Keisuke Horiuchi

Thermal Characteristics of Mixed Electroosmotic and Pressure-Driven Microflows

Analytical solutions for the temperature distribution, heat transfer coefficient, and Nusselt number of steady electroosmotic flows with an arbitrary pressure gradient are obtained for two-dimensional straight microchannels. The thermal analysis considers interaction among advective, diffusive, and Joule heating terms in order to obtain the thermally developing behavior of mixed electroosmotic and pressure-driven flows with isothermal boundary conditions. Heat transfer characteristics are obtained for low Reynolds number microflows where the viscous and electric field terms are very dominant. The electroosmotic component of the flow velocity is modelled with Helmholtz-Smoluchowski slip velocity, and the mixed flow velocity is presented as linear superposition of pure electroosmotic velocity and plane Poiseuille velocity. In mixed flow cases, the governing equation for energy is not separable in general. Therefore, we introduced a method that considers the extended Graetz problem. Analytical results show that the heat transfer coefficient of mixed electroosmotic and pressure-driven flow is smaller than that of pure electroosmotic flow. For the parameter range studied here (Re<0.7), the fully developed Nusselt number is independent of the thermal Peclet number for both pure electroosmotic and mixed electroosmotic-pressure driven microflows. In mixed electroosmotic and pressure-driven flows, the thermal entrance length increases with the imposed pressure gradient.

Electrokinetic flow control in microfluidic chips using a field-effect transistor

A field-effect transistor is developed to control flow in microfluidic chips by modifying the surface charge condition. In this investigation, zeta potential at a particular location is altered locally by applying a gate voltage, while zeta potential at other locations is maintained at its original value. This non-uniform zeta potential results in a secondary electroosmotic flow in the lateral direction, which is used for flow control in microgeometries. Here, microchannel structures and field-effect transistors are formed on polydimethylsiloxane (PDMS) using soft lithography techniques, and a micro particle image velocimetry technique is used to obtain high resolution velocity distribution in the controlled region. The flow control is observed at relatively low gate voltage (less than 50 V), and this local flow control is primarily due to current leakage through the interface between PDMS and glass layers. A leakage capacitance model is introduced to estimate the modified zeta potential for the straight channel case, and excellent agreement is obtained between the predicted and experimental zeta potential results. This leakage-current based field-effect is then applied to a T-channel junction to control flow in the branch channel. Experiments show that the amount of discharge in the branch channel can be controlled by modulating gate voltage.

Heat Transfer Characteristics of Steady Electroosmotic Flows in Two-Dimensional Straight Microchannels

Analytical solutions for the temperature distributions, heat transfer coefficients and Nusselt numbers of steady electroosmotic flows are obtained for two-dimensional straight micro-channels. This analysis is based on infinitesimal electric double layer (EDL) in which flow velocity becomes “plug-like” uniform except very close to the wall. Both constant surface temperature and constant surface heat flux conditions are considered in this study. Separation of variables techniques are applied to obtain analytical solutions of temperature distributions from the energy equation in which Joule heating is a significant contributor due to the applied electric field. The thermal analysis considers interaction among inertial, diffusive and joule heating terms in order to obtain the thermally developing behavior of electroosmotic flows. Heat transfer characteristics are presented for low Reynolds number microflows where the viscous and electric field terms are very dominant. For the parameter range studied here (Re ≤ 0.7), the Nusselt number is independent of the thermal Peclet number, except in the thermally developing region. In both isothermal and constant surface heat flux boundary conditions, the Nusselt number becomes constant in the fully developed region for a uniform volumetric heat generation. Analytical results for no Joule heating cases are also compared with the classical heat transfer results, and in the thermally fully developed region an excellent agreement is obtained between them.© 2003 ASME

Multistage Isoelectric Focusing: A Novel On-Chip Bio-Separation Technique

This experimental study reports a method to increase the resolving power of isoelectric focusing (IEF) on a polymeric microfluidic chip. Microfluidic chip is formed on poly-di-methyl siloxane (PDMS) using soft lithography and multilayer bonding technique. In this novel bioseparation technique, IEF is staged by first focusing protein species in a straight channel using broad-range ampholytes and then refocusing segments of that first channel into secondary channels that branch out from the first one. Experiments demonstrated that three fluorescent protein species within a segment of pH gradient in the first stage were refocused in the second stage with much higher resolution in a shallower pH gradient. A serially performed two-stage IEF was completed in less than 25 minutes under particularly small electric field strength up to 100 V/cm.Copyright

Heat Transfer Characteristics of Mixed Electroosmotic and Pressure Driven Flows Under Constant Heat Flux

