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Joule heating is inevitable when an electric field is applied across a conducting medium. It would impose limitations on the performance of electrokinetic microfluidic devices. This article presents a 3-D mathematical model for Joule heating and its effects on the EOF and electrophoretic transport of solutes in microfluidic channels. The governing equations were numerically solved using the finite-volume method. Experiments were carried out to investigate the Joule heating associated phenomena and to verify the numerical models. A rhodamine B-based thermometry technique was employed to measure the solution temperature distributions in microfluidic channels. The microparticle image velocimetry technique was used to measure the velocity profiles of EOF under the influence of Joule heating. The numerical solutions were compared with experimental results, and reasonable agreement was found. It is found that with the presence of Joule heating, the EOF velocity deviates from its normal "plug-like" profile. The numerical simulations show that Joule heating not only accelerates the sample transport but also distorts the shape of the sample band.

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Air bubble formation during polymerase chain reaction (PCR) thermocycling in microreactors has been reported as one of the major causes for PCR failure. In this paper we investigate the locations, mechanisms and other characteristics of the micro bubble formation inside a PCR microreactor array chip made by polydimethylsiloxane (PDMS) bonded with glass. The bubble formation is found to be strongly related to the micro features inside the microreactors and inside the chip bonding interface, especially near the inner corners of the microreactors, which are dependent on the micro-fabrication methods used. Gas permeability of PDMS and the wetting property of PCR sample also have influence on the air bubble formation. After investigation of various methods to control the bubble formation, we present the two most viable ones through micro bubble absorption and chip bonding interface modification. Finally, a bubble-free PCR in PDMS microreactors is demonstrated, in which the micro bubbles are suppressed with a bonding interface cladding technique.

We present a novel process (through cutting and pattern transfer processes) for rapidly prototyping polydimethylsiloxane (PDMS) microfluidic structures without a replication template using a CO2 laser. The process typically takes less than 30 min to make a PDMS microfluidic chip from idea to device. In addition to time saving, the process also drastically cuts down equipment and operating costs by eliminating the use of masks, templates, wafer fabrication equipment and consumables needed in the template-making process. We further demonstrate the capability of the process in the rapid prototyping of a variety of microstructures from a 2 µm thin layer up to a 3.6 mm high structure on a single PDMS layer with accurate thickness control as well as smooth top and bottom surfaces. Various process characteristics and challenges for the PDMS laser prototyping process are addressed in this note.