Microfluidic temperature gradient focusing by electrokinetically induced joule heating

Microfluidics has been developed over the past decade due to its promising applications in biotechnology, medicine, and chemistry. Towards these applications, enhancing concentration sensitivity and detection resolution are needed to meet the detection limits because of the very dilute solution, ultra-small volumes and short detection length in micro-devices. A variety of microfluidic concentration techniques of analytes has been developed. Among them, temperature gradient focusing (TGF) is a recently developed field gradient focusing technique. It concentrates and separates the sample analytes at their stagnant points, where the analyte’ electrophoretic motion is balanced against the bulk flow of buffer solution. The simple structure and the high concentration in a relatively short channel make TGF well suitable for integrated microfluidic systems. Furthermore, using Joule heating for generating temperature gradient shows more advantages than using external heating. The former consumes less power, simplifies the device construction and operation, and makes the whole device more portable without need of bulky external heating unit. This dissertation presents a fundamental, systematic and in-depth investigation on Joule heating induced TGF to provide better understanding of the physical processes associated with TGF and to improve the concentration enhancement. TGF of sample solutes was studied in microchannels with a sudden expansion the cross-section area. A comprehensive mathematical model was developed to describe Joule heating induced TGF under either a sole DC field or a combined AC with DC electric field. The parametric effects of applied voltage, buffer concentration and channel width ratio were studied and summarised by using a dimensionless Joule number. The general trend shows that increasing the Joule number can enhance the TGF, and thus improve concentration efficiency. Moreover, TGF in microdevices made of two different substrates, PDMS and Glass, was compared. The results showed that the device with smaller thermal conductivity would enhance the concentration effect. The proposed numerical model gave reasonable agreement with the experimental data. A concentration enhancement of 450-fold was obtained within 75 seconds by appropriately choosing the parameters in the PDMS/PDMS micro-device. Analyses showed that the performance of Joule heating induced TGF is limited due to the induced hydrostatic backpressure. An improved electrokinetic technique using a combined AC and DC field was presented. Effect of reservoir size was also studied. The induced backpressure can be greatly reduced by using a combined AC and DC field with a larger reservoir size. Thus, the residence time of sample analytes in the narrow channel region can be increased and a higher concentration enhancement factor can be achieved. Finally, by imposing a combined 10 kHz AC (with 400 V) and 450 V DC field over a 10 mm long PDMS channel with two square-shaped reservoirs of 16 mm × 16 mm, concentration enhancement of more than 4500-fold of 0.05 µM Fluorescein-Na solute dissolved in a 180 mM Tris-borate buffer was achieved within 14 min. Such concentration enhancement factor is more than one order of magnitude higher than that ever reported by existing literature studies using the Joule heating induced microfluidic TGF technique. Electrokinetic induced Joule heating for microfluidic TGF technique has been extended for concentration of micro- and nano-particles, and experimental investigations were performed in microchannels with a convergent-divergent structure. Effects of applied voltage and particle size were studied. Velocity and temperature profiles were analysed to illustrate the focusing mechanisms. The dielectrophoretic (DEP) and thermophoretic (TP) velocities are found to have the same magnitude compared with the sum of the electrophoretic velocity and bulk velocity near the constriction section, suggesting considerable effects of DEP and TP on TGF of microparticles in terms of both focusing location and concentration enhancement. Finally, an efficient and rapid concentration enhancement of DNA is demonstrated using the improved TGF technique. A peak of 480-fold concentration enhancement of ssDNA analytes was achieved within 40 s when a combined 283 V AC with 450 V DC was applied to a 10 mm long PDMS/PDMS microchannel. Such concentration speed is comparable or even faster than the performance of microfluidic focusing of DNA by other techniques, such as DEP trapping or the thermophoresis trapping. Moreover, the focused DNA band is as narrow as 0.2 mm, making it suitable for integration into a microfluidic chip for on-line analysis.Doctor of Philosophy (MAE