Parallel Plate Microchannel Research Articles

Entropy Generation Due to Laminar Incompressible Forced Convection Flow Through Parallel-Plates Microchannel

The entropy generation due to steady laminar forced convection fluid flow through parallel plates microchannel is investigated numerically. The effect of Knudsen, Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference on entropy generation within the microchannel is discussed. The fraction of the entropy generation due to heat transfer to the total entropy generation within the microchannel is studied in terms of Bejan number. The entropy generation within the microchannel is found to decrease as Knudsen number increases, and it is found to increase as Reynolds, Prandtl, Eckert numbers and the nondimensional temperature difference increase. The contribution of the viscous dissipation in the total entropy generation increases as Knudsen number increases over wide ranges of the flow controlling parameters.

Conformation and dynamics of single DNA molecules in parallel-plate slit microchannels

The conformation and diffusion of a single DNA molecule confined between two parallel plates are examined using both single molecule experiments and Brownian dynamics simulations accounting for hydrodynamic interactions. The degree of chain stretching and the diffusivity are characterized as a function of the chain confinement and the channel geometry. Good agreement is found between the simulations, experiments, and scaling theory predictions.

Flow Field Effect on Electric Double Layer during Streaming Potential Measurements

It is typically assumed that flow field has very little or no effect on the electric double layer distribution. Here, we study this effect by means of a dilute electrolyte (deionized water) in a streaming potential mode for a 40-μm parallel-plate microchannel. By measuring the electrical potential downstream along the channel surface, we have shown that the potential distribution along a streamwise direction can be nonlinear. By means of a current continuity equation, we also presented numerical simulation results of potential distribution for pressure-driven flow with electrokinetic effects. The simulated results agree well with those from experiment. Results indicate that the conduction current in the streamwise direction is only a fraction of the streaming current. As a result, the ζ-potential values determined from streaming potential measurements are generally smaller than those from streaming current measurements. It is expected that this discrepancy would be smaller for a more concentrated electrolyte.

Analytical treatment of electrokinetic microfluidics in hydrophobic microchannels

Microfluidics in microelectromechanical systems (MEMS) and nanoelectromechanical systems (NEMS) devices is complex due to the large surface area to volume ratio. Thus, surface properties play an important role in flow behavior. In this paper, we summarize the effects of electric double layer and surface hydrophobicity of rectangular microchannels on time-dependent electrokinetic flow. Theoretically, we have shown that flow resistance can, in principal, be significantly reduced so that a larger flow rate can be obtained for pressure-driven flow or electric-field-driven flow. This relies on the ability to change surface charges and surface hydrophobicity independently. Our theoretical results provide guidelines for the design and operation of microfluidic flow in rectangular microchannels. Because of liquid slippage, zeta potential determination by traditional method could be overestimated. Taking into account the effect of hydrophobicity, a modified method is proposed to determine the zeta potential and slip coefficient for parallel-plate microchannels with hydrophobic surfaces.

The effect of viscous dissipation in thermally fully-developed electro-osmotic heat transfer in microchannels

The influence of viscous dissipation on thermally fully-developed, electro-osmotically generated flow has been analyzed for a parallel plate microchannel and circular microtube under imposed constant wall heat flux and constant wall temperature boundary conditions. Such a flow is established not by an imposed pressure gradient, but by a voltage potential gradient along the length of the tube. The result is a combination of unique electro-osmotic velocity profiles and volumetric heating in the fluid due to the imposed voltage gradient. For large ratio of the microtube radius (or microchannel half-width) to Debye length, the wall-normal fluid velocity gradients can be extremely high, which has the potential for significant viscous heating. The solution for the fully-developed, dimensionless temperature profile and corresponding Nusselt number have been determined for both geometries and for both thermal boundary conditions. It is shown that three dimensionless parameters govern the thermal transport: the relative duct radius (ratio of the duct radius or plate gap half-width to Debye length), the dimensionless volumetric source (ratio of Joule heating to wall heat flux), and a dimensionless parameter that relates the magnitude of the viscous heating to the Joule heating. Surprisingly, it is shown that the influence of viscous dissipation is only important at low values of the relative duct radius. For magnitudes of the dimensionless parameters which characterize most practical electro-osmotic flow applications, the effect of viscous dissipation is negligible.

