Characterization of the Mixing Quality in Micromixers

The laminar flow patterns and mixing performance of two different micromixers have been investigated and quantified using CFD. The micromixer geometries consist of a channel with either diagonal or asymmetric herringbone grooves on the channel floor. The numerical results show that a single helical flow is produced for the diagonal mixer, whereas the herringbone mixer creates a double helical flow, composed of an alternating large and small vortex. Particle tracking of a tracer shows that very little convective mixing occurs in the diagonal mixer. However, in the herringbone mixer, very good mixing occurs. Quantitative analysis methods that are traditionally used for characterizing macro‐scale static mixers have been employed. Calculation of the variance of tracer dispersion and the stretching has shown to be well adapted for quantifying the mixing in the micromixers. However, methods based on the deformation rate appear to be less suitable. The results are in excellent agreement with previous experimental findings.     

汽液两相流中流体诱导振动的研究

流体横掠单管或管束时产生所谓诱导振动。这种诱导振动常常是引起热交换器损坏的主 要原困之一。P。idous幻s’“综述了由于流动所引起的诱导振动而导致热交换器损坏的50多种 情况。由仕可看出对诱导振动研究的必婴什。     

Polarization dependent optical absorption properties of single-walled carbon nanotubes and methodology for the evaluation of their morphology

Polarization dependence of the optical absorption properties of SWNTs is presented and investigated in detail for the energy range 0.5-6 eV. We found that the absorption peaks in the UV region at approximately 4.5 and 5.25 eV exhibit remarkable and different dependencies on the morphology of the SWNT film, or equivalently, on the incident light polarization relative to the SWNT axis. An analytical pathway to evaluate the physical degree of SWNT alignment for a vertically aligned SWNT film is developed with both transition dipoles parallel and perpendicular to the SWNT axis taken into account. This analytical procedure, coupled with polarized optical absorption measurements performed on the vertically aligned SWNT film grown on substrates, leads to the determination of the bare optical absorption cross-section of SWNTs for both parallel and perpendicular to SWNT axis. In the end, the proposed methodology for evaluating the SWNT film morphology is applied to investigate the transient change of the degree of alignment in the growth process of our vertically aligned SWNT films.     

Investigation of the convective motion through a staggered herringbone micromixer at low Reynolds number flow

A computational study is presented of the complex flow through a staggered herringbone micromixer (SHM), which utilises sequences of asymmetrical herringbone grooves in cycles where a set of topologically similar grooves represent a half cycle. It was analysed using finite-element (method) based software to elucidate the fluid flow within the channel and characterise the effect of the grooves at moving fluid across the channel thus creating non-axial fluid movement. Three separate physical systems were modelled: a channel containing a single groove, a half cycle of infinite grooves and an infinite system with one groove per half cycle. A range of groove heights were investigated for the single groove for the Reynolds number range 0-15 to identify the mechanics through which fluid is transported across the channel by the grooves, the effect that inertial and viscous forces have on the process and to identify a groove height range for optimised cross channel fluid transfer. The flow field within the grooves at various heights was analysed and their relationship with non-axial flow within the bulk channel identified. The culminating effect of increasing grooves per half cycle on their ability to transport fluid across the channel is analysed by comparing the entrainment of fluid into and across the groove for both a single and infinite grooves. The maximum increase in fluid entrainment per groove for the addition of extra grooves to a cycle was found to be 14%. The helicity (or swirl) of the flow within the channel is found to be small for all three systems, while increased helicity within the flow was found to correspond to an increase in energy dissipation.     

Two-step design protocol for patterned groove micromixers

Micromixers are essential components of microreactor technology. In this paper, a simple two-step design protocol for patterned groove micromixers based on numerical simulations is presented. In the first step, one groove of the staggered herringbone micromixer (SHM) is designed based on the average magnitude of transversal velocity v_AVGyz at the end of the groove. In the second step, different configurations of six grooves are investigated. A slightly better mixing is achieved compared to the established SHM and significantly fewer grooves are needed. Due to fewer grooves and rounded groove corners, the new design is easier to be produced by microengineering technologies (MET). Additionally, good mixing was also achieved with a modified slanted groove micromixer (SGM) configuration with the largest rounding radius at the edges. A SGM prototype was machined by micro EDM milling. The simulation results were experimentally verified with flow visualization and a good agreement was observed. The presented protocol vastly reduces the number of optimal patterned groove geometry configuration candidates to be evaluated; it is simple and effective for practical applications.     

