Formation of bubbles and droplets in parallel, coupled flow-focusing geometries

This paper describes the mechanism of formation of bubbles of nitrogen in water containing Tween 20 as a surfactant, and of droplets of water in hexadecane containing Span 80 as a surfactant. The study of these microfluidic systems compares two or four flow-focusing generators coupled through shared inlets, supplying the continuous phase, and through a common outlet channel. The processes that form bubbles in neighboring generators interact for a wide range of flow parameters; the formation of bubbles alternates in time and space, and the bubbles assemble into complex patterns in the outlet channel. The dynamics of formation of bubbles in these systems are stable for long time (at least 10 min). For a certain range of flow parameters, the coupled flow-focusing generators exhibit two stable modes of operation for a single set of flow parameters. The dynamics of formation of droplets of water in hexadecane by the coupled flow-focusing generators are simpler--the adjacent generators produce only monodisperse droplets over the entire range of flow parameters that are explored. These observations suggest that the mechanism of interaction between coupled flow-focusing generators relies on the compressibility of the dispersed phase (e.g., the gas or liquid), and on variations in pressure at the flow-focusing orifices induced by the breakup of bubbles or droplets.     

Monitoring the Dynamic Process of Formation of Plasmonic Molecular Junctions during Single Nanoparticle Collisions

The capability to study the dynamic formation of plasmonic molecular junction is of fundamental importance, and it will provide new insights into molecular electronics/plasmonics, single‐entity electrochemistry, and nanooptoelectronics. Here, a facile method to form plasmonic molecular junctions is reported by utilizing single gold nanoparticle (NP) collision events at a highly curved gold nanoelectrode modified with a self‐assembled monolayer. By using time‐resolved electrochemical current measurement and surface‐enhanced Raman scattering spectroscopy, the current changes and the evolution of interfacial chemical bonding are successfully observed in the newly formed molecular tunnel junctions during and after the gold NP “hit‐n‐stay” and “hit‐n‐run” collision events. The results lead to an in‐depth understanding of the single NP motion and the associated molecular level changes during the formation of the plasmonic molecular junctions in a single NP collision event. This method also provides a new platform to study molecular changes at the single molecule level during electron transport in a dynamic molecular tunnel junction.     

Preparation of polycrystalline ice specimens for laboratory experiments

A perennial problem in preparing fine grained ice specimens is the convenient production of low porosity material which is homogeneous, of random c-axis orientation and of controlled grain size.A review of the nature of bubble formation in ice leads directly to the two factors which must be controlled in order to minimize gas bubble formation, and thus porosity. The factors are (1) reduction of the number of bubble nucleation sites and (2) reduction of the amount of dissolved gas available at the freezing front.An ice specimen preparation technique developed at CRREL addresses both these factors. Two techniques have proven effective in reducing the number of nucleation sites; (1) a purge technique using carbon dioxide gas and (2) a vacuum technique. A newly developed flushing technique reduces the amount of dissolved gas in the pore water during freezing.The resulting material is optically clear with some very small bubbles dispersed throughout. The radial freezing technique employed occasionally results in a narrow column of fine bubbles along the central axis of the specimen. The density is 0.917 ± 0.002 Mg/m³.An overview of techniques for forming, flooding and freezing ice specimens is presented and several major preparation methods are reviewed in detail.     

Formation of Single Carbon Nanotube T‐Junctions

Abstract

We investigate the formation of single wall carbon nanotube T‐junctions via the fusing of two nanotubes. We propose energetically efficient pathways for formation (9, 0)–(10, 0)–(9, 0) and (5, 5)–(10, 0)–(5, 5) T‐junctions. In the proposed scheme all carbon atoms maintain their sp ² arrangements throughout and transformation is achieved through creation/annihilation of topological defects. We use tight‐binding molecular dynamics simulation to investigate energetic of proposed mechanism.     

Low‐Energy Facets on CdS Allomorph Junctions with Optimal Phase Ratio to Boost Charge Directional Transfer for Photocatalytic H2 Fuel Evolution

Low‐energy facets on CdS allomorph junctions with optimal phase ratio are designed to boost charge directional transfer for photocatalytic H₂ fuel evolution. Fermi energy level difference between low‐energy facets as driving force promotes electrons directional transfer to hexagonal CdS(102) facet and holes to cubic CdS(111) facet. The optimal allomorphs CdS presents superior photocatalytic H₂ evolution rate of 32.95 mmol g^?1 h^?1 with release in a large amount of visible H₂ bubbles, which is much higher than single‐phase CdS with high‐energy facets and even supports noble metal photocatalysts. This scientific perspective on low‐energy facets of allomorph junctions with optimal phase ratio breaks the long‐held view of pursuing high‐energy crystal surfaces, which will break the understanding on surface structure crystal facet engineering of photocatalytic materials.     

