method to fit the microPIV measured velocity distribution of the tracer particles. Measurements were carried out with a microPIV system to determine the zeta potentials of the channel wall and the fluorescent tracer particles in deionized water and sodium chloride and boric acid solutions of various concentrations.
Publisher: IOP Publishing
Date: 04-2006
Publisher: Royal Society of Chemistry (RSC)
Date: 2016
DOI: 10.1039/C5LC01137J
Abstract: A concentration gradient device generates stagnation flows which confine combinational concentration gradients within velocity wells, thereby suppressing shear stress effects.
Publisher: ASMEDC
Date: 2008
Abstract: This paper presents theoretical and experimental investigations of the pressure-driven two-liquid flow in microchannels with the electroosmosis effect. For a fully developed, steady state, laminar flow of two liquids combined the pressure gradient, electroosmosis and surface charges at the liquid-liquid interface, we have derived analytical solutions that relate the velocity profiles and flow rates to the liquid holdup, the aspect ratio of the microchannel, the viscosity ratio of the two liquids and the externally applied electric field. It was shown that adjusting the externally applied electric field could control the fluid interface position precisely. The prediction from the proposed model compares very well with measured data.
Publisher: Springer Science and Business Media LLC
Date: 11-05-2005
Publisher: Royal Society of Chemistry (RSC)
Date: 2015
DOI: 10.1039/C5RA07491F
Abstract: We reports the continuous generation of droplets with concentrated s le conditioned ion concentration polarization.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 04-2014
Publisher: Wiley
Date: 23-07-2010
Abstract: Soft lithography technology allows for the development of numerous PDMS-based microfluidic devices for manipulation of particles and cells. However, integrating metallic electrodes with PDMS-based channel structures is challenging due to weak adhesion between metal and PDMS. To overcome this issue, we develop a new PDMS-based microfluidic device for continuous sorting and separation of microparticles by size using AC dielectrophoresis (DEP) with 3-D conducting PDMS composites as sidewall electrodes. The composites are synthesized by mixing silver powders with PDMS gel and such composite electrodes can easily be integrated with the PDMS microchannels. Furthermore, the sidewall electrodes also allow DEP forces to distribute three dimensionally, thus enhancing DEP effects in the entire region of channels. The capability of such PDMS-based microfluidic device is demonstrated for continuously sorting and separating 10 and 15 mum particles, and also for separating 5 from 10 mum particles. Together with experimental results, analysis of particle's trajectory based on Lagrangian approach provides insights into how microparticles transport under the effects of hydrodynamic and DEP forces in the present PDMS-based microfluidic device.
Publisher: Royal Society of Chemistry (RSC)
Date: 2009
DOI: 10.1039/B813639D
Abstract: This paper presents a novel approach of mixing two laminar flowing streams in microchannels. The mixer consists of a pair of electrodes disposed along a fluidic channel. By energizing the electrodes with a DC-biased (2.5 V) AC voltage (20 Vpp), an electrokinetic flow is induced with a flow profile perpendicular to that of the incoming laminar streams of liquids to be mixed. As a result, the flow lines of the incoming streams and the induced flow are forced to crossover and very efficient stirring and mixing at short mixing length can be achieved. The mixer can be operated from the AC-electroosmotic (ACEO) (sigma=1 mS/m, f=100 kHz) to the AC-electrothermal (ACET) (sigma=500 mS/m, f=500 kHz) flow regimes. The mixing efficiency in the ACEO regime was 92%, with a mixing length of 600 microm (Q=2 microL/min), an estimated mixing time of 69 ms and an induced ACEO flow velocity of approximately 725 microm/s. The mixing efficiency in the ACET regime was 65% for a mixing length of approximately 1200 microm. The mixer is efficient and suitable for mixing reagents in a fluid media from low to high conductivity as required in erse microfluidic applications.
