The aim of microfluidic mixing is to achieve a thorough and rapid mixing of multiple samples in microscale devices. In such devices, sample mixing is essentially achieved by enhancing the diffusion effect between the different species flows. Broadly speaking, microfluidic mixing schemes can be categorized as either “active”, where an external energy force is applied to perturb the sample species, or “passive”, where the contact area and contact time of the species samples are increased through specially-designed microchannel configurations. Many mixers have been proposed to facilitate this task over the past 10 years. Accordingly, this paper commences by providing a high level overview of the field of microfluidic mixing devices before describing some of the more significant proposals for active and passive mixers.

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A commercially available CO 2 laser scriber is used to perform the direct-writing ablation of polymethylmethacrylate (PMMA) substrates for microfluidic applications. The microfluidic designs are created using commercial layout software and are converted into the command signals required to drive the laser scriber in such a way as to reproduce the desired microchannel configuration on the surface of a PMMA substrate. The aspect ratio and surface quality of the ablated microchannels are examined using scanning electron microscopy and atomic force microscopy surface measurement techniques. The results show that a smooth channel wall can be obtained without the need for a post-machining annealing operation by performing the scribing process with the CO 2 laser beam in an unfocused condition. The practicality of the proposed approach is demonstrated by fabricating two microfluidic chips, namely a cytometer, and an integrating microfluidic chip for methanol detection, respectively. The results confirm that the proposed unfocused ablation technique represents a viable solution for the rapid and economic fabrication of a wide variety of PMMA-based microfluidic chips.

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