A numerical analysis of passive scalar transport in turbulent shear flows

Abstract

The development of the formation and vortex pairing process in a two‐dimensional shear flow and the associated passive scalar (mass concentration or energy) transport process was numerically simulated by using the Vortex‐in‐Cell (VIC) Method combined with the Upwind Finite Difference Method. The visualized temporal distributions of passive scalars resemble the vortex structures and the turbulent passive scalar fluxes showed a definite connection with the occurrence of entrainment during the formation and pairing interaction of large‐scale vortex structures. The profiles of spatial‐averaged passive scalar ø, turbulent passive scalar fluxes, u'ø’ and v'ø’, turbulent diffusivity of mean‐squared scalar fluctuation, v'ø‘ ², mean‐squared turbulent passive scalar fluctuation, √ø‘ ², skewness, and flatness factor of the probability density function of scalar fluctuation ø’ at three different times are calculated. With the lateral dimension scaled by the momentum thickness and the velocity scaled by the velocity difference across the shear layer, these profiles were shown to be self‐preserved. The probability density function of turbulent scalar fluctuation was found to be asymmetric and double‐peaked.     

From an Organometallic Monolayer to an Organic Monolayer Covered by Metal Nanoislands: A Simple Thermal Protocol for the Fabrication of the Top Contact Electrode in Molecular Electronic Devices

In this contribution, a novel method for practical uses in the fabrication of the top contact electrode in a metal/organic monolayer/metal device is presented. The procedure involves the thermally induced decomposition of an organometallic compound, abbreviated as the TIDOC method. Monolayers incorporating the metal organic compounds (MOCs) [[4‐{(4‐carboxy)ethynyl}phenyl]ethynyl]‐(triphenylphosphine)‐gold, 1, or [1‐isocyano‐4‐methoxybenzene]‐[4‐amino‐phenylethynyl]‐gold, 2, were annealed at moderate temperatures (1: 150 °C for 2h and 2: 100 °C for 2 h), resulting in cleavage of the Au‐P or Au‐C bond and reduction of Au(I) to Au(0) as metallic gold nanoparticles (GNPs). These particles are distributed on the surface of the film resulting in formation of metal/molecule/GNP sandwich structures. Electrical properties of these nascent devices were determined by recording I–V curves with a conductive‐AFM. The I–V curves collected from these metal/organic monolayer/GNPs sandwich structures are typical of metal‐molecule‐metal junctions, with no low resistance traces characteristic of metallic short circuits observed over a wide range of set‐point forces. The TIDOC method is therefore an effective procedure for the fabrication of molecular junctions for the emerging area of molecular electronics.     

Fabrication of Reproducible,Integration‐Compatible Hybrid Molecular/Si Electronics

Reproducible molecular junctions can be integrated within standard CMOS technology. Metal–molecule–semiconductor junctions are fabricated by direct Si–C binding of hexadecane or methyl‐styrene onto oxide‐free H‐Si(111) surfaces, with the lateral size of the junctions defined by an etched SiO₂ well and with evaporated Pb as the top contact. The current density, J, is highly reproducible with a standard deviation in log(J) of 0.2 over a junction diameter change from 3 to 100 μm. Reproducibility over such a large range indicates that transport is truly across the molecules and does not result from artifacts like edge effects or defects in the molecular monolayer. Device fabrication is tested for two n‐Si doping levels. With highly doped Si, transport is dominated by tunneling and reveals sharp conductance onsets at room temperature. Using the temperature dependence of current across medium‐doped n‐Si, the molecular tunneling barrier can be separated from the Si‐Schottky one, which is a 0.47 eV, in agreement with the molecular‐modified surface dipole and quite different from the bare Si–H junction. This indicates that Pb evaporation does not cause significant chemical changes to the molecules. The ability to manufacture reliable devices constitutes important progress toward possible future hybrid Si‐based molecular electronics.     

Nanoparticle Linker‐Controlled Molecular Wire Devices Based on Double Molecular Monolayers

Highly conductive molecular wires are an important component for realizing molecular electronic devices and have to be explored in terms of interactions between molecules and electrodes in their molecular junctions. Here, new molecular wire junctions are reported to enhance charge transport through gold nanoparticle (AuNP)‐linked double self‐assembled monolayers (SAMs) of cobalt (II) bis‐terpyridine molecules (e.g., Co(II)(tpyphS)₂). Electrical characteristics of the double‐SAM devices are explored in terms of the existence of AuNP. The AuNP linker in the Co(II)(tpyphS)₂–AuNP–Co(II)(tpyphS)₂ junction acts as an electronic contact that is transparent to electrons. The weak temperature dependency of the AuNP‐linked molecular junctions strongly indicates sequential tunneling conduction through the highest occupied molecular orbitals (HOMOs) of Co(II)(tpyphS)₂ molecules. The electrochemical characteristics of the AuNP–Co(II)(tpyphS)₂ SAMs reveal fast electron transfer through molecules linked by AuNP. Density functional theory calculations reveal that the molecular HOMO levels are dominantly affected by the formation of junctions. The intermolecular charge transport, controlled by the AuNP linker, can provide a rational design for molecular connection that achieves a reliable electrical connectivity of molecular electronic components for construction of molecular electronic circuits.     