Publisher: IOP Publishing
Date: 09-10-2015
Publisher: AIP Publishing
Date: 2007
DOI: 10.1063/1.2430502
Abstract: Measuring the electro-osmotic velocity distributions in microchannels is usually performed for steady-state electrokinetic flows. Characterizing time-dependent electrokinetic flows is of importance to the development of microfluidic devices such as rapid capillary electrophoretic separation system, ac pumps, novel micromixers, etc. In this paper, we use a micron-resolution particle image velocimetry (micro-PIV) based phase locking technique with an ordinary PIV charge coupled device (CCD) camera to carry out an experimental study of the transient electrokinetic flow in microchannels by synchronizing different trigger signals for the laser, CCD camera, and in-house designed high-voltage switch. With the transient micro-PIV technique, we further propose a method to decouple the particle electrophoretic velocity from the micro-PIV measured velocity and to determine the zeta potential of the channel wall. The time evolution of the full-field, electro-osmotic velocity distributions in both open- and closed-end rectangular microchannels is obtained. Using the slip velocity approach and the measured channel zeta potential, the theoretical predictions of the transient electro-osmotic flow in the open- and closed-end microchannels are obtained, and they are found to be in good agreement with the experimental results.
Publisher: Institute of Electrical and Electronics Engineers (IEEE)
Date: 03-2015
Publisher: ASMEDC
Date: 2009
Abstract: Capillary filling is the key phenomenon in planar chromatography techniques such as paper chromatography and thin layer chromatography. While capillary filling in channels of micrometers scale are experimentally verified that obeys well to Washburn’s law, there is evident show that the speed of capillary filling in nanochannels is noticeable lower than described by Washburn’s formula. This paper describes a model for capillary filling phenomenon in nanochannel. Experiments on the filling of electrolytic and nonelectrolytic solutions in polymeric nanochannels were carried out. The filling processes were observed and recorded. Filling distances were measured, from which filling speeds were derived. Formation of air bubbles was also observed with some channel’s geometry. A mathematical model to calculate the electroviscous effect was established. This model shows that contribution of electroviscous effect in the reduction of filling speed is small. This result also agrees well with other previous theoretical works on the electroviscous effect. That means beside electroviscous effect, there are other phenomena that contribute in the reduction of capillary filling speed in nanochannel, such as air bubbles formation. Those phenomena need to be described qualitatively and quantitatively in order to understand more on the capillary filling in nanochannel.
Publisher: Springer Science and Business Media LLC
Date: 12-2012
Publisher: ASME International
Date: 09-2015
DOI: 10.1115/1.4030240
Abstract: Electro-osmotic flow (EOF) is widely used in microfluidic systems. Here, we report an analysis of the thermal effect on EOF under an imposed temperature difference. Our model not only considers the temperature-dependent thermophysical and electrical properties but also includes ion thermodiffusion. The inclusion of ion thermodiffusion affects ionic distribution, local electrical potential, as well as free charge density, and thus has effect on EOF. In particular, we formulate an analytical model for the thermal effect on a steady, fully developed EOF in slit microchannel. Using the regular perturbation method, we solve the model analytically to allow for decoupling several physical mechanisms contributing to the thermal effect on EOF. The parametric studies show that the presence of imposed temperature difference/gradient causes a deviation of the ionic concentration, electrical potential, and electro-osmotic velocity profiles from their isothermal counterparts, thereby giving rise to faster EOF. It is the thermodiffusion induced free charge density that plays a key role in the thermodiffusion induced electro-osmotic velocity.
Publisher: American Chemical Society (ACS)
Date: 17-08-2007
DOI: 10.1021/AC070810U
Abstract: This study presents a new cell manipulation method using a moving dielectrophoretic force to transport or fractionate cells along a microfluidic channel. The proposed moving dielectrophoresis (mDEP) is generated by sequentially energizing a single electrode or an array of electrodes to form an electric field that moves cells continuously along the microchannel. Cell fractionation is controlled by the applied electrical frequency, and cell transportation is controlled by the interelectrode activation time. The applicability of this method was demonstrated to simultaneously fractionate and transport Saccharomyces cerevisiae yeast cells, both viable and nonviable, by operating at conditions under which the cells were subjected to positive and negative dielectrophoresis, respectively. Compared to the conventional dielectrophoresis (cDEP and traveling wave dielectrophoresis (twDEP), moving dielectrophoresis allows cells to be separated on the basis of the real part of the Clausius-Mossotti factor, as in cDEP, but yet allows the direct transportation of separated cells without using fluid flow, as in twDEP. This dielectrophoresis technique provides a new way to manipulate cells and can be readily implemented on programmable multielectrode devices.
