Μm Length Scale Research Articles

Rapid crystallization of precious-metal-mineralized layers in mafic magmatic systems

The solidified remnants of mafic magmatic systems host the greatest concentrations of platinum-group metals in the Earth’s crust. Our understanding of precious-metal mineralization in these intrusive bodies is underpinned by a traditional view of magma chamber processes and crystal mush solidification. However, considerable uncertainty remains regarding the physical and temporal controls on concentrating these critical metals, despite their importance to modern society. We present high-precision 87Sr/86Sr analyses of plagioclase and clinopyroxene from within centimetre-thick precious-metal-enriched layers in the Palaeogene open-system Rum layered intrusion (northwest Scotland). Isotopic heterogeneity is present between plagioclase crystals, between clinopyroxene and plagioclase and within plagioclase crystals throughout the studied section. On the basis of these observations, we demonstrate that platinum-group element mineralization formed by repeated small-volume reactive melt percolation events. The preservation of strontium isotope heterogeneities at 10–100 µm length scales implies cooling of the melts that formed the precious-metal-rich layers occurred at rates greater than 1 °C per year, and cooling to diffusive closure within tens to hundreds of years. Our data highlight the importance of cyclic dissolution–recrystallization events within the crystal mush and raise the prospect that precious-metal-bearing mafic intrusions may form by repeated self-intrusion during cooling and solidification. Mineralization of platinum-group elements in mafic intrusions occurs due to repeated self-intrusion of magma, according to strontium isotope heterogeneities preserved in the Rum layered intrusion, Scotland.

Dynamic Leidenfrost temperature of saturated water drops on textured surfaces

In this study, we propose an analytical model to predict dynamic Leidenfrost temperature for saturated water drops impacting on superheated smooth and textured surfaces. To define the Leidenfrost triggering mechanism, this model postulates a balance relation between the downward pressure by the drop itself and the resistant pressure arising from vaporization from the base of the drop; the former is expressed as the sum of dynamic and water hammer pressures induced by the drop motion while the latter is modeled with pressure buildup effect due to vapor flow within the thin film under the drop. The textured surfaces have uniformly distributed circular pillars with ~10 µm length scale, and the center-to-center pitch of the pillars varies from 15 to 120 µm. The experimental results show that the Leidenfrost temperature on textured surfaces increases at the same Weber number (We), as the pillar pitch becomes coarser. However, the Leidenfrost temperatures on the textured surfaces with relatively fine pitch were found to be rather lower than that on the smooth surface at the same We. Those experimental data are well predicted by the theoretical model, in which two simple equations with two unknowns (Leidenfrost temperature and thickness of thin vapor layer) are derived; one is based on the pressure balance relation and the other postulates an initial transient phase during drop impact.

Microstructure of polymer-imprinted metal–organic frameworks determined by absorption edge tomography

Abstract Mechanochemically synthesized metal–organic framework material HKUST-1 in combination with acrylonitrile butadiene styrene polymer was used to form a polymer metal–organic framework composite material by a simple extruder. This composite filament was used for 3D printing. X-ray diffraction measurements were used to prove the homogeneous distribution of the metal–organic framework in the polymer on a centimeter scale, whereas X-ray Absorption Edge Tomography using a synchrotron radiation source was able to evaluate the 3D distribution of the metal–organic framework material both in the filament and the resultant printed sample with a resolution of a few μm. Our very first data indicate that, apart from a few clusters having significantly higher Cu concentration, HKUST-1 is distributed homogeneously down to the 100 μm length scale in both polymer bulk materials in the form of clusters with a size of a few μm. Absorption Edge Tomography in combination with data fusion also allows for the calculation of the metal–organic framework amount located on the external polymer surface.

Hierarchical structure and the origins of coercivity in exchange-coupled Co-Pt nanochessboards

Abstract We report the first detailed investigation into how exchange-coupled magnetic properties evolve during the formation of the Co-Pt L10 + L12 nanochessboards, which self-assemble by a pseudo-spinodal mechanism below the eutectoid isotherm. The maximum observed coercivities exceed 3 kOe, but these values are more than 5 times lower than the largest values predicted by micromagnetics simulations of ideal, single-colony chessboards. For magnetization reversal controlled by nucleation, a simple analysis readily reconciles this discrepancy by accounting for misorientation associated with the polycrystalline structure that exists at the 50 µm lengthscale, demagnetizing fields associated with eutectoid colonies at the 0.5 µm lengthscale, and non-idealities in the chessboard structure at the tiling lengthscale of about 20 nm.

Diffusiophoresis in ionic surfactants: effect of micelle formation.

