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Porous electrode capacitors are used extensively in systems which store energy, harvest mixing energy, or desalinate water. These electrodes can possess a hierarchical pore structure with larger macroscale pores allowing for facile ion and fluid transport, and smaller, nanometer-scale pores enabling significant ion storage. We here present a combined theoretical (linear circuit model) and experimental (electrochemical impedance spectroscopy) study of porous carbon electrode capacitors which integrate nanoscale pores into a micron-scale porous network. Our experiments are performed on a set of customfabricated hierarchical carbon aerogel electrodes with varying pore structure, including electrodes with sub-nanometer (sub-nm) pores. Our combined theory and experimental approach allows us to demonstrate the utility of our model, perform detailed characterizations of our electrodes, study the effects of pore structure variations on impedance, and propose hierarchical electrode design and characterization guidelines. Further, we demonstrate that our approach is promising toward the detailed study of ion storage mechanisms in sub-nm pores.

On-chip monodisperse droplet analysis systems are ideal for low concentration and single molecule applications because they partition reagents into identical picoliter or smaller reactor volumes that can be observed in real-time. We present a novel trapping method with droplet stopping times of approximately 38 ms that is applicable to most on-chip droplet generators. The technique greatly extends optical interrogation times without droplet motion or coalescence; and will allow on-chip single molecule detection of nanoparticle emitters with simple optics. The method maintains droplet monodispersity without chemistry-altering surfactants, and has been shown to preserve a stationary droplet stream through repetitive high-temperature thermal cycling with no additional energy input required to maintain droplet position.

Metallic nanoparticles suspended in aqueous solutions and functionalized with chemical and biological surface coatings are important elements in basic and applied nanoscience research. Many applications require an understanding of the electrokinetic or colloidal properties of such particles. We describe the results of experiments to measure the zeta potential of metallic nanorod particles in aqueous saline solutions, including the effects of pH, ionic strength, metallic composition, and surface functionalization state. Particle substrates tested include gold, silver, and palladium monometallic particles as well as gold/silver bimetallic particles. Surface functionalization conditions included 11-mercaptoundecanoic acid (MUA), mercaptoethanol (ME), and mercaptoethanesulfonic acid (MESA) self-assembled monolayers (SAMs), as well as MUA layers subsequently derivatized with proteins. For comparison, we present zeta potential data for typical charge-stabilized polystyrene particles. We compare experimental zeta potential data with theoretically predicted values for SAM-coated and bimetallic particles. The results of these studies are useful in predicting and controlling the aggregation, adhesion, and transport of functionalized metallic nanoparticles within microfluidic devices and other systems.

N-type porous silicon can be used to realize electroosmotic pumps with high flow rates per applied potential difference. The porosity and pore size of porous silicon membranes can be tuned, the pore geometry has near-unity tortuosity, and membranes can be made thin and with integrated support structures. The size of hexagonally packed pores is modified by low-pressure chemical vapor deposition (LPCVD) polysilicon deposition, followed by wet oxidation of the polysilicon layer, resulting in a pore radius varying from 1 to 3 /spl mu/m. Pumping performance of these devices is experimentally studied as a function of pore size and compared with theory. These 350-/spl mu/m-thick silicon membranes exhibit a maximum flow rate per applied field of 0.13 ml/min/cm/sup 2//V. This figure of merit is five times larger than previously demonstrated porous glass EO pumps.

A polymer-based packaging platform for creating hybrid microfluidic systems is presented. Polydimethylsiloxane (PDMS) is cast into an acrylic mold frame with suspended elements that are removed after curing to form chip cavities, inlet and outlet ports, microchannels, and reservoirs. The packaging approach enables the integration of off-the-shelf components such as pumps and valves with glass microfluidic devices, electronic chips, sample reservoirs, and flow channels. A particle pre-concentration module with a glass capture chip and integrated micropump is shown as an example. A pneumatically driven microfluidic pumping module is also shown. Custom microfluidic interconnects for interfacing to micro-scale fluidic systems are presented. The connectors are capable of withstanding more than 1000 psi and allow microdevices to be rapidly connected to macroscopic devices and systems, without the use of tools.

