Mixing Enhancement in Microfluidic devices

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Abstract

Microscale mixing is paramount for processing targeted drug delivery, chemical production and medical diagnostics. Mixing is often performed in a micromixer, which is a microfluidic device where the fluids are confined in micro sized channels in the order of 100-500 μm. Micromixing has many advantages over its macroscale counterpart, which include small sample consumption, portability, low cost, handling of dangerous materials, compact size and disposability. However, there are also many challenges for mixing enhancement that include low diffusivity rates, high surface-volume ratio, laminar flow, viscous effects, fluid confinement and surface defects are all significant challenges. This aim of this study is to design an effective and efficient micromixer that overcomes these limitations. The literature review summarises the different approaches that have been reported to address these technical challenges, which included numerous sub-processes and micromixer designs. The most common technique involved forcing the liquid samples to mix through a complex microchannel pattern. However, this approach was limited by the high-pressure drop, complex manufacturing of the microchannel, cleaning difficulties and long mixing distances. Other mixing techniques made use of external energy sources such as sound waves, electromagnetic fields, pulsing the flow inlets, temperature gradients, in an attempt to enhance mixing. Although some were effective in specific cases, they did not offer a broad solution for many applications. In order to address these issues, three novel micromixer designs were investigated and validated using a combination of numerical simulations and experiments; these included: 1) a micromixer with a modified geometry and synthetic jets, 2) a micromixer that exploited the multiphase flow principle and 3) a micromixer with a straight channel and a spinning disk. The results confirmed that it was not feasible to develop a micromixer by scaling down a macromixer. However, by modifying the geometry and adding synthetic jets, it was possible to achieve the desired mixing degree of 90% in just 3 seconds at 350 μm downstream with a stroke length of 10.5 (ÃŽ”pp=263 μm), Strouhal number of 0.525 (f=6 Hz) and Reynolds number of 0.25. However, the final design suffered technical issues and became complex. The second proposed solution relied on the multiphase flow principle that did not require a complex channel pattern, any external energy source or moving parts to effectively enhance mixing. A mixing quality of 95% was achieved within 0.2 seconds at 350 μm downstream with typical Re<1. Conversely, this design suffered poor mixing performance at Re>2, which was addressed by incorporating a straight channel and spinning disk that achieved a mixing quality of 90% for any Re<10 at a spinning frequency of 15 Hz. The promising results obtained with the multiphase principle and spinning disk make them serious candidates for being implemented in practical applications.

Keyword(s)

microchannels; mixing; multiphase flow; synthetic jets

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