Powder—wall friction: The effects of orientation of wall grooves and wall lubricants

The friction between compacted ferric oxide powders and cemented tungsten carbide walls has been determined. The influence of the surface structure of the walls on powder—wall friction was studied by comparing the results obtained with polished, spark-eroded, and ground walls having grooves oriented perpendicular or parallel to the direction of sliding of the wall. Typical differences in powder—wall friction, appear to be present between ground walls with grooves oriented parallel to the direction of sliding and the other walls. The effect of wall lubrication was investigaed by covering walls with a thin film of stearic acid having a thickness in the range from 0.2 to 2.4 μm. Typical diferences are found between the cases where the coating thickness is larger and smaller than the mean particle size of the powder. In practice it follows that powder—wall friction can be reduced by grinding die walls in the axial direction and by the application of lubricant film thicknesses much larger than the particle size.     

Directed crystallization of poly(3-hexylthiophene) in micrometre channels under confinement and in electric fields

We present two methods for controlling the in-plane alignment of polymer chains in poly(3-hexylthiophene) (P3HT) films within the channel of a field effect transistor device. Solvent-induced dewetting into the transistor channels of a prepatterned, bottom-gate bottom-contact transistor channel followed by controlled, low nucleation density recrystallization under confinement resulted in a preferential orientation of the π-stacked nanocrystalline lamellae parallel to the resulting P3HT micrometre-sized lines. The contrasting alignment, of perpendicular lamellae, was induced by application of an electric field during recrystallization. Preliminary measurements of the dependence of charge transport mobility on the global orientation of the polymer chain direction are consistent with faster charge transport perpendicular to the π-stacked lamellae (direction parallel to the polymer chains) compared to along the π-stacking direction in the common edge-on oriented morphology.     

Magnetic‐field‐assisted electrospinning highly aligned composite nanofibers containing well‐aligned multiwalled carbon nanotubes

Composite nanofiber meshes of well‐aligned polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP) nanofibers containing multiwalled carbon nanotubes (MWCNTs) were successfully fabricated by a magnetic‐field‐assisted electrospinning (MFAES) technology, which was confirmed to be a favorable method for preparation of aligned composite nanofibers in this article. The MFAES experiments showed that the diameters of composite nanofibers decreased first and then increased with the increase of voltage and MWCNTs content. With the increase of voltage, the degree of alignment of the composite nanofibers decreased, whereas it increased with increasing MWCNTs concentration. Transmission electron microscopy observation showed that MWCNTs were parallel and oriented along the axes of the nanofibers under the low concentration. A maximum enhancement of 178% in tensile strength was manifested by adding 2 wt % MWCNTs in well‐aligned composite nanofibers. In addition, the storage modulus of PAN/PVP/MWCNTs composite nanofibers was significantly higher than that of the PAN/PVP nanofibers. Besides, due to the highly ordered alignment structure, the composite nanofiber meshes showed large anisotropic surface resistance, that is, the surface resistance of the composite nanofiber films along the fiber axis was about 10 times smaller than that perpendicular to the axis direction. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41995.     

Highly aligned,large pore ordered mesoporous carbon films by solvent vapor annealing with soft shear

Macroscopic alignment of block copolymer (BCP)-templated mesoporous carbon films is challenging, especially for large pores (>10 nm), due to the slow dynamics of the polymer segments that impede re-orientation of the ordered domains. Here, we demonstrate a facile method, solvent vapor annealing with soft shear (SVA–SS), to fabricate unidirectionally aligned, ordered mesoporous carbon films using two different BCP templates, poly(ethylene oxide)-block-poly(n-butyl acrylate) and polystyrene-block-poly(N,N-dimethyl-n-octadecylammonium p-styrenesulfonate), and we illustrate the efficacy of this technique for both cylindrical and spherical morphologies with relatively large accessible pores (≈15 nm). This alignment is preserved through the thermopolymerization of resol and carbonization. The alignment of the mesopores impacts several key properties of these carbon films, especially for the unidirectional cylindrical mesostructures. The highly aligned mesoporous carbon films exhibit a more narrow pore size distribution than the analogous unaligned ordered mesoporous carbon as determined by ellipsometric porosimetry. Moreover, the electrical conductivity becomes anisotropic with nearly 40% difference in conductivity between parallel and perpendicular directions of the cylindrical mesopores. In the parallel orientation, the electrical conductivity is over 20% greater than the analogous unoriented (random) films. These results illustrate the applicability of SVA–SS to obtain unidirectional aligned mesoporous carbon films over large areas without additional physical or chemical templating.     