Publisher: Springer Science and Business Media LLC
Date: 24-07-2013
Publisher: Wiley
Date: 03-2011
DOI: 10.1002/APJ.428
Publisher: Springer Science and Business Media LLC
Date: 29-01-2009
Publisher: Springer Science and Business Media LLC
Date: 28-07-2007
Publisher: Wiley
Date: 09-2009
Abstract: This study reports improved electrokinetically driven microfluidic T-mixers to enhance their mixing efficiency. Enhancement of electrokinetic microfluidic T-mixers is achieved using (i) an active approach of utilizing a pulsating EOF, and (ii) a passive approach of using the channel geometry effect with patterned blocks. PDMS-based electrokinetic T-mixers of different designs were fabricated. Experimental measurements were carried out using Rhodamine B to examine the mixing performance and the micro-particle image velocimetry technique to characterize the electrokinetic flow velocity field. Scaling analysis provides an effective frequency range of applied AC electric field. Results show that for a T-mixer of 10 mm mixing length, utilizing frequency modulated electric field and channel geometry effects can increase the mixing efficiency from 50 to 90%. In addition, numerical simulations were performed to analyze the mixing process in the electrokinetic T-mixers with various designs. The simulation results were compared with the experimental data, and reasonable agreement was found.
Publisher: World Scientific Pub Co Pte Lt
Date: 12-2008
DOI: 10.1142/S0219581X08005493
Abstract: This paper reports an experimental investigation into force convective heat transfer of nanofluids flowing through a cylindrical minichannel under laminar flow and constant wall heat flux conditions. S le nanofluids were prepared by dispersing different volumetric concentrations (0.2–0.8%) of nanoparticles in deionized water. The results showed that both the convective heat transfer coefficient and the Nusselt number of the nanofluid increase considerably with the nanoparticle volume fraction as well as the Reynolds number. Along with the enhanced thermal conductivity of nanofluids, the migration, interactions, and Brownian motion of nanoparticles and the resulting disturbance of the boundary layer are responsible for the observed enhancement of heat transfer coefficients of nanofluids.
Publisher: Elsevier BV
Date: 07-2015
Publisher: Springer Science and Business Media LLC
Date: 20-08-2018
DOI: 10.1038/S41598-018-31019-Y
Abstract: Rapidly evolving cell-based therapies towards clinical trials demand alternative approaches for efficient expansion of adherent cell types such as human mesenchymal stem cells (hMSCs). Using microcarriers (100–300 µm) in a stirred tank bioreactor offers considerably enhanced surface to volume ratio of culture environment. However, downstream purification of the harvested cell product needs to be addressed carefully due to distinctive features and fragility of these cell products. This work demonstrates a novel alternative approach which utilizes inertial focusing to separate microcarriers (MCs) from the final cell suspension. First, we systematically investigated MC focusing dynamics inside scaled-up curved channels with trapezoidal and rectangular cross-sections. A trapezoidal spiral channel with ultra-low-slope (Tan(α) = 0.0375) was found to contribute to strong MC focusing (~300 Re ~400) while managing high MC volume fractions up to ~1.68%. Accordingly, the high-throughput trapezoidal spiral channel successfully separated MCs from hMSC suspension with total cell yield~94% (after two passes) at a high volumetric flow rate of ~30 mL/min (Re~326.5).
