Recent years have seen an explosion of interest in the field of microfluidics. Many possible applications exist for low cost, portable fluidic systems; examples include drug delivery devices, chemical reactors, and DNA sequencers. In many of these systems, the thorough mixing of small amounts of two or more fluids will be required. While it may at first seem that diffusion will be sufficient to quickly mix fluids in a sub-millimeter scale channel, this is often not the case. To meet this need, a new mixing process for microfluidics is introduced. This work has been based upon a need for a lightweight, wearable, computer controlled drug delivery device which can inject medicine at a precise concentration and rate; however, the ideas developed herein are also applicable to other microfluidic applications.
The mixing process utilizes the alternating, pulsed flow of two fluids into a com-mon mixing channel. A sequence of fluid packets is thus produced. As the fluid in the mixing channel is forced downstream by each entering pulse, a velocity field characterized by a parabolic-like profile develops. Packets of fluid are stretched since the center-line fluid moves much faster than the fluid near the walls. The subsequent distortion leads to a much larger interface over which diffusion can operate and therefore much faster mixing.
In addition to analytical and numerical modeling of the alternating, pulsed flow mixing process, an actual working micromixer was desired. Such a device was created out of silicon and quartz, using many integrated circuit processes such as lithography, plasma etching, and chemical vapor deposition. The fabrication steps as well as the experimental results (which prove the validity of the numerical models) are presented.
Furthermore, in order to drive two fluids and enable mixing, two micropumps were integrated into each device. These pumps were also of a novel design and proved to be a practical method for microscale fluid transport. Displacement of fluid inside the pumps was performed by an oscillating bubble; check valves rectified the resultant flow. These check valves were also of a new design, and can be used for microfluidic control independent of the pumps. Analytical and experimental work describing both the pumps and valves is included.
April 30, 2001
Deshmukh, A. A. (2001). Continuous Microfluidic Mixing Using Pulsatile Micropumps. United States: University of California, Berkeley.