Analysis of interfacial mixing zone and mixing index in microfluidic channels

Maintaining the interfacial mixing zone is very important for many micro-engineering and nano-technological applications. Many applications including controlled separation of nanoparticles in microfluidic devices, chemical reactions, mechanical separations, cell sorting and various biomedical applications require desired width of the interfacial mixing zone. Co-laminar flow and the flow rate of the liquid streams have been found to influence the interfacial mixing zone in the microfluidic flow. Passive microdevices utilise no energy inputs except the pressure head used to drive the flow at constant flow rates. In considering such situations, the flow has a laminar flow pattern, and hence mixing relies due to the convection-diffusion effect to separate and increase the contact time between flowing streams. In this paper, the behaviour of fluid mixing is demonstrated in T-shaped microdevices (micromixers) with a rectangular cross-sectional at low Reynolds numbers. Three water-soluble dyes and deionised water having almost the same densities and viscosities were used to characterize the interfacial mixing zones which formed at the centre of the microchannel. Results showed that interfacial mixing is related to diffusivities of flowing streams, flow rates and geometries of microchannels. Mixing performance and analytical evaluation of diffusive mixing are also reported in the paper. The present work provides a platform for the design of novel microfluidic devices to control diffusion processes for applications such as absorption, extraction, crystallization, capture of molecules or nanoparticles. [Graphical Abstract]