Analytical solution for the heat transfer characteristics of steady electroosmotic flows with an arbitrary pressure gradient are obtained for two-dimensional straight micro-channels. Both thermally developing and fully developed regions are considered for hydraulically fully developed mixed flows under isoflux channel wall conditions. In mixed flow cases, the governing equation for energy is not separable in general. Therefore, we introduced a new method that considers the extended Graetz problem. Heat transfer characteristics are presented for low Reynolds number micro-flows where the viscous and electric field terms are very dominant. Our analytical techniques are verifled by obtaining an excellent agreement with existing literature for slug, pressure driven, and pure electroosmotic flow cases. In the fully developed region, the Nusselt number of mixed flow is independent of the thermal Peclet number for a particular Joule heat and imposed surface heat flux. The entry length of mixed flow significantly depends on the applied pressure gradient to the electroosmotic flow.Copyright

Thermal Analysis of Mixed Electroosmotic and Pressure Driven Flows in Two-Dimensional Straight Microchannels

Analytical solution for the temperature distributions, heat transfer coefficients, and Nusselt numbers of steady electroosmotic flows with an arbitrary pressure gradient are obtained for two-dimensional straight micro-channels. The thermal analysis considers interaction among inertial, diffusive and Joule heating terms in order to obtain the thermally developing behavior of mixed electroosmotic and pressure driven flows. In mixed flow cases, the governing equation for energy is not separable in general. Therefore, we introduced a new method that considers the extended Graetz problem. Heat transfer characteristics are presented for low Reynolds number micro-flows where the viscous and electric field terms are very dominant. Analytical results show that the heat transfer coefficient of mixed-electroosmotic and pressure driven flow is smaller than that of pure electroosmotic flow. For the parameter range studied here (Re<0.7), the fully developed Nusselt number is independent of the thermal Peclet number and pressure gradient. Moreover, in mixed electroosmotic and pressure driven flows, the thermal entrance length increases with the imposed pressure gradient.Copyright

Development of Microfluidic Flow Sensor in a Polymeric Microchip

A microfluidic flow sensor has been developed to precisely measure the flow rate in a micro/nanofluidic channel for lab-on-a-chip applications. Mixed electroosmotic and pressure driven microflows are investigated using this sensor. Our microflow sensor consists of two components: fluidic circuit and electronic circuit. The fluidic circuit is embedded into the microfluidic chip, which is formed during the microfabrication sequences. On the other hand, the electronic circuit is a microelectronic chip that works as a logical switch. We have tested the microflow sensor in a hybrid poly di-methyl-siloxane (PDMS)-glass microchip using de-ionized (DI) water. Softlithography techniques are used to form the basic microflow structure on a PDMS layer, and all sensing electrodes are deposited on a glass plate using sputtering technique. In this investigation, the microchannel thickness is varied between 3.5 and 10 microns, and the externally applied electric field is ranged between 100V/mm and 200V/mm. The thickness of the gold electrodes is kept below 100nm, and hence the flow disturbance due to the electrodes is very minimal. Fairly repeatable flow results are obtained for all the channel dimensions and electric fields. Moreover, for a particular electric field strength, there is an appreciable change in the flow velocity with the change of the channel thickness.Copyright

Band Deformation at a T-Junction While Electrofocusing in a Dog-Leg Microchannel

On-chip isoelectric focusing (IEF) has been performed in both straight and dog-leg microchannels. Three-dimensional microfluidic chips were fabricated on poly di-methyl-siloxane (PDMS) using soft lithography and multilayer bonding techniques. Plasma oxidized PDMS channel surfaces were dynamically coated with methyl cellulose to discourage electroosmotic flow during separation and purification processes. In a straight microchannel, IEF was completed within 5 minutes at an applied electric field strength of 50 V/cm using broad range ampholytes. The focused bands were generally well-formed with sharp edges and were less than 100 microns across yielding a putative peak capacity in excess of 100 peaks in a 2-cm long channel. However, the conventional IEF protocol shifts the focused bands toward the cathodic well. This cathodic drift can be effectively minimized by placing highly viscous polymer solutions in the electrode reservoirs. In dog-leg microchannels, initially well formed focused band dispersed at the Tee-channel junction, but refocused at the dog-leg channels with relatively lower resolution.Copyright

High Resolution Separation of Proteins in a Polymeric Micro-Fluidic Chip

An integrated micro-fluidic chip has been developed using Poly-di-methyl siloxane (PDMS) to separate proteins by isoelectric focusing (IEF). Soft lithography techniques, which offer rapid prototyping, easy multilayer fabrication, mass production capability and biocompatibility, were utilized to fabricate various parts of the micro-fluidic chip. Separately molded PDMS layers were bonded together to form three-dimensional microfluidic chips. The microfluidic chips were prepared for IEF by conditioning the channel with 1 M NaOH and then loading it with a solution of fluorescent proteins made using 0.4% MC, 4% broad-range ampholyte and 0.018 mg/ml protein in 18 MOhm water. Relatively large reservoirs on the acidic and basic ends of the channel were filled with anolyte (50 mM phosphoric acid) and catholyte (50 mM sodium hydroxide), respectively, and then current was applied along the axis of the channel until one or more bands of protein focused, usually in just a few minutes even at relatively low voltages. The focused bands were generally well-formed with sharp edges and were less than 100 microns across yielding a putative peak capacity in excess of 100 peaks in a 2-cm long channel.© 2003 ASME