Papers by Author: Yee Cheong Lam

Abstract: This paper presents an experimental investigation on the DC-biased AC-electrokinetic (ACEK) flow vortex in a rectangular microchannel. This is a new flow phenomenon in AC-electrokinetics and had been successfully utilized for microfluidic mixing as reported in our previous work. The microchannel is aligned parallel to a pair of energized coplanar microelectrodes such that the generated flow vortex is perpendicular to the pressure-driven flowing samples. With the application of a DC bias AC voltage, incipient Faradaic reactions occur above the electrodes and thus producing ions and creating a net dominant flow vortex. Further investigation to understand the generation of the DC-biased ACEK flow was conducted. This is performed by direct visualization of such flow vortex formation from the cross-section of a rectangular microchannel.

Abstract: Two phenomena for flows in microchannels, namely visco-elastic instability and Faradaic charging effects, are discussed. Aqueous solutions, which do not exhibit elastic behaviors, will behave visco-elastically with the addition of a small amount of polymers. Visco-elastic flow instability which promote mixing has been observed in the flow of these visco-elastic solutions through a constriction in microchannels. In addition, we recently show experimentally the existance of Faradaic charging in AC-electrokinetic (ACEK) flow. Lastly, surface roughness effects in micro- plastic injection molding are highlighted. The micro-injection molding process involves the flow of polymer melt in micro-cavities. Our recent investigations demonstrate that the flow model for macro-cavities could be employed satisfactorily to simulate the flow in micro-cavities if roughness effects are considered in an appropriate manner.

Abstract: UV micro-casting is a promising mass production method for replication of polymeric microdevices due to the non-stringent process conditions and fast curing time. This paper describes a potential method to mass produce polymeric microdevices. The first generation mold for UV micro-casting was fabricated by using chemically micro-etched copper clad laminate (CCL) base substrate. Subsequently a two part silicone rubber was cast over the CCL micro-feature mold. Photosensitive resin was dispensed onto the silicone rubber mold and a transparent Mylar thin film was placed on top of the UV curable prepolymer. After the silicone rubber mold-resin-Mylar assembly was UV irradiated for tens of seconds, the crosslinked polymer, together with the Mylar film was peeled off from the mold. The cross-linked polymer was placed on top of a second layer of Mylar film dispensed with the similar UV curable resin. In this way, a complete polymeric micro device could be efficiently produced.

Abstract: The effects of compatibilization and mould geometry on the fibrillation of liquid crystalline polymer were investigated in this study. The blend is composed of polycarbonate (PC) and LC5000, a thermotropic liquid crystalline polymer consisting of 80/20 wt% of hydroxybenzoic acid and poly(terephthalate). The effect of compatibilization was investigated by studying the morphological properties of injection moulded plaques and significant enhancement in the fibrillation of LCP was observed in the moulded samples after addition of the compatibilizer. The effect of mould geometry was studied by comparing results obtained from three types of specimens, namely, specimens cut from large rectangular plaques, moulded rectangular bar specimens and molded dog-bone specimens. It was found that mould geometry had a direct influence on the magnitude of the elongational and shear stresses in the melt during molding and thus affected the degree of LCP fibrillation. Moreover, it was observed that the constriction at the “neck” of the dogbone geometry actually facilitated development of LCP fibre in the blend by increasing shear stress during moulding and promoting LCP fibre coalescence.

Abstract: The flow of a suspension system with glass microspheres in polymer EVA (Ethylene Vinyl Acetate) melts system was studied in a capillary rheometer. The slip velocity was determined by Mooney technique. A modified slip law describing the slip velocity as a function of the wall shear stress and particle concentration was proposed and employed to describe the flow behavior of the suspension system.