Water Splitting Photovoltaic-Photoelectrochemical GaAs/InGaAsP - WO3/BiVO4 Tandem Cell with Extremely Thin Absorber Photoanode Structure

We demonstrate highly efficient solar hydrogen generation via water splitting by photovoltaicphotoelectrochemical (PV-PEC) tandem device based on GaAs/InGaAsP (PV cell) and WO3/BiVO4 core/shell nanorods (PEC cell). We utilized extremely thin absorber (ETA) concept to design the WO3/BiVO4 core/shell heterojunction nanorods and obtained the highest efficiencies of photo-induced charge carriers generation, separation and transfer that are possible for the WO3/BiVO4 material combination. The PV-PEC tandem shows stable water splitting photocurrent of 6.56 mA cm-2 under standard AM1.5G solar light that corresponds to the record solar-to-hydrogen (STH) conversion efficiency of 8.1%

Equipment grant for interfacial velocimetry and 3d liquid-phase thermometry in microfluidic devices

Issued as final reportIn terms of colloid science, these experiments have demonstrated that an electric field applied parallel to the wall creates an additional nonlinear electrokinetic force that repels near-wall (i.e., those less than 300 nm from the wall) particles of radii ranging from 0.2 um to 0.5 um. The measurements verify previous theoretical predictions of a force that scales with the square of the electric field magnitude and the square of the particle radius, albeit with a magnitude one to two orders of magnitude greater than that predicted by the theory (note that the original theory was developed for 'remote wall-sphere interactions'). In terms of fluid mechanics, this result suggests that knowledge of the near-wall particle distribution will be required to accurately measure near-wall velocity fields with particle velocimetry techniques (e.g. micro-PIV and evanescent-wave particle velocimetry). Without this knowledge, using tracers of different diameters in these techniques will give different results for the velocity field for a shear flow where there is an electric field parallel to the wall, such as combined electroosmotic and Poiseuille flow.National Science Foundation (U.S.

High-Pressure Acceleration of Nanoliter Droplets in the Gas Phase in a Microchannel

Microfluidics has been used to perform various chemical operations for pL–nL volumes of samples, such as mixing, reaction and separation, by exploiting diffusion, viscous forces, and surface tension, which are dominant in spaces with dimensions on the micrometer scale. To further develop this field, we previously developed a novel microfluidic device, termed a microdroplet collider, which exploits spatially and temporally localized kinetic energy. This device accelerates a microdroplet in the gas phase along a microchannel until it collides with a target. We demonstrated 6000-fold faster mixing compared to mixing by diffusion; however, the droplet acceleration was not optimized, because the experiments were conducted for only one droplet size and at pressures in the 10–100 kPa range. In this study, we investigated the acceleration of a microdroplet using a high-pressure (MPa) control system, in order to achieve higher acceleration and kinetic energy. The motion of the nL droplet was observed using a high-speed complementary metal oxide semiconductor (CMOS) camera. A maximum droplet velocity of ~5 m/s was achieved at a pressure of 1–2 MPa. Despite the higher fluid resistance, longer droplets yielded higher acceleration and kinetic energy, because droplet splitting was a determining factor in the acceleration and using a longer droplet helped prevent it. The results provide design guidelines for achieving higher kinetic energies in the microdroplet collider for various microfluidic applications

Nanofluidic gas/liquid switching utilizing a nanochannel open/close valve based on glass deformation

There has been much progress in the field of nanofluidics, and novel applications, such as single-cell analysis, have been achieved. In such cases, controlling the location of the gas/liquid interface is vital and partial hydrophobic modification is frequently used to pin the position of this interface. However, because the fluid manipulating pressure in such devices is comparable to the Laplace pressure at the interface of approximately 0.1 MPa, the interface cannot be maintained stably. The present work demonstrates a method of controlling the gas/liquid interface using a hydrophobic nanochannel open/close valve. The high Laplace pressure at this valve (on the order of 1 MPa) fixes the location of the interface even during fluid manipulation. In addition, the interface can be moved at any time simply by closing the valve to generate an impulsive pressure higher than the Laplace pressure. A device incorporating this nanochannel open/close valve was fabricated, and the surface of the valve chamber was modified with hydrophobic molecules. Gas/liquid replacement in association with the operation of this valve was verified using microscopic observations. It was verified that this replacement was triggered by the valve operation, with a replacement time of 1.2 s. Using this process, gas/liquid switching can be performed when desired and this control method could expand the use of gas/liquid two-phase systems to realize further integration of chemical processes in nanofluidics

A Simple Low-Temperature Glass Bonding Process with Surface Activation by Oxygen Plasma for Micro/Nanofluidic Devices

The bonding of glass substrates is necessary when constructing micro/nanofluidic devices for sealing micro- and nanochannels. Recently, a low-temperature glass bonding method utilizing surface activation with plasma was developed to realize micro/nanofluidic devices for various applications, but it still has issues for general use. Here, we propose a simple process of low-temperature glass bonding utilizing typical facilities available in clean rooms and applied it to the fabrication of micro/nanofluidic devices made of different glasses. In the process, the substrate surface was activated with oxygen plasma, and the glass substrates were placed in contact in a class ISO 5 clean room. The pre-bonded substrates were heated for annealing. We found an optimal concentration of oxygen plasma and achieved a bonding energy of 0.33–0.48 J/m2 in fused-silica/fused-silica glass bonding. The process was applied to the bonding of fused-silica glass and borosilicate glass, which is generally used in optical microscopy, and revealed higher bonding energy than fused-silica/fused-silica glass bonding. An annealing temperature lower than 200 °C was necessary to avoid crack generation by thermal stress due to the different thermal properties of the glasses. A fabricated micro/nanofluidic device exhibited a pressure resistance higher than 600 kPa. This work will contribute to the advancement of micro/nanofluidics

Transport of a Micro Liquid Plug in a Gas-Phase Flow in a Microchannel

Micro liquid droplets and plugs in the gas-phase in microchannels have been utilized in microfluidics for chemical analysis and synthesis. While higher velocities of droplets and plugs are expected to enable chemical processing at higher efficiency and higher throughput, we recently reported that there is a limit of the liquid plug velocity owing to splitting caused by unstable wetting to the channel wall. This study expands our experimental work to examine the dynamics of a micro liquid plug in the gas phase in a microchannel. The motion of a single liquid plug, 0.4–58 nL in volume, with precise size control in 39- to 116-m-diameter hydrophobic microchannels was investigated. The maximum velocity of the liquid plug was 1.5 m/s, and increased to 5 m/s with splitting. The plug velocity was 20% of that calculated using the Hagen-Poiseuille equation. It was found that the liquid plug starts splitting when the inertial force exerted by the fluid overcomes the surface tension, i.e., the Weber number (ratio of the inertial force to the surface tension) is higher than 1. The results can be applied in the design of microfluidic devices for various applications that utilize liquid droplets and plugs in the gas phase