Characterizing intrinsic charges in top gated bilayer graphene device by Raman spectroscopy

In this work we study the behavior of the optical phonon modes in bilayer graphene devices by applying top gate voltage, using Raman scattering. We observe the splitting of the Raman G band as we tune the Fermi level of the sample, which is explained in terms of mixing of the Raman (E_g) and infrared (E_u) phonon modes, due to different doping in the two layers. We theoretically analyze our data in terms of the bilayer graphene phonon self-energy which includes non-homogeneous charge carrier doping between the graphene layers. We show that the comparison between the experiment and theoretical model not only gives information about the total charge concentration in the bilayer graphene device, but also allows to separately quantify the amount of unintentional charge coming from the top and the bottom of the system, and therefore to characterize the intrinsic charges of bilayer graphene with its surrounding environment.     

Pulling platinum atomic chains by carbon monoxide molecules

The interaction of carbon monoxide molecules with atomic-scale platinum nanojunctions is investigated by low temperature mechanically controllable break junction experiments. Combining plateau length analysis, two-dimensional conductance-displacement histograms and conditional correlation analysis a comprehensive microscopic picture is proposed about the formation and evolution of Pt-CO-Pt single-molecule configurations. Our analysis implies that before pure Pt monoatomic chains are formed a CO molecule infiltrates the junction, first in a configuration that is perpendicular to the contact axis. This molecular junction is strong enough to pull a monoatomic platinum chain with the molecule being incorporated in the chain. Along the chain formation the molecule can either stay in the perpendicular configuration, or rotate to a parallel configuration. The evolution of the single-molecule configurations along the junction displacement shows quantitative agreement with theoretical predictions, justifying the interpretation in terms of perpendicular and parallel molecular alignment. Our analysis demonstrates that the combination of two-dimensional conductance-displacement histograms with conditional correlation analysis is a useful tool to analyze separately fundamentally different types of junction trajectories in single molecule break junction experiments.     

Preparation of large-scale 2D zeolite crystal array structures to achieve optical functionality

We examined a method to refine a technique to prepare close-packed monolayers of cylindrical zeolite L crystals tethered on a substrate through covalent linkages, which was developed recently for popular use. We attempted to fabricate both vertical and horizontal–unidirectional alignments of the zeolite particles. The vertical alignment for centimeter scale large areas was achieved on a flat glass substrate by using flat-based zeolite crystals with an aspect ratio (length divided by width) of 1.1 while the crystals with an aspect ratio of 1.7 failed to align vertically. Thus the parameters important for successful alignment were determined. On the other hand, the unidirectional alignment in the horizontal packing structures was achieved by employing pregrooved substrates. We found that the relationship between the sizes of zeolite crystals and grooves is a crucial factor for the preparation of a close-packed structure. In this respect, our present preparation requires further improvements because it resulted in structures with many void spaces. Furthermore, we prepared the zeolite films loaded with unidirectionally aligned dyes to achieve optical functionalities.     

Effect of Grinding on Flaw Geometry and Fracture of Glass

Fracture mechanics is combined with fracture surface analysis to analyze brittle failure of glass bars which were tested relative to the direction of grinding. Grinding essentially produces two sets of flaws from which failure occurs. In the most severe set, formed basically parallel to the grinding direction, the ratio of the average depth ( a ) to the half-width ( b ) is 0.5. In the less severe set, formed perpendicular to the grinding direction, the average a / b ratio is 1.6. In both sets the most severe flaws are generally associated with a particularly deep grinding groove or gouge. The strength reduction resulting from testing perpendicular to the grinding direction results from the larger flaw size and slightly higher stress-intensity factor resulting from the greater ellipticity of the flaws formed parallel to the grinding grooves and perpendicular to the tensile axis. Detailed analysis of these 2 sets of flaws causing failure of appropriately oriented specimens shows that (1) the fracture mirror radius, r , occurs at a constant stress-intensity level independent of flaw geometry; (2) unsymmetric fracture mirrors result from unsymmetric, irregular flaws leading to unsymmetric stress-intensity distributions; (3) is constant for semielliptical flaws; and (4) fracture energy calculated from an expression including mirror constants, the flaw-to-mirror size ratio, and the flaw geometry agrees with measured values over a wide range of a / b values.     