Effect of liquid slip in electrokinetic parallel-plate microchannel flow

Liquid slip at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in parallel-plate microchannels by combining the electrokinetic transport phenomena with Navier's slip condition. Our parametric results suggest that electrokinetic transport phenomena and liquid slip at channel walls are both important and should be considered simultaneously. Their significance depends on channel wall material, electrolyte concentration, and pH. For pressure-driven-flow, liquid slip counteracts the effect by the electrical double layer and induces a larger flow rate. A higher apparent viscosity would be predicted if slip is neglected. For electroosmotic flow, liquid slip alters the flow rate by about 20% for a thick electrical double layer. Our results provide design guidelines to precisely control time-dependent microflow in hydrophobic microfluidic microelectromechanical system devices.

Fully developed electro-osmotic heat transfer in microchannels

Thermally fully developed, electro-osmotically generated convective transport has been analyzed for a parallel plate microchannel and circular microtube under imposed constant wall heat flux and constant wall temperature boundary conditions. Such a flow is established not by an imposed pressure gradient, but by a voltage potential gradient along the length of the tube. The result is a combination of unique electro-osmotic velocity profiles and volumetric heating in the fluid due to the imposed voltage gradient. The exact solution for the fully developed, dimensionless temperature profile and corresponding Nusselt number have been determined analytically for both geometries and both thermal boundary conditions. The fully developed temperature profiles and Nusselt number are found to depend on the relative duct radius (ratio of the Debye length to duct radius or plate gap half-width) and the magnitude of the dimensionless volumetric source.

Laminar heat transfer of gas in a parallel‐plate microchannel with one wall temperature constant and the other adiabatic

AbstractBased on “wall‐adjacent layer” effect, the convective heat transfer for developed laminar flow of gas through parallel‐plate microchannel with one wall temperature constant and the other adiabatic was analyzed theoretically. Considering the change in thermal conductivity and viscosity of gas in the region adjacent to the solid wall, mathematical models were built and the dimensionless temperature distribution and the corresponding heat transfer characteristics were simulated numerically. The results indicate that the laminar heat transfer coefficient is less than that of the larger passages, while the dimensionless temperature is greater than that of the larger passages. Compared with the monotonous boundary conditions, the varied heat transfer coefficient amplitude of mixed boundary condition is lower. © 2002 Wiley Periodicals, Inc. Heat Trans Asian Res, 32(1): 58–64, 2003; Published online in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/htj.10075

A New Method to Determine Zeta Potential and Slip Coefficient Simultaneously

Liquid slip at hydrophobic surfaces in microchannels has frequently been observed. We present here an analytical solution for oscillating flow in a parallel-plate microchannel by combining electrokinetic transport phenomena with Navier's slip condition. Results suggest that the apparent and true zeta potential for a typical electrolye solution can differ by more than an order of magnitude with only a 1% slip. From our analysis, we propose a new method to measure simultaneously the true zeta potential and slip coefficient.

The ζ-Potential of Glass Surface in Contact with Aqueous Solutions

In this paper, the streaming potential technique is utilized to measure the ζ-potential of glass surface in contact with a variety of aqueous solutions. A flow cell made of a parallel-plate microchannel is devised to conduct the streaming potential measurements. Particular efforts are made to characterize the well-recognized effect of surface conductance and to determine its role in the electrokinetic phenomenon. Instead of determining the ζ-potential directly from the classical streaming potential equation, an alternative strategy is employed to determine the ζ-potential and the surface conductivity λs at the glass–water interfaces simultaneously from the streaming potential equation in which the surface conductance is accounted for. A series of measured streaming potential data for the parallel-plate microchannels of different gaps is acquired and used to fit the streaming potential equation. This method is then applied to study the effects of surface conductance, pH values, ionic features, concentrations of two electrolytes, and two ionic surfactants on the measured ζ-potential at the solid–liquid interfaces.

Interaction of neoplastic cells with glass surface under flow conditions

The adhesion to glass of L 1210 cells flowing in transparent parallel plate microchannel was studied by a cinematographic method. Most cells settle on the surface when their velocity immediately preceding attachment does not exceed approx. 100 μm/sec, the greatest adhesion rate accompanying relatively small velocities. The arrest of cells on the glass surface is either permanent or temporary and in a certain range of fluid velocities numerous cells are arrested several times consecutively for brief periods. Two types of surface attachment may be distinguished: cells are either totally immobilized on the surface (firm adhesion) or are able to perform under the influence of the fluid impulses some movements around the attachment site (loose adhesion). When the adherent cells are subjected to the shearing force of rapidly flowing fluid, they detach from the surface, the tearing away being frequently preceded by an accelerating gliding movement. The influence of hydrodynamic forces on the cell-surface interaction and adhesion processes is discussed, as well as some problems concerning possible mechanisms of the cell binding to the surface under dynamic conditions.