Gongyue Tang

Numerical modeling of Joule heating‐induced temperature gradient focusing in microfluidic channels

Temperature gradient focusing (TGF) is a recently developed technique for spatially focusing and separating ionic analytes in microchannels. The temperature gradient required for TGF can be generated either by an imposed temperature gradient or by Joule heating resulting from an applied electric field that also drives the flow. In this study, a comprehensive numerical model describing the Joule heating induced temperature development and TGF is developed. The model consists of a set of governing equations including the Poisson–Boltzmann equation, the Laplace equation, the Navier–Stokes equations, the energy equations and the mass transport equation. As the thermophysical and electrical properties including the liquid dielectric constant, viscosity, and electric conductivity are temperature‐dependent, these governing equations are coupled, and therefore the coupled governing equations are solved numerically by using a CFD‐based numerical method. The numerical simulations agree well with the experimental results, suggesting the valid mathematical model presented in this study.

On Electrokinetic Mass Transport in a Microchannel With Joule Heating Effects

We present 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 sample plug

Geometry Effect on the Electrokinetic Instability of the Electroosmotic Flow in Microfluidic Channels

To study the effect of geometry on electroosmotic flow in micro channels, we fabricated PDMS-glass microchannels of different designs, which have patterned channels with abrupt contraction of different sizes. Using fluorescent imaging technology, we demonstrated the effect of geometry on the instability of DC driven electroosmotic flow in microfluidic channels. For certain geometry and conductivity of the electrolyte solution (Sodium Bicarbonate), there is a threshold voltage for electroosmotic instability, exhibiting itself as “ripple”. Generally, the factors which affect the threshold voltage include channel width, channel geometry, and electrolyte conductivity. Narrower channel resulted in higher onset voltage. As conductivity of the electrolyte increases, the threshold voltage tends to increase. Early transition to unstable electroosmotic flow in microfluidic channels was observed under relatively low Re.Copyright

Joule Heating Induced Heat Transfer and Its Effects on Electrokinetic Mixing in T-Shape Microfluidic Channels

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.Copyright

Joule Heating Induced Thermal and Hydrodynamic Development in Microfluidic Electroosmotic Flow

Joule heating is present in electrokinetically driven flow and mass transport in microfluidic systems. Specifically, in the cases of high applied voltages and concentrated buffer solutions, the thermal management may become a problem. In this study, a mathematical model is developed to describe the Joule heating and its effects on electroosmotic flow and mass species transport in microchannels. The proposed model includes the Poisson equation, the modified Navier-Stokes equation, and the conjugate energy equation (for the liquid solution and the capillary wall). Specifically, the ionic concentration distributions are modeled using (i) the general Nernst-Planck equation, and (ii) the simple Boltzmann distribution. These governing equations are coupled through temperature-dependent phenomenological thermal-physical coefficients, and hence they are numerically solved using a finite-volume based CFD technique. A comparison has been made for the results of the ionic concentration distributions and the electroosmotic flow velocity and temperature fields obtained from the Nernst-Planck equation and the Boltzmann equation. The time and spatial developments for both the electroosmotic flow fields and the Joule heating induced temperature fields are presented. In addition, sample species concentration is obtained by numerically solving the mass transport equation, taking into account of the temperature-dependent mass diffusivity and electrophoresis mobility. The results show that the presence of the Joule heating can result in significantly different electroosomotic flow and mass species transport characteristics.Copyright

Electroosmotic Flow and Mass Species Transport in a Microcapillary Under Influences of Joule Heating

This study presents a numerical analysis of Joule heating effect on the electroosmotic flow and species transport, which has a direct application in the capillary electrophoresis based BioChip technology. A rigorous mathematic model for describing the Joule heating in an electroosmotic flow including Poisson-Boltzmann equation, modified Navier-Stokers equations and energy equation is developed. All these equations are coupled together through the temperature-dependent parameters. By numerically solving aforementioned equations simultaneously, the electroosmotic flow field and the temperature distributions in a cylindrical microcapillary are obtained. A systematic study is carried out under influences of different geometry sizes, buffer solution concentrations, applied electric field strengths, and heat transfer coefficients. In addition, sample species transport in a microcapillary is also investigated by numerically solving the mass transfer equation with consideration of temperature-dependant diffusion coefficient and electrophoresis mobility. The characteristics of the Joule heating, electroosmotic flow, and sample species transport in microcapillaries are discussed. The simulations reveal that the presence of the Joule heating could have a great impact on the electroosmotic flow and sample species transport.Copyright