2.2 Planar (XY) Mixing Evaluation
To demonstrate the mixing produced by active posts at the macroscale, we
have developed planar and axial mixing assays wherein a colored liquid
is added to a chamber/well containing a clear liquid and the time
required to mix the two is calculated. Each well within the microwell
plate was filled with a clear 1% solution of Triton X-100 (Thermo
Scientific™ 85111). Afterward, a diluted blue food dye solution was
prepared (McCormick) and a volume of 4 µL was delivered via pipet to the
well. The 4-well chip was then placed over a 1-inch drive magnet (120
mT). At this time, the drive system was engaged at the pre-determined
rpm (3000, 6000, 9000), or left off in the case of diffusion. Videos of
the four independent mixing wells driven simultaneously with identical
drive conditions were captured using LCD digital microscope. The data
collected via mixing video analysis was analyzed using the relative
mixing index (RMI) as outlined in Hashmi, 2014 [14]. A computer
program was written to pull in each frame of a video, select the
appropriate ROI, and perform the analysis for each well in the frame.
For this study, the threshold for a mixed state was set at an RMI of
0.85. The resulting graphs show the average RMI for each independent
drive speed evaluation (n=4).