Vorticity evolution mechanism in near‐field of a transverse jet

Abstract

Flow structure and vorticity evolution processes in the near field of an elevated jet in a crossflow are experimentally studied in a wind tunnel. The instantaneous and time‐averaged flow field characteristics are observed and measured by using a flow visualization technique and a high‐speed Particle Image Velocimeter (PIV). Time histories of the instantaneous velocity of the vortical flows in the shear‐layer are recorded by a hot‐wire anemometer and a high‐speed data acquisition system in order to analyze the frequency characteristics of the traveling coherent structure in the shear‐layer. Experiments are performed between two different jet‐to‐crossflow momentum flux ratios R = 0.08 and 0.56, which are selected from two regimes with different kinds of flow patterns at a fixed crossflow Reynolds number 2051. The behaviors and mechanisms of the vortical flow structure and the vorticity evolution mechanisms appear to be distinct in different flow regimes. By analyzing the pictures of the smoke flow visualization and the instantaneous vorticity contour maps, two kinds of vorticity evolution mechanisms, “shear‐induced vortices” and “swing‐induced vortices”, can be identified in the shear‐layer evolving from the jet exit. The time‐averaged velocity field and vorticity properties are also discussed in this paper.     

Realization of In‐Plane p–n Junctions with Continuous Lattice of a Homogeneous Material

Two‐dimensional (2D) in‐plane p–n junctions with a continuous interface have great potential in next‐generation devices. To date, the general fabrication strategies rely on lateral epitaxial growth of p‐ and n‐type 2D semiconductors. An in‐plane p–n junction is fabricated with homogeneous monolayer Te at the step edge on graphene/6H‐SiC(0001). Scanning tunneling spectroscopy reveals that Te on the terrace of trilayer graphene is p‐type, and it is n‐type on monolayer graphene. Atomic‐resolution images demonstrate the continuous lattice of the junction, and mappings of the electronic states visualize the type‐II band bending across the space‐charge region of 6.2 nm with a build‐in field of 4 × 10⁵ V cm^?1. The reported strategy can be extended to other 2D semiconductors on patternable substrates for designed fabrication of in‐plane junctions.     

Study on bubble formation at a submerged micro‐hole by a two‐stage model

Abstract

In order to increase the accuracy of detached bubble volume prediction and to correct the theoretical weakness in models proposed previously, a modified two‐stage spherical bubble formation model is proposed in this study. This modified model takes account of an important factor, the length‐to‐diameter ratio of the micro‐hole by calculating the orifice constant with an entrance flow effect and expands the applicability of the modified model. Also, experimental tests were conducted to form bubbles at a submerged micro‐hole with diameters ranging from 60 to 1200 μm under variable pressure conditions due to continuous liquid drainage with drain rate ranging from 0.006 to 0.100 ml/s. The improved model indeed increases the ability to predict the detached bubble volume in the present study. Moreover, the results show that the condition for bubble formation in the present study could be shifted from a constant flow condition to a constant pressure condition depending on the orifice constant.     

A device for the fabrication of multifunctional particles from microbubble suspensions

A novel V-junction microfluidic (VJM) device has been used to generate several types of particles from the break-up of bubbles. The flow rates of selected solutions and the gas pressure were adjusted in order to successfully generate monodisperse polymer coated microbubbles, which serve as a platform for particle generation. Uniform particles in both the nano and micro size ranges with different shapes were spontaneously generated from bubbles and have the potential to be used in several advanced technological applications.     

Fabrication and Doping Methods for Silicon Nano‐ and Micropillar Arrays for Solar‐Cell Applications: A Review

Silicon is one of the main components of commercial solar cells and is used in many other solar‐light‐harvesting devices. The overall efficiency of these devices can be increased by the use of structured surfaces that contain nanometer‐ to micrometer‐sized pillars with radial p/n junctions. High densities of such structures greatly enhance the light‐absorbing properties of the device, whereas the 3D p/n junction geometry shortens the diffusion length of minority carriers and diminishes recombination. Due to the vast silicon nano‐ and microfabrication toolbox that exists nowadays, many versatile methods for the preparation of such highly structured samples are available. Furthermore, the formation of p/n junctions on structured surfaces is possible by a variety of doping techniques, in large part transferred from microelectronic circuit technology. The right choice of doping method, to achieve good control of junction depth and doping level, can contribute to an improvement of the overall efficiency that can be obtained in devices for energy applications. A review of the state‐of‐the‐art of the fabrication and doping of silicon micro and nanopillars is presented here, as well as of the analysis of the properties and geometry of thus‐formed 3D‐structured p/n junctions.     

