Dennis Desheng Meng

A degassing plate with hydrophobic bubble capture and distributed venting for microfluidic devices

This paper introduces a microfluidic wall plate that allows the removal of gas bubbles from a gas/liquid mixture in a distributed fashion, i.e., throughout the flow path, eliminating the need for discrete separators common in macroscopic practice. Integrated into a microfluidic system at critical locations, such a degassing plate prevents the build up of gas bubbles in microchannels so as to maximize the effective reaction area, decrease the flow resistance and keep the chamber pressure under check. Furthermore, the plate surface is designed to capture the gas bubbles preferentially on designated venting sites, so that the rest of the surface can be dedicated to other functions, such as the catalyst or electrodes. The mechanism of bubble capture is explained by surface energy minimization, and two types of bubble sinks are proposed and verified. Once captured, the bubbles can be vented out through hydrophobic venting holes small enough (e.g. sub-micron) to block the liquid by surface tension. By chemically generating CO2 inside a small chamber (30 mm ? 50 mm ? 1.5 mm) sealed by the degassing plate, the process of bubble capture and removal is visually demonstrated. A porous polypropylene membrane with ~0.2 ?m diameter holes shows that gas can be removed with only several kPa of internal pressure while water stays free of leakage even under 2.4 ? 105 Pa (35 psi). Venting is effective in any gravitational orientation, paving the way for portable microfluidic devices.

Scalable High-Power Redox Capacitors with Aligned Nanoforests of Crystalline MnO2 Nanorods by High Voltage Electrophoretic Deposition

It is commonly perceived that reduction-oxidation (redox) capacitors have to sacrifice power density to achieve higher energy density than carbon-based electric double layer capacitors. In this work, we report the synergetic advantages of combining the high crystallinity of hydrothermally synthesized α-MnO2 nanorods with alignment for high performance redox capacitors. Such an approach is enabled by high voltage electrophoretic deposition (HVEPD) technology which can obtain vertically aligned nanoforests with great process versatility. The scalable nanomanufacturing process is demonstrated by roll-printing an aligned forest of α-MnO2 nanorods on a large flexible substrate (1 inch by 1 foot). The electrodes show very high power density (340 kW/kg at an energy density of 4.7 Wh/kg) and excellent cyclability (over 92% capacitance retention over 2000 cycles). Pretreatment of the substrate and use of a conductive holding layer have also been shown to significantly reduce the contact resistance between the aligned nanoforests and the substrates. High areal specific capacitances of around 8500 μF/cm(2) have been obtained for each electrode with a two-electrode device configuration. Over 93% capacitance retention was observed when the cycling current densities were increased from 0.25 to 10 mA/cm(2), indicating high rate capabilities of the fabricated electrodes and resulting in the very high attainable power density. The high performance of the electrodes is attributed to the crystallographic structure, 1D morphology, aligned orientation, and low contact resistance.

Superhydrophilic Surfaces for Antifogging and Antifouling Microfluidic Devices

Superhydrophilic surfaces are investigated for their potential to provide antifogging and antifouling properties for microfluidic devices. Two types of exemplary superhydrophilic surfaces are prepared, including polyester films treated by oxygen plasma and indium tin oxide-coated glasses treated by an electrochemical method. The superhydrophilicity of the treated surfaces presented herein is confirmed by their near-zero water contact angles. Their corresponding antifogging and antifouling capability is examined. The fluorescence microscopic study has confirmed the significantly reduced adhesion of the fluorescein and fluorescent proteins after the surfaces are treated to be superhydrophilic, indicating their potential for antifouling applications. The degradation of the superhydrophilicity under different humidity conditions is also investigated.

A Methanol-Tolerant Gas-Venting Microchannel for a Microdirect Methanol Fuel Cell

As a byproduct, gas is constantly generated from the electrochemical reactions of direct methanol fuel cells (DMFCs). In the anodic channel of a DMFC, the gas forms bubbles, which leads to bubble clogging and pressure buildup if the device is miniaturized. Bubble clogging increases the flow resistance in microchannels, calling for excessive power consumption for fuel delivery. Pressure buildup aggravates the undesired crossover of methanol. In order to solve those problems, this paper introduces a gas-venting microchannel that directly removes gas bubbles from the two-phase flows of gas and methanol solution without leakage. By employing a hydrophobic nanoporous membrane, successful venting is achieved for both water and methanol fuel with a concentration of as high as 10 M. The fuel is contained without leakage under overpressures of as high as 200 kPa for both water and 10-M methanol, fulfilling the requirement of the current- as well as next-generation microdirect methanol fuel cells. A 1-D venting rate model is developed and experimentally verified for elongated bubbles. The reported bubble removal approach is also useful for other microfluidic devices, in which the accidental introduction of gas bubbles is prevalent.

