Microfluidic concentration enhancement of bio-analyte by temperature gradient focusing via Joule heating by DC plus AC field: A numerical approach

We present a detailed numerical analysis of electrophoresis induced concentration of a bio-analyte facilitated by temperature gradient focusing in a phosphate buffer solution via Joule heating inside a converging-diverging microchannel. The purpose is to study the effects of frequency of AC field and channel width variation on the concentration of target analyte. We tune the buffer viscosity, conductivity and electrophoretic mobility of the analyte such that the electrophoretic velocity of the analyte locally balances the electroosmotic flow of the buffer, resulting in a local build-up of the analyte concentration in a target region. An AC field is superimposed on the applied DC field within the microchannel in such a way that the back pressure effect is minimized, resulting in minimum dispersion and high concentration of the target analyte. Axial transport of fluorescein-Na in the phosphate buffer solution is controlled by inducing temperature gradient through Joule heating. The technique leverages the fact that the buffer's ionic strength and viscosity depends on temperature, which in turn guides the analyte transport. A numerical model is proposed and a finite element-based solution of the coupled electric field, mass, momentum, energy and species equations are carried out. Simulation predict peak of 670-fold concentration of fluorescein-Na is achieved. The peak concentration is found to increase sharply as the channel throat width, while the axial spread of concentrated analyte increases at lower frequency of AC field. The results of the work may improve the design of micro concentrator.

Study of Liquid-Metal Based Heating Method for Temperature Gradient Focusing Purpose

Temperature gradient focusing (TGF) is a highly efficient focusing technique for the concentration and separation of charged analytes in microfluidic channels. The design of an appropriate temperature gradient is very important for the focusing efficiency. In this study, we proposed a new technique to generate the temperature gradient. This technique utilizes a microchannel filled with liquid-metal as an electrical heater in a microfluidic chip. By applying an electric current, the liquid-metal heater generates Joule heat, forming the temperature gradient in the microchannel. To optimize the temperature gradient and find out the optimal design for the TGF chip, numerical simulations on four typical designs were studied. The results showed that design 1 can provide a best focusing method, which has the largest temperature gradient. For this best design, the temperature is almost linearly distributed along the focusing microchannel. The numerical simulations were then validated both theoretically and experimentally. The following experiment and theoretical analysis on the best design also provide a useful guidance for designing and fabricating the liquid-metal based TGF microchip.

Design of Liquid-Metals Heating-Induced Temperature Gradient Focusing in Microfluidic Channels

Temperature gradient focusing (TGF) is a highly efficient focusing technique for the concentration and separation of charged analytes in microfluidic channels. Design and control of an appropriate temperature gradient are very important for protein concentration and separation. In this study, we propose a new technique to generate the temperature gradient for the focusing purpose. This technique utilizes a microchannel filled with liquid-metals as a heater in the microfluidic chip. By applying electrical current, the liquid-metal microchannel generates Joule heat to form temperature gradient in the microfluidic chip. To optimize the temperature gradient, several typical designs were investigated. The results show the best design which provides the largest linear temperature gradient. The parametric studies present a clear guideline for designing aTGF microfluidic chip.