Publisher: SPIE
Date: 21-01-2006
DOI: 10.1117/12.645917
Publisher: ASMEDC
Date: 2008
Abstract: In this paper, we report numerical and experimental studies of the Joule heating-induced heat transfer in fabricated T-shape microfluidic channels. We have developed comprehensive 3D mathematical models describing the temperature development due to Joule heating and its effects on electrokinetic flow. The models consist of a set of governing equations including the Poisson-Boltzmann equation for the electric double layer potential profiles, the Laplace equation for the applied electric field, the modified Navier-Stokes equations for the electrokinetic flow field, and the energy equations for the Joule heating induced conjugated temperature distributions in both the liquid and the channel walls. Specifically, the Joule number is introduced to characterize Joule heating, to account for the effects of the electric field strength, electrolyte concentration, channel dimension, and heat transfer coefficient outside channel surface. As the thermophysical and electrical properties including the liquid dielectric constant, viscosity and electric conductivity are temperature-dependent, these governing equations are strongly coupled. We therefore have used the finite volume based CFD method to numerically solve the coupled governing equations. The numerical simulations show that the Joule heating effect is more significant for the microfluidic system with a larger Joule number and/or a lower thermal conductivity of substrates. It is found that the presence of Joule heating makes the electroosmotic flow deviate from its normal “plug-like” profiles, and cause different mixing characteristics. The T-shape microfluidic channels were fabricated using rapid prototyping techniques, including the Photolithography technique for the master fabrication and the Soft Lithography technique for the channel replication. A rhodamine B based thermometry technique, was used for direct “in-channel” measurements of liquid solution temperature distributions in microfluidic channels, fabricated by the PDMS/PDMS and Glass/PDMS substrates. The experimental results were compared with the numerical simulations, and reasonable agreement was found.
Publisher: IEEE
Date: 06-2014
Publisher: Elsevier BV
Date: 10-2014
Publisher: ASMEDC
Date: 2009
Abstract: Nanofluidics is the science and technology involving a fluid flowing in or around structures with a least one dimension in the nanoscale, which is defined as the range from 1 nm to 100 nm. In this paper, we present the fabrication and characterization of nanochannels in silicon and glass. Since the lateral dimension of the channels is limited by the wavelength of UV light used in photolithography, the channel width can only be fabricated in the micrometer scale. However, the depth of the channel can be controlled precisely by etch rate of reactive ion etching (RIE). Microchannels and access holes were etched with deep reactive ion etching (DRIE). Both nanonochannel and microchannel were sealed by a Pyrex glass wafer using anodic bonding. The fabricated nanochannels were characterized by capillary filling and evaporation experiments. Due to the small channel height and weak fluorescent signal, fluorescent techniques are not suitable for the characterization of the nanochannels. A long exposure time due to the limited amount of fluorescent molecules inhibit the measurement of transient and dynamic processes. However, as the channel height shorter than all visible wavelengths, the contrast in refractive indices of air and liquid allow clear visualization of nanochannels filled with liquids. Automatic image processing with MATLAB allows the evaluation of capillary filling in nanochannels. Interesting phenomena and discrepancies with conventional theories are presented.
Publisher: AIP Publishing
Date: 11-01-2006
DOI: 10.1063/1.2162533
Abstract: We have demonstrated a transient micro particle image velocimetry (micro-PIV) technique to measure the temporal development of electroosmotic flow in microchannels. Synchronization of different trigger signals for the laser, the CCD camera, and the high-voltage switch makes this measurement possible with a conventional micro-PIV setup. Using the transient micro-PIV technique, we have further proposed a method on the basis of inertial decoupling between the particle electrophoretic motion and the fluid electroosmotic flow to determine the electrophoretic component in the particle velocity and the zeta potential of the channel wall. It is shown that using the measured zeta potentials, the theoretical predictions agree well with the transient response of the electroosmotic velocities measured in this work.
Publisher: Institution of Engineering and Technology (IET)
Date: 2006
Abstract: A novel microfluidic sensor for measuring dynamic gas-liquid interfacial tension is reported. The device consists of a microfluidic chip with a microchannel network and an optical detection system. The s le is introduced into a main channel, while air is injected through a T-junction. Owing to the fixed flow rate ratio used for the sensor, surface tension is the only parameter determining bubble formation frequency, which can be measured by optical detection. Although the bubble is represented by a pulse in the output signal, the formation frequency is simply the frequency of the output signal. Measurements were carried out for aqueous solutions with different concentrations of the ionic surfactant cetyl trimethyl ammonium bromide. Surface tensions of these solutions were calibrated with a commercial tensiometer. The measurement results show a clear relationship between surface tension and formation frequency. The sensor can be used to identify the critical micelle concentration of the surfactant. The sensor potentially allows the use of a minute amount of s le compared with the relatively large amount required for existing commercial systems.
Publisher: SPIE
Date: 28-12-2005
DOI: 10.1117/12.639035
No related grants have been discovered for Chun Yang.