  Here, we describe a simple and unique architecture of a microfluidic mixer
that can mix two streams (Water and Fluorescein Isothiocyanate (FITC) buffer)
with 30% efficiency. The overall mixing in this design is dominated by 1D
diffusion, and to enhance mixing, we used three different types of geometric
obstacles inside the channel. Comsol multiphysics simulation software was used
to validate the theoretical mixing efficiency (at Reynolds Number, Re ~ 0.1, 1,
and 10) of this device. We utilized soft lithography and replica molding
techniques to fabricate the device out of Polydimethylsiloxane (PDMS, a
commonly used polymer) on a glass substrate. The effective length of our
microfluidic mixer is 5mm, and the channel width is about 200 microns with 50
micron height. It is composed of three different shapes of obstacles (e.g., 6
cones, 3 arrays of rectangular bars, and 5 circular posts), and all of these
are placed inside the main channel. FITC buffer (Diffusion co-efficient, D =
0.5 x 10-9 m2/s) and DI water were used to investigate the mixer performance at
Re~1. Simulated and experimental results are based on approximately 1.1 mm2
flow area and suggest that 30% mixing is achievable with the current design.
