Passive mixing by applying geometric variations were studied in this research. In respect to the nature of laminar flow in a microchannel, the geometric variations were designed to try to improve the lateral convection. By doing this, the dispersion of solute was not only contributed by diffusion, but also, and more importantly, the convection in the lateral direction. Geometric parameters versus the mixing performance were investigated systematically in T-type channels, by applying a known computational fluidic dynamic (CFD) solver for microfluidics. Various obstacle shapes, sizes and layouts were studied. As the ratio of the height to obstacles to the depth of channel became negative, it was the special case that obstacles became grooves. The mechanism for obstacles to enhance mixing was to create convective effects. However, the asymmetric arrangement of grooves applied a different mechanism to enhance mixing by create helical shaped recirculation of fluids. The stretching and folding of fluids of this mixing mechanism provided a efficient way to reduce the diffusion path in microchannels. The mixing performance of mixers with obstacles were evaluated by mass fraction, and mixers with grooved surfaces were evaluated by particle tracing techniques. The results illustrated that both of the strategies provided potential solutions to microfluidic mixing.
Taylor-Aris dispersion is an unwanted effect in some applications, such as chromatography, because of its rapid dispersion along channel axis direction to cause the difficulties to separation. However, this effect can be used in solving mixing problems. In this presentation, the authors studied mixing of two streams of food dye solution in a microchannel. The two liquids sandwiched in the axis direction, and due to non-slip boundary condition, the slugs of liquids stretched and thus increase their interfacial area. This phenomena was firstly studied by Taylor about the dispersion of solute in a circular capillary, and then improved by Aris. Numerical analysis was applied to design mixing section in microchannels, and simplified experiments were conducted to illustrate the concept.
Conventional methods of producing micro-scale components for BioMEMS applications such as microfluidic devices are limited to relatively simple geometries and are inefficient for prototype production. Rapid prototyping techniques may be applied to overcome these limitations. Fused Deposition Modelling is one such rapid prototyping process, which can build parts using layer by layer deposition technique with layers as low as 0.178 mm thick and using a select group of thermoplastic building materials. This paper presents the potential of Fused Deposition Modelling (FDM) system, available at IRIS, in building prototypes of scaled microchannels for experimental study and verification of fluid flow in microfluidic devices. The scope and application of FDM system as a powerful and flexible rapid prototyping device is described. Microchannels of different geometries are produced in ABS material on the FDM3000 rapid prototyping system and a methodology is presented for experimental study of the mixing of fluids in microchannels in conjunction with the theoretical analysis using FlumeCAD system.
In general, the Reynolds number is low in microfluidic channels. This means that the viscous force plays a dominant role. As a result, the flow is most likely to be laminar under normal conditions, especially for liquids. Therefore, diffusion, rather than turbulence affects the mixing. In this work, the commercial computational fluid dynamics tool for microfluidics, known as FlumeCAD, is used to study the mixing of two liquids in a Y channel and the results are presented. To improve mixing, obstacles have been placed in the channel to try to disrupt flow and reduce the lamella width. Ideally, properly designed geometric parameters, such as layout and number of obstacles, improve the mixing performance without sacrificing the pressure drop too much. In addition, various liquid properties, such as viscosity, diffusion constant, are also evaluated for their effect on mixing. The results indicate that layout of the obstacle has more effect on the mixing than the number of the obstacles. Placing obstacles or textures in the microchannels is a novel method for mixing in microfluidic devices, and the results can provide useful information in the design of these devices.
A study of pressure-driven liquid flow in microchannels is presented with the aim of providing a simple model for microfluidics. The paper presents the initial research effort, which covers a survey of CFD packages, the general principles for fluid dynamics, and a simple model of flow in microchannels formulated from these governing equations. The model demonstrates how the capillary force affects the flow and the applicability of boundary layer theory to the flow in a microchannel. The simulation of textures in a microchannel, and difficulties in modelling, are then discussed.