Thermal diffusivity of heteroepitaxial diamond films: Experimental setup and measurements

The thermal diffusivity of heteroepitaxial CVD diamond films grown on iridium buffer layers has been measured using a combined laser flash and converging thermal wave setup. Absolute values and anisotropy for a fiber-textured reference sample were in the range of former reports in the literature. The in-plane thermal conductivity for three heteroepitaxial samples grown on Ir/YSZ/Si(001) as deduced from the diffusivity measurements was around 20 W/cm K, similar to high purity large grain polycrystalline films. Laser flash measurements of the perpendicular diffusivity suggest that the defect rich first microns of the heteroepitaxial films represent a thermal series resistance which limits the perpendicular heat transport especially for thin films. For the parallel component of the diffusivity the contribution of this shunt resistance is negligible. The absolute values for the parallel component in the heteroepitaxial films with in-plane angular spread of the crystal lattice below 0.5° were discussed in the framework of the model proposed by Klemens for phonon scattering by grain boundaries. The present data indicate that the remaining defects in heteroepitaxial diamond films with low mosaic spread are significantly less detrimental for the heat transport than large angle grain boundaries. In addition we speculate that the exclusive deposition on the {100} growth sector may also reduce the influence of nitrogen in the gas phase on the heat transport properties.     

Magnetically processed carbon nanotube/epoxy nanocomposites: Morphology, thermal, and mechanical properties

The processing-structure-property relationships of multiwalled carbon nanotubes (MWNTs)/epoxy nanocomposites processed with a magnetic field have been studied. Samples were prepared by dispersing the nanotube in the epoxy and curing under an applied magnetic field. The nanocomposite morphology was characterized with Raman spectroscopy and wide angle X-ray scattering, and correlated with thermo-mechanical properties. The modulus parallel to the alignment direction, as measured by dynamic mechanical analysis, showed significant anisotropy, with a 72% increase over the neat resin, and a 24% increase over the sample tested perpendicular to the alignment direction. A modest enhancement in the coefficient of thermal expansion (CTE) parallel to the alignment direction was also observed. These enhancements were achieved even though the nanotubes were not fully aligned, as determined by Raman spectroscopy. The partial nanotube alignment is attributed to resin a gel time that is faster than the nanotube orientation dynamics. Thermal conductivity results are also presented.     

A phenomenological model for the viscosity reductions in blends of HMMPE containing a small quantity of thermotropic liquid crystalline copolyester HBA/HQ/SA

In this paper, we present a phenomenological model for the viscosity changes in bulk high molecular mass polyethylene (HMMPE) due to the addition of a very small quantity of a main chain longitudinal thermotropic liquid crystalline polymer (TLCP) containing flexible spacers. The chain alignment in the elongated TLCP domains causes chain alignment and disengagement in the neighboring HMMPE melt. In converging capillary flows, this occurs at a certain critical centerline velocity. After the onset of such transition, melt of elongated chain conformations forms from the center core and expands towards the capillary wall with increasing flow rates. The model successfully predicts both the drastic viscosity reduction effects and the critical yield stress in the HMMPE+TLCP blends without any adjustable parameters. Our model is also applicable to other systems that undergo flow-induced phase transitions, e.g. in biphasic liquid crystalline polymer melts.     

Correlation between polymer architecture, mesoscale structure and photovoltaic performance in side-chain-modified poly(p-arylene-ethynylene)-alt-poly(p-arylene-vinylene): PCBM bulk-heterojunction solar cells

