Mixing in flows past confined microfluidic cylinders: Effects of pin and fluid interface offsetting

Abstract

Inserting obstacles such as cylindrical pins in a micromixer has the potential to significantly enhance scalar transport and improve species mixing between two co-flowing streams. However, it remains unclear how the mixing efficiency in confined microchannel flows is affected by the positioning of the fluid interface and the cylindrical pin with respect to the wall or to each other. The present work investigates the mixing induced by a single cylindrical pin placed at different gap distances from the wall of a Y-type micromixer. Two fluid interface positions are considered by mixing the fluid streams at different ratios; one located at the channel centreline and one shifted towards one of the walls. Micro particle image velocimetry (μPIV) is applied to investigate the velocity fields and streamline patterns for the different pin locations, and micro laser-induced fluorescence (μLIF) to acquire the instantaneous concentration fields and assess the mixing performance, utilising the intensity of segregation technique. Prior to the onset of vortex-shedding and when the fluid interface coincides with the channel centreline, slightly offsetting the pin from the centreline is found to yield the best mixing performance compared to centreline or near wall pin locations. However, when vortex-shedding is present, a centreline pin location exhibits the best mixing performance. The present measurements indicate that single micropins can enhance mixing, even in the absence of vortex-shedding, and when vortex-shedding occurs, they are most efficient when the pin axis and fluid interface are aligned.