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Biologically Enabled Synthesis of Ceramic Microdevices Application: Microfluidic Mixer Chamber
Microfluidic devices for analytical chemistry applications have received much scientific interest recently due to improved design tools, modeling tools, and understanding of their potential applications, including Lab-on-a-chip processes involving single molecule detection, analysis and manipulation, drug delivery and control and DNA manipulation and transport. Microfluidic devices have the potential to reduce the size of analytical chemistry tools dramatically, thus improving point of care diagnostic devices and battlefield warrior care. Recent work at The Ohio State University (OSU) by Hansford and Conlisk has demonstrated that devices containing fluidic channels with a minimum dimension in the range of 1-100 nm (nanochannels) have the potential to further improve the analytical capabilities of these devices1. Potential applications of nanochannels being investigated at OSU include:
- electroosmotic pumps contained within microfluidic devices for controlling motion of fluid;
- integrated filters for sample preparation aspects of analytical chemistry; and
- shear-based assay systems for mechanical mass separation and chirality separations of biological molecules.
The focus of the present work is on the development of an electroosmotic mixing device using diatoms in several different ways. As with all microfluidic devices, devices fabricated with nanochannels can produce only laminar flow and are thus limited in their ability to mix different fluid streams, which is an important step in many chemical applications. Several different approaches have been made to develop a “micro-mixer” component for microfluidics, based mostly on design of three-dimensional tortuous paths to induce chaotic advective mixing within the fluid stream. A few groups have started looking at modification of surface properties through the use of monolayers of charged molecules to introduce spatial differences in the surface charge, thereby providing instability in the flow lines, leading to mixing. These approaches, however, are limited due to the temporary nature of the molecular monolayers, and are therefore only useful in limited applications. Finally, other attempts have been made to induce mixing through the use of applied fields to change the wall potential locally. This is the most promising of currently research micro-mixers, but increases the fabrication complexity greatly and requires the introduction of additional energy into the system.
In this part of the MURI, we propose to investigate the permanent transformation of a portion of a surface of silica, an acidic oxide, into MgO or CaO, basic oxides, for the purpose of developing a simple micro-mixing chamber within a micro- or nanofluidic device. The extent of transformation and effects of patterning oppositely charged oxides in intimate contact have not been explored previously, and will require an in depth analysis of processing conditions and fluid stream design for full exploitation. Modeling of the micro-mixer will be conducted using a simplified geometry for an analytical solution, and using numerical solvers to obtain a full three-dimensional model. It is anticipated that this research will lead to simplified approaches to integrated micro-mixing chambers for microfluidic devices, thus simplifying the design and fabrication of these devices.
Key Questions to be Addressed
The basic approach of using oppositely charged oxides within a confined chamber for induced mixing will be examined in this application. In order to achieve a perfectly repeatable micro-mixer, the following basic questions will be addressed:
- Can a simple and reliable fabrication protocol for an integrated microfluidic mixer be developed from the basic transformation process described in Thrust #3?
- Can oxides of opposite signs be patterned to be intimately in contact and retain their individual chemical characteristics?
- How thick must the basic oxide layer be to mask out the charge/potential of the acidic oxide it is replacing?
- Does the micro-mixer behave in the predicted manner according to standard electroosmotic models?
Fabrication of Micro-Mixer by Selective Transformation of SiO2
The application will be demonstrated with a simple microfluidic system that can easily be optically interrogated, made from microchannels in polydimethyl siloxane (PDMS, silicone rubber) bonded to a monolithic SiO2 substrate with a patterned region transformed into the basic oxide. The microchannels will be fabricated using standard photolithography, followed by pouring the PDMS with a cross-linking agent over the wafer. After the PDMS has cured into its rubber state, it is pealed off the wafer leaving microfluidic channels in the negative of the photoresist. Fluidic connections will be made through tubing placed through the PDMS mold, and electrical connection to the reservoirs will be made through the fluidic connections.
The critical component of the microfluidic mixer is the mixed oxide substrate, which will be produced using a selected area version of the transformation process described in Thrust #3. Openings will be produced in an amorphous carbon layer on the SiO2 substrate using a lift-off process. This carbon layer will act as a mask so that only open regions will be exposed to the gaseous Mg and therefore transformed into MgO. By aligning this patterned basic oxide with the PDMS microfluidic component, a mixing chamber with both basic and acidic oxide surfaces can be produced. Nanometer-scale fluidic mixing chambers can be produced in an analogous protocol, with the difference being in the thickness of the initial polymer layer for the PDMS mold.
Design and Optimization of Micromixer Chambers
The micro-mixing chamber obtains its function from the spatial distribution of the surface charge within the chamber, leading to the accumulation of oppositely charged electric double layers (EDL) which will exert oppositely signed body forces within the fluid in the same signed field. This leads to the development of reversed flow which mixes the component streams with no required additional control mechanisms. The simple micro-mixing chamber described above can demonstrate the principle, but for applications that require minimal energy usage, the geometrical design of the structure will require optimization.
Analytical/Computational Model of Mixing in Nanochannels
The fluidic system under consideration is a dilute aqueous solution of strong electrolytes. The surfaces of the channel wall are negatively charged, which causes the accumulation of the positive ions near the surface, forming electrical double layer. The thickness of the EDL (l) can be calculated by
(1)
where e is the dielectric constant of the solution, e0 is the permittivity of free space, R is the ideal gas constant, T is the temperature, F is the Faraday constant, and I is the ionic strength of the solution. A variable wall potential will cause will cause the flow to lose its unidirectionality, top hat profile as in standard electroosmotic flow. Computational studies of several geometries are ongoing and the results show the creation of strong vertical structures within the flow. The evaluation of mixing efficiency and optimization of the prescribed geometry is underway.
Major MURI Participants
The main personnel on this thrust are:
A. Terrence Conlisk, Jr., Mechanical Engineering Dept., The Ohio State University
Derek J. Hansford, Biomedical Engineering Center, The Ohio State University
We will be working closely with:
Mark M. Hildebrand, Microbiology Research Dept., Scripps Inst. Oceanography, University of California/San Diego
Rajesh R. Naik, Survivability & Sensor Materials Divn., Materials & Manufacturing Directorate, AFRL/WPAFB
Brian P. Palenik, Marine Biology Research Divn., Scripps Inst. Oceanography, University of California/San Diego
in the characterization of natural diatoms and with:
Ken H. Sandhage, School of Materials Science & Engineering, Georgia Tech.
Robert A. Rapp, Materials Science & Engineering Dept., The Ohio State University
Robert L. Snyder , School of Materials Science & Engineering, Georgia Tech.
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