Recent investigations have shown that an anthracene containing poly(p-arylene-ethynylene)-alt-poly(p-arylene-vinylene) statistical copolymer consisting of a well defined conjugated backbone, along which linear and branched alkoxy side chains are attached in a random manner, yields, compared to its counterparts with regular side chain substitution, an improved performance in polymer [6,6]:-phenyl-C₆₁-butyric acid methyl ester (PCBM) bulk-heterojunction solar cells. The microscopic origin for the improved power conversion efficiency (η ≈ 3.8%) of the statistical copolymer - which is the best in its material class - has not been resolved. We conducted grazing incidence wide-angle x-ray scattering investigations in order to correlate the nanomorphology of the active layers to the photovoltaic performance of the device. A comparison of the results obtained for the statistical copolymer to those obtained for the corresponding regular copolymers shows that the improved performance of the former may be attributed to a combination of the following structural characteristics: 1. well, ordered stacked domains that promote backbone planarization, 2. partly face-on alignment of domains relative to the electrodes for an improved active layer-electrode charge transfer, and 3. a more isotropic domain orientation throughout the active layer that ensures that the backbone alignment direction has components perpendicular and parallel to the electrodes in order to compromise between light absorption and efficient intra-chain charge transport. The regular copolymers exhibiting inferior performance lack either sufficient stacking order or face-on alignment of the domains. None of them shows isotropic domain orientation.     

Perpendicular Alignment of 2D Nanoplatelet Emitters in Electrospun Fibers: A Result of the Barus Effect?

Stable jet electrospinning (SJES) is a special form of optical fiber generation that prevents chaotic fiber whipping typical for conventional electrospinning procedures. Incorporation of highly emissive semiconductor nanoplatelets (NPLs) in such fibers has very high potential in optical data transmission, optological circuits, fiber lasers, solar light concentrators and many other fields because NPLs exhibit strongly directed emission from their surface plane due to various in-plane transition dipole moments. However, potential orientation control of 2D-NPLs in SJES is entirely unknown as electric fields and various mechanical forces contribute in a complex manner simultaneously. Here, the observation of counter-intuitive yet very beneficial orientation of rectangular CdSe/CdS 2D-NLP in SJES perpendicular to the fiber drawing axis is reported. Scanning electron microscopy, 3D-single particle excitation polarization microscopy, 3D-photogoniometry, polarized emission spectroscopy and small angle X-ray scattering (SAXS) demonstrate aggregation free perpendicular alignment of the NPLs in poly(methyl methacrylate) (PMMA) fibers, resulting in dominant emission in directions parallel to the fiber. It is suggested that the observed vertical alignment is due to normal forces resulting from viscoelastic expansion when the polymer solution leaves the cannula (Barus effect) and that using such perpendicular nano-emitter alignment forces allows for the generation of novel materials also beyond fibers.     

Focus on the interlude between topographic transition and cell response on shape-memory surfaces

Shape-memory surfaces with on demand, tunable nano-patterns have been developed to observe time dependent changes in fibroblast cell alignment using temperature-responsive poly(ε-caprolactone) (PCL) films. The PCL films were prepared by crosslinking tetra-branched and linear PCLs, each with acrylate end-groups. Permanent surface patterns were generated by crosslinking the PCLs in a mold. Temporary surface patterns were later embossed into the crosslinked PCL. NIH 3T3 cells cultured on the temporal nanopatterns showed marked alignment along the pattern direction, regardless of their ridge and groove widths. Then, the direction of grooves was suddenly transitioned 90° to the temporary ridges. Holographic microscope revealed that the application of heat quickly and completely transitioned temporal to permanent patterns within 30 s. However, it took more than 2 h for cells on substrate with 500 nm grooves to change their orientation, while it took more than 8 h on substrate with 2000 nm grooves. This different alignment behavior can be explained by the different adhesion strength and reorganization of cytoskeletal proteins on nano-v.s. micro-patterns. Dynamically tunable nano-structured surfaces, therefore, can be used to study the effects of surface nano-geometries on time-dependent cytoskeleton remodeling under biological relevant conditions.     

Symmetries and selection rules in the measurement of the phonon spectrum of graphene and related materials

When the phonon spectrum of a material is measured in a scattering experiment, selection rules preclude the observation of phonons that are odd under reflection by the scattering plane. Understanding these rules is crucial to correctly interpret experiments and to detect broken symmetries. Taking graphene as a case study, in this work we derive the complete set of selection rules for the honeycomb lattice, showing that some of them have been missed or misinterpreted in the literature. Focusing on the technique of high-resolution electron energy loss spectroscopy (HREELS), we calculate the scattering intensity for a simple force constant model to illustrate these rules. In addition, we present HREELS measurements of the phonon dispersion for graphene on Ru(0 0 0 1) and find excellent agreement with the theory. We also illustrate the effect of different symmetry breaking scenarios in the selection rules and discuss previous experiments in light of our results. Finally we clarify why the shear horizontal label is not equivalent to odd parity, and how this can be misleading in the identification of selection rules.