Impact of cross-sectional geometry on mixing performance of spiral microfluidic channels characterized by swirling strength of Dean-vortices

Abstract

Mixing in a microfluidic system is challenging due to dominant diffusion effects at a microscale (low Reynolds number). In this work, we report the improvement of mixing performance in spiral microchannels of varying cross-sectional geometry and hydraulic diameter. The formation of secondary flow interactions in spiral channels, known as Dean vortices, aid the primary diffusion process. The evolution of these Dean vortices was experimentally visualized along the length of the microchannel by confocal microscopy, and then compared to numerical studies. The cross-sectional geometries of the spiral channels, especially in the case of irregular shapes such as the semi-circular and trapezoidal profiles, were found to be an important factor in tuning the strength of Dean vortices, which in turn dictate the mixing performance, as opposed to diffusion which is more prominent at lower Re. This experiment-based finding has been validated via the evaluation of swirling strength of the working fluid, obtained using a numerical study. The results thus obtained show a mixing performance greater than 90% above a Reynolds number of 20 for most spiral channel designs, making this system suitable for high throughput operation with reduced pressure drop. This work is the first to experimentally and numerically demonstrate, within this operating range (20 < Re < 277), the impact on mixing performance in curved microchannels of varying cross-sectional geometries of constant cross-sectional area, and of varying hydraulic diameters for square shaped channels. The capability of these channels to operate at a moderately high Re with enhanced mixing performance and reduced pressure drop would be of great use in large-scale industrial operations, such as complex integrated micro-reactors wherein pressure drop plays a key role.