Curved passive mixing structures: a robust design to obtain efficient mixing and mass transfer in microfluidic channels

Abstract

Analyte mixing and delivery to a functionalized sensor surface are important to realize several advantages associated with biosensors integrated with microfluidic channels. Here, we present a comparison between a herringbone structure (HBS) and a curved passive mixing structure of their efficiency at facilitating mixing and surface saturation using fluorescein included in one of the inlets of a Y-channel microfluidic device. We performed a large parametric study to assess the effects of varying the height of the microfluidic channel as well as the height, width, and spacing of the passive mixing structures. Scanning confocal microscopy combined with a custom-designed image-analysis procedure were utilized to visualize and quantify the observed changes in efficiency in inducing solute mixing by the different designs. The flow patterns within the channels were found to vary significantly with changes in the geometry of the passive mixing structures, which in turn affected the efficiency of the channel at mixing the fluid and saturating the surface opposite the mixing structures. The solute mixing as a function of the channel length was also determined; an initial slow mixing rate does not always coincide with a low mixing index (MI). We found that the range of MIs for the curved mixing structure 1 cm downstream from the inlet was 0.85–0.99 whilst for our HBS it was 0.74–0.98, depending on the design parameters of the passive mixing structures. Overall, this study shows that the curved passive mixing structure family is more robust in inducing efficient mixing than the HBSs.

Characterization of Mixing Performance Induced by Double Curved Passive Mixing Structures in Microfluidic Channels

“…The development of the flow patterns was realized in both the transverse and the axial direction, resulting in the possibility of a pseudo 3D analysis. The addition of the double curve design changes the flow pattern drastically, compared to the CMS previously studied [ 56 ], in both dimensions ( Figure 2 ).…”

“…Two different groups of passive mixing designs were studied. These groups were a curved mixing structure (CMS) as introduced previously [ 56 ], and a double curved mixing structure (DCMS) extending from the CMS by adding a second curve to the overall geometry ( Figure 1 ). A total of 16 different channels with passive mixing designs were fabricated for each group.…”

“…XYZT-stack images were obtained for specific locations downstream from the inlet employing a 10× (NA = 0.4) HC PL Apo CS dry objective. The image acquisition process is further detailed in our previous study [ 56 ].…”

“…The analysis shows that a double exponential function with 4 parameters, where a + c < 1, was adequate to account for the trends in MI(x) while at the same time limiting the number of parameters. Further aspects of the selection of this procedure was reported previously [ 56 ]. The image processing, calculation of the MI based on the observed fluorescence distributions and the further representation of MI using Equation (2), were conducted for the layers adjacent to a possible sensor location (termed sensor layer, SL, adjacent to the bottom in Figure 1 c) or were based on the mean over the height of the channel (channel layers, CL).…”

“…We previously reported on a comparison of the mixing efficiency of a curved passive mixing structure (CMS) with that of herringbone structures (HBS) and found that the CMS represented a more robust group of design geometries in maintaining efficient mixing than the HBS [ 56 ]. Specifically, we reported a mixing index (MI) 1 cm downstream from the inlet in the range of 0.85–0.99 for various parameter values of the designs in the CMS group, compared to the HBS group that resulted in an MI range of 0.74–0.98.…”