Highly Reliable Ag Nanowire Flexible Transparent Electrode with Mechanically Welded Junctions

Deformation behavior of the Ag nanowire flexible transparent electrode under bending strain is studied and results in a novel approach for highly reliable Ag nanowire network with mechanically welded junctions. Bending fatigue tests up to 500 000 cycles are used to evaluate the in situ resistance change while imposing fixed, uniform bending strain. In the initial stages of bending cycles, the thermally annealed Ag nanowire networks show a reduction in fractional resistance followed by a transient and steady‐state increase at later stages of cycling. SEM analysis reveals that the initial reduction in resistance is caused by mechanical welding as a result of applied bending strain, and the increase in resistance at later stages of cycling is determined to be due to the failure at the thermally locked‐in junctions. Based on the observations from this study, a new methodology for highly reliable Ag nanowire network is proposed: formation of Ag nanowire networks with no prior thermal annealing but localized junction formation through simple application of mechanical bending strain. The non‐annealed, mechanically welded Ag nanowire network shows significantly enhanced cyclic reliability with essentially 0% increase in resistance due to effective formation of localized wire‐to‐wire contact.     

Non‐destructive Examination of Loads in Regular and Self‐locking Spiralock® Threads through Energy‐resolved Neutron Imaging

Energy‐resolved neutron transmission imaging is utilised for in situ comparisons of strain distributions in fastened assemblies with regular and self‐locking Spiralock® female threads. The strain maps measured within torqued steel bolts indicate that for a Spiralock® thread, the load is distributed over a larger section of the fastener, making this type of thread more suitable for fastening of assemblies subject to transverse vibrations.     

Preparation of Microcellular Epoxy Foams through a Limited‐Foaming Process: A Contradiction with the Time–Temperature–Transformation Cure Diagram

3D cross‐linking networks are generated through chemical reactions between thermosetting epoxy resin and hardener during curing. The curing degree of epoxy material can be increased by increasing curing temperature and/or time. The epoxy material must then be fully cured through a postcuring process to optimize its material characteristics. Here, a limited‐foaming method is introduced for the preparation of microcellular epoxy foams (Lim‐foams) with improved cell morphology, high thermal expansion coefficient, and good compressive properties. Lim‐foams exhibit a lower glass transition temperature (T_g) and curing degree than epoxy foams fabricated through free‐foaming process (Fre‐foams). Surprisingly, however, the T_g of Lim‐foams is unaffected by postcuring temperature and time. This phenomenon, which is related to high gas pressure in the bubbles, contradicts that indicated by the time–temperature–transformation cure diagram. High bubble pressure promotes the movement of molecular chains under heating at low temperature and simultaneously suppresses the etherification cross‐linking reaction during post‐curing.     

Ultrafast Solar‐Blind Ultraviolet Detection by Inorganic Perovskite CsPbX3 Quantum Dots Radial Junction Architecture

Inorganic CsPbX₃ (X = Cl, Br, I, or hybrid among them) perovskite quantum dots (IPQDs) are promising building blocks for exploring high performance optoelectronic applications. In this work, the authors report a new hybrid structure that marries CsPbX₃ IPQDs to silicon nanowires (SiNWs) radial junction structures to achieve ultrafast and highly sensitive ultraviolet (UV) detection in solar‐blind spectrum. A compact and uniform deployment of CsPbX₃ IPQDs upon the sidewall of low‐reflective 3D radial junctions enables a strong light field excitation and efficient down‐conversion of the ultraviolet incidences, which are directly tailored into emission bands optimized for a rapid photodetection in surrounding ultrathin radial p‐i‐n junctions. A fast solar‐blind UV detection has been demonstrated in this hybrid IPQD‐NW detectors, with rise/fall response time scales of 0.48/1.03 ms and a high responsivity of 54 mA W^?1@200 nm (or 32 mA W^?1@270 nm), without the need of any external power supply. These results pave the way toward large area manufacturing of high performance Si‐based perovskite UV detectors in a scalable and low‐cost procedure.     

Generalized Couette flow in channels of irregular cross‐section

Abstract

The flow behavior of non‐Newtonian power‐law fluids in channels of irregular cross‐section is examined. The driving force of the flow may be a constant pressure gradient (Poiseuille flow), a moving boundary (Couette flow) or the combination of the two (generalized Couette flow). There are three factors that influence the fluid motion in a channel, namely, the power‐law index n, the channel geometry and a dimensionless quantity E which can be viewed as the ratio of drag flow to pressure flow. The effects of these variables on velocity distributions and volumetric flow rates for various channel geometries are analyzed. The direct application of the numerical results on extruder design and operation is discussed.