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Last update:

Wed, 9/26/07 2:27 PM

 

On-going Research Projects @ MuSES Lab:

 

Embedded Self-Circulation of Liquid Reactants for Micro Fuel Cells

Micro direct methanol fuel cells (mDMFCs) are widely anticipated as the next-generation high energy density micro power source for portable electronics and MEMS. However, enormous challenges have been slowing down its commercialization, including methanol crossover, cathode flooding, MEA dry-out, and the need for complex peripheral components, which results in a significant packaging penalty. While electrochemists make tremendous efforts to improve MEA performance, we believe that MEMS and nanotechnologhies can provide a comprehensive microfluidic management paradigm to solve the above-mentioned problems of mDMFCs. A key component of this microfluidic approach is reactant regulation with integrated compact structures and little or no power consumption.  We have developed a bubble-driven micro pumping mechanism that utilizes gas byproducts to transport liquid reactants in the micro fuel cell without consuming electrical power. Moreover, this pumping mechanism can be embedded into the microchannels of mDMFCs without occupying much of the overall volume, greatly reducing the packaging penalty. 

 
MicroPump FCPerformance
Bubble motion the pumping section
Flowrate
Measurement of the flow rate Continuous mDMFC operation w/o embedded self-circulation of methanol fuel
 

Universal gas bubble removal by hydrophobic venting

Blocking of microchannels by gas bubbles has been a daunting problem for microfluidics. Our study shows that this problem can be solved by making small hydrophobic holes in the wall of a microfluidic reactor. Ours is the first demonstrated approach that can quickly remove any gas bubbles from a packaged microfluidic device, whether the gas is soluble in the solvent or not. The leakage prevention pressure of the hydrophobic holes is determined by its surface properties and size. By using hydrophobic nanoporous polypropylene with pore size of ~ 100 nm in radius, we demonstrated leakage prevention pressure as high as 200 kPa for both water and 10 M methanol.

 
VentingMech BubblePic
Principle of the universal gas bubble removal mechanism by hydrophobic venting A degassing plate with bubble traps and distributed hydrophobic venting
BubbleTrain
A gas-venting microchannel is removing bubbles from 10M methanol
 

Comparative Study of Electrolysis and Boiling for Bubble-Driven Microactuations

Boiling to generate thermal bubbles has been the most popular way for bubble-driven microactuation, mainly because no additional structures are needed for bubble removal. With our hydrophobic venting technology, it is now possible to remove insoluble gas bubbles quickly with a very simple structure.  Therefore, electrochemically-generated gas bubbles are finally viable for microactuation, and it is time to reevaluate electrochemical bubbles as compared to thermal bubbles for bubble-driven microdevices. In this study, we show that the power consumption of electrolysis microactuation is several orders of magnitude lower than that of boiling. Analyses of controllability, bio-compatibility and scaling effects also show the advantages of electrolysis.

BubbleActu
Thermal_Electrolysis
Schematic view of test chips for two kinds of bubble-driven microactuation Experimental results shows lower power consumption and better controllability of electrolysis actuation
 

Bubble Capturing Potential FBc: the Quantity to Evaluate The Surface’s “Affinity” for Bubbles

It has been demonstrated that gas bubbles can serve as pressure sensors, imaging particles, signal sources for MRI, drug delivery vessels, and microlenses. Guided by surface free energy, gas bubbles in a liquid environment can automatically attach to energetically favorable locations (bubble-traps) and align into a prescribed array pattern to perform their functionalities in parallel. In this study, bubble capturing potential Fbc is proposed as the quantity to evaluate the surface’s “affinity” for bubbles. A bubble-trap can therefore be viewed as an area with local maximum Fbc. Two types of bubble-traps are proposed and evaluated.  Type I bubble-traps are hydrophobic patterns on a hydrophilic flat surface. Type II bubble-traps are concave pits surrounded by a hydrophilic flat surface. Simulation of bubble capturing potential Fbc explains the bubble-capturing behavior for both cases and predicts better performance for type II bubble-traps. Experiments agree well with the theoretical prediction and suggest promising applications.

 
3PhaseBubble BCP
Bubbles in three-phase system a: floating; b,c and d: attached on different surface structures Simulation result of bubble capturing potential

bubblearray1

Bubblearray2

Gas bubbles captured on arrayed type I bubble-traps Gas bubbles captured on arrayed type II bubble-traps