On-chip monodisperse droplet generation and analysis is ideal for low concentration and single molecule applications because it enables reagents to be partitioned into identical nanoliter or smaller reactor volumes while allowing real-time optical interrogation. Typically, these systems operate in a continuous droplet generation mode to maximize sample throughput. We have observed that on-chip droplet production for water-in-oil emulsions causes downstream droplet velocity fluctuations similar to those reported for gas-in-liquid emulsion systems with a periodicity equivalent to the droplet generation rate. This phenomenon can affect the accuracy of data collection due to variation in photon integration times as the moving droplets are observed. Here we perform high speed imaging and frequency domain analysis to describe the periodic velocity fluctuations due to monodisperse droplet generation.

This article presents analysis and optimization of a microfluidic particle filter that uses acoustic radiation forces to remove particles larger than a selected size by adjusting the driving conditions of the piezoelectric transducer (PZT). Operationally, the acoustic filter concentrates microparticles to the center of the microchannel, minimizing undesirable particle adsorption to the microchannel walls. Finite element models predict the complex two-dimensional acoustic radiation force field perpendicular to the flow direction in microfluidic devices. We compare these results with experimental parametric studies including variations of the PZT driving frequencies and voltages as well as various particle sizes (0.5-5.0 microm in diameter). These results provide insight into the optimal operating conditions and show the efficacy of our device as a filter with an adjustable effective pore size. We demonstrate the separation of Saccharomyces cerevisiae from MS2 bacteriophage using our acoustic device. With optimized design of our microfluidic flow system, we achieved yields of greater than 90% for the MS2 with greater than 80% removal of the S. cerevisiae in this continuous-flow sample preparation device.

We present a theoretical and experimental study of the role of hydrodynamic interactions on the motion and dispersion of metal rodlike particles in the presence of an externally applied electric field. In these systems, the electric field polarizes the particles and induces an electroosmotic flow relative to the surface of each particle. The simulations include the effect of the gravitational body force, buoyancy, far-field hydrodynamic interactions, and near-field lubrication forces. The particles in the simulations and experiments were observed to experience repeated pairing interactions in which they come together axially with their ends approaching each other, slide past one another until their centers approach, and then push apart. These interactions were confirmed in measurements of particle orientations and velocities, pair distribution functions, and net dispersion of the suspension. For large electric fields, the pair distribution functions show accumulation and depletion regions consistent with many pairing events. For particle concentrations of 10 8 particles/mL and higher, dispersion within the suspension dramatically increases with increased field strength.

The first lab-on-chip system for picoliter droplet generation and PCR amplification with real-time fluorescence detection has performed PCR in isolated droplets at volumes 106 smaller than commercial real-time PCR instruments. The system utilized a shearing T-junction in a silicon device to generate a stream of monodisperse picoliter droplets that were isolated from the microfluidic channel walls and each other by the oil-phase carrier. An off-chip valving system stopped the droplets on-chip, allowing them to be thermally cycled through the PCR protocol without droplet motion. With this system, a 10-pL droplet, encapsulating less than one copy of viral genomic DNA through Poisson statistics, showed real-time PCR amplification curves with a cycle threshold of approximately 18, 20 cycles earlier than commercial instruments. This combination of the established real-time PCR assay with digital microfluidics is ideal for isolating single-copy nucleic acids in a complex environment.

Analytical models are developed for the translation and rotation of metallic rods in a uniform electric field. The limits of thin and thick electric double layers are considered. These models include the effect of stripes of different metals along the length of the particle. Modeling results are compared to experimental measurements for metallic rods. Experiments demonstrate the increased alignment of particles with increasing field strength and the increase in degree of alignment of thin versus thick electric double layers. The metal rods polarize in the applied field and align parallel to its direction due to torques on the polarized charge. The torque due to polarization has a second-order dependence on the electric field strength. The particles are also shown to have an additional alignment torque component due to nonuniform densities along their length. The orientation distributions of dilute suspensions of particles are also shown to agree well with results predicted by a rotational convective-diffusion equation.

We present the fabrication and electromechanical characterization of a class of polymeric high-elongation strain sensors. Samples of polydimethylsiloxane were coated with Creative Materials, Inc.'s 123-27 Electrically Conductive Silicone Ink and the resistance behavior was evaluated in uniaxial tensile tests. Large strains (up to 100%) were observed with monotonically increasing resistance changes. A clear, linear trend up to 65% strain dominated the resistance vs. strain behavior then resistance increased non-linearly. Image processing of the film coupled with a finite element conduction simulation indicate the change in resistance is primarily a geometric effect. Both the conduction path and the polydimethylsiloxane substrate break completely around 100% strain. The samples exhibit a gauge factor of approximately 10.