High-Voltage Electrophoretic Deposition for Vertically Aligned Forests of One-Dimensional Nanoparticles

Deposition of aligned forests of 1D nanoparticles (carbon nanotubes and MnO(2) nanorods) on conductive, including flexible and transparent, substrates has been achieved at room temperature. The process, named high-voltage electrophoretic deposition (HVEPD), has been enabled by three key elements: high deposition voltage for alignment, low dispersion concentration of the nanoparticles to avoid aggregation, and simultaneous formation of a holding layer by electrodeposition. The effects of key parameters are investigated. The alignment on the vertical direction has been revealed by scanning electron microscopy of the samples, their superhydrophobicity, electrochemical performance, and capability to electrically connect two separated electrodes. Compared with their randomly oriented counterparts, the aligned nanoforests showed higher electrochemical capacitance, lower electrical resistance, and the capability to achieve superhydrophobicity, implicating their potential in a broad range of applications.

Formation of metal nanoparticles by short-distance sputter deposition in a reactive ion etching chamber

A new method is reported to form metal nanoparticles by sputter deposition inside a reactive ion etching chamber with a very short target-substrate distance. The distribution and morphology of nanoparticles are found to be affected by the distance, the ion concentration, and the sputtering time. Densely distributed nanoparticles of various compositions were fabricated on the substrates that were kept at a distance of 130 μm or smaller from the target. When the distance was increased to 510 μm, island structures were formed, indicating the tendency to form continuous thin film with longer distance. The observed trend for nanoparticle formation is opposite to the previously reported mechanism for the formation of nanoparticles by sputtering. A new mechanism based on the seeding effect of the substrate is proposed to interpret the experimental results.

A self-adaptive thermal switch array for rapid temperature stabilization under various thermal power inputs

A self-adaptive thermal switch array (TSA) based on actuation by low-melting-point alloy droplets is reported to stabilize the temperature of a heat-generating microelectromechanical system (MEMS) device at a predetermined range (i.e. the optimal working temperature of the device) with neither a control circuit nor electrical power consumption. When the temperature is below this range, the TSA stays off and works as a thermal insulator. Therefore, the MEMS device can quickly heat itself up to its optimal working temperature during startup. Once this temperature is reached, TSA is automatically turned on to increase the thermal conductance, working as an effective thermal spreader. As a result, the MEMS device tends to stay at its optimal working temperature without complex thermal management components and the associated parasitic power loss. A prototype TSA was fabricated and characterized to prove the concept. The stabilization temperatures under various power inputs have been studied both experimentally and theoretically. Under the increment of power input from 3.8 to 5.8 W, the temperature of the device increased only by 2.5 °C due to the stabilization effect of TSA.

An integrated microfluidic self-regulating and self-circulating hydrogen generator for fuel cells

This paper introduces a micro-hydrogen generator with self-circulation and self-regulation mechanisms for delivering alkaline sodium borohydride solution without parasitic power consumption. The self-circulation of the sodium borohydride solution was achieved by using directional growth and selective venting of hydrogen bubbles in microchannels, which leads to agitation and addition of fresh solution without power consumption. The pumping rate can be automatically regulated by the pressure at the gas outlet, which directly corresponds to the current output of the fuel cell, and thus the electrical load. Design, fabrication, and testing results of a prototype system are described. A maximum hydrogen generation rate of 0.5 sccm and self-circulation/self-regulation mechanisms have been demonstrated in this paper.

Embedded self-circulation of liquid fuel for a micro direct methanol fuel cell

This paper introduces a micro direct methanol fuel cell (muDMFC) with an embedded self-pumping mechanism to deliver liquid fuel. The fuel is propelled by the CO2 bubbles generated by the fuel-cell electrochemical reaction, and the bubbles are removed from the system during the self-pumping process. Furthermore, the pumping rate is self- regulated by the reaction, i.e., by the electric load. By eliminating the need for a pump and gas/liquid separator, our design allows much simpler fuel-cell systems, which is especially beneficial for miniaturization. Although we test with muDMFC in this paper, the mechanism applies to other membrane electrode assembly (MEA)-based fuel cells with organic liquid fuels as well.

Superhydrophilic anti-fog polyester film by oxygen plasma treatment

Superhydorphilic polyester film has been obtained by oxygen plasma treatment to maintain optical clarity under high relative humidity. The original and treated films are simultaneously exposed to the vapor from hot water. The latter keeps its optical clarity because the condensed water formed a thin film on it instead of droplets for the former. Compared with the widely used coating method, plasma treatment to obtain anti-fog polymer film has the potential for mass production, lower cost, better compatibility with thermal extrusion process, and safer for food packaging. It can also be employed to improve the visualization of microfluidic reactors under high relative humidity.