This paper focuses on the design considerations, fabrication processes, and preliminary testing of a retinal prosthesis that has the potential to aid in vision restoration to millions of blind patients. We are developing an implantable, stretchable micro-electrode array using polymer-based microfabrication techniques. The device will serve as the interface between an electronic imaging system and the human eye, directly stimulating retinal neurons via thin film conducting traces and electroplated electrodes. The metal features are embedded within a thin (/spl sim/50 /spl mu/m) substrate fabricated using poly (dimethylsiloxane) (PDMS), a biocompatible elastomeric material that has high oxygen permeability and low water permeability. The conformable nature of PDMS is critical for ensuring uniform contact with the curved surface of the retina. To fabricate the device, we developed unique processes for metalizing PDMS to produce robust traces capable of maintaining conductivity when stretched (strain = 7%, SD 1), and for selectively passivating the conductive elements. An in situ substrate curvature measurement taken while curing the PDMS revealed a tensile residual strain of 10%, explaining the stretchable nature of the thin metalized devices.

Fluorapatite nanostructures of various shapes (prismatic, ellipsoidal, star, and aggregate) were synthesized and their structures correlated with the supersaturation of the system. Reagent concentration and pH were adjusted and the change in supersaturation was simulated by the Geochemist's Workbench® software and the MINTEQ database. A higher pH caused changes to the FAP surface charge and was shown to be the dominant force behind aggregate formation. This led to nanorod aggregates and when combined with an increase in reagent concentration, FAP stars were generated. Increasing reaction temperature (room temperature to 100 °C) allowed release of calcium by the chelating agent, EDTA, which steadily increased the supersaturation as demonstrated by simulation. This condition led to ellipsoidal nanorods. As the crystal growth continued with an increasing reaction temperature of up to 150 °C, ellipsoidal nanorods transformed to prismatic nanowires. This transformation was explained by the decreasing supersaturation of the system as the growth nutrients were consumed. Microwave irradiation, the role of fluorite, and control of monodispersity for the FAP synthesis are also discussed.

Capacitive desalination (CD) is a promising desalination technique as, relative to reverse osmosis (RO), it requires no membrane components, can operate at low (sub-osmotic) pressures, and can potentially utilize less energy for brackish water desalination. In a typical CD cell, the feed water flows through the separator layer between two electrically charged, nanoporous carbon electrodes. This architecture results in significant performance limitations, including an inability to easily (in a single charge) desalinate moderate brackish water feeds and slow, diffusion-limited desalination. We here describe an alternative architecture, where the feed flows directly through electrodes along the primary electric field direction, which we term flow-through electrode (FTE) capacitive desalination. Using macroscopic porous electrode theory, we show that FTE CD enables significant reductions in desalination time and can desalinate higher salinity feeds per charge. We then demonstrate these benefits using a custom-built FTE CD cell containing novel hierarchical carbon aerogel monoliths as an electrode material. The pore structure of our electrodes includes both micron-scale and sub-10 nm pores, allowing our electrodes to exhibit both low flow resistance and very high specific capacitance (>100 F g−1). Our cell demonstrates feed concentration reductions of up to 70 mM NaCl per charge and a mean sorption rate of nearly 1 mg NaCl per g aerogel per min, 4 to 10 times higher than that demonstrated by the typical CD cell architecture. We also show that, as predicted by our model, our cell desalinates the feed at the cell's RC timescale rather than the significantly longer diffusive timescale characteristic of typical CD cells.

Carbon aerogels are a unique class of high-surface-area materials derived by sol–gel chemistry. Their high mass-specific surface area and electrical conductivity, environmental compatibility and chemical inertness make them very promising materials for many energy related applications, specifically in view of recent developments in controlling their morphology. In this perspective we will review the synthesis of monolithic resorcinol–formaldehyde based carbon aerogels with hierarchical porosities for energy applications, including carbon nanotube and graphene composite carbon aerogels, as well as their functionalization by surface engineering. Applications that we will discuss include hydrogen and electrical energy storage, desalination and catalysis.