We explore the consequences of micelle formation for diffusiophoresis of charged colloidal particles in ionic surfactant concentration gradients, using a quasi-chemical association model for surfactant self assembly. The electrophoretic contribution to diffusiophoresis is determined by re-arranging the Nernst-Planck equations, and the chemiphoretic contribution is estimated by making plausible approximations for the density profiles in the electrical double layer surrounding the particle. For sub-micellar solutions we find that a particle will typically be propelled down the concentration gradient, although electrophoresis and chemiphoresis are finely balanced and the effect is sensitive to the detailed parameter choices and simplifying assumptions in the model. Above the critical micelle concentration (CMC), diffusiophoresis becomes much weaker and may even reverse sign, due to the fact that added surfactant goes into building micelles and not augmenting the monomer or counterion concentrations. We present detailed calculations for sodium dodecyl sulfate (SDS), finding that the typical drift speed for a colloidal particle in a ∼100 μm length scale SDS gradient is ∼1 μm s-1 below the CMC, falling to ⪅0.2 μm s-1 above the CMC. These predictions are broadly in agreement with recent experimental work.

Necklace-like microstructure in shallow-quenched aqueous solutions of poly(n-isopropylacrylamide), detected by advanced small-angle neutron scattering methods.

The microstructure of aqueous poly(N-isopropyl acrylamide) (PNIPA) gel and solution was investigated by small-angle neutron scattering (SANS) in the vicinity of the gel volume phase transition at TV (= 34 °C). The SANS technique was reinforced by refractive neutron lenses and perfect single crystals in order to get access to μm length scales. At 31 °C SANS shows Ornstein-Zernike (OZ) type scattering in the swollen gel which at 32 °C starts to deviate from the OZ-formalism, exhibiting excess scattering and at the wave number qc≅ 5 × 10-3Å-1 a crossover to Porod's asymptotic q-4 power law. For shallow quenches of ΔT < 1.0 K above TV the excess scattering intensity is further increasing whereas qc is shifting toward lower values. Based on this observation and analysis of the SANS q-profiles, we propose a necklace-like microstructure consisting of PNIPA-rich globules of R≅ 100 Å size which are connected by swollen PNIPA chains and stabilized for more than a day by pinning of chain connectivity. The formation of PNIPA globules near TV is discussed in terms of partially cooperative dehydration which is crucial to explain the "miscibility square phase behavior" of aqueous PNIPA solutions. Globule-like structure was also found in aqueous PNIPA solution of size slightly larger than in gels. At deeper quenches of gels above TV (ΔT > 1.0 K) the globules are aggregating to larger objects of R≅ 0.24 μm size as determined from a strong intensity upturn in the small q-region of USANS.

Angle-dependent sputter yield measurements of keV D ions on W and Fe and comparison with SDTrimSP and SDTrimSP-3D

The influence of surface roughness on the sputter yield was investigated as a function of the angle of incidence. In this work, nm-smooth and rough samples with roughnesses on the µm length scale were produced by depositing thin Fe and W films on smooth and rough Si substrates via magnetron sputtering. The surface morphology of the samples was determined by atomic force microscopy. The samples were exposed to a 6 keV D3+ ion beam (2 keV/D) under various angles of incidence ranging from 0° to 75° with respect to the surface normal to fluences of the order of 1022 D/m2. The layer thickness was measured by Rutherford backscattering spectrometry (RBS) before and after erosion. The resulting sputter yields were compared to simulations performed with SDTrimSP (static and dynamic) and SDTrimSP-3D (static), showing good qualitative agreement in all cases, as well as agreement with literature data at normal incidence. This constitutes the first experimental benchmark of SDTrimSP-3D. A discrepancy in the value of the sputter yield for smooth W at normal incidence was observed between the SDTrimSP simulations and the experimental values obtained in this work and found in literature. Analogous experiments were performed to study the sputter yield at normal incidence of 2 keV/D on smooth Au and 6 keV He on smooth W. These sputter yields were also compared to SDTrimSP simulations and literature, showing good agreement in all cases.

Determination of Ce3+ and Co2+ Diffusion Coefficients in NR211 Membrane

The proton exchange membrane fuel cell (PEMFC) is a promising clean energy propulsion system for automobiles and other vehicles. A critical component of the PEMFC is the polymeric ionomer membrane which enables proton conduction and provides a crucial barrier to both reactant gas crossover and electrical shorting. Many fuel cell systems employ thin (12-25 µm) perfluorosulfonic acid (PFSA) based membranes due to their superior mechanical and chemical stability properties. The primary product of a PEMFC is water, generated via the four-electron oxygen reduction reaction. However, smaller amounts of highly oxidizing species such as hydroxyl radical (•OH), and hydrogen peroxide (H2O2) are generated either chemically or electrochemically. These, and other, oxidizing species can react with non-fluorocarbon end groups or with acid side chain functional groups to degrade and weaken the polymeric membrane until causing a physical breach. In the absence of effective chemical mitigation, proton exchange membranes will survive no more than a few hundred hours of operation under typical automotive duty cycles involving large humidity and potential variations. Fortunately, PFSA lifetimes can be extended up to and beyond 10,000 hours of operation using •OH redox quenchers such as Ce3+ and Mn2+ that efficiently reduce •OH to H2O. The remarkable effectiveness of these redox quenchers is due to their high competitive activity toward •OH reduction and the rapid and benign reduction of the oxidized metal to the reduced form via reaction with ubiquitous H2O2. Within an ionomer, cations, including Ce3+, associate (ion pair) with the negatively charged sulfonate groups to varying degrees and have been observed to diffuse in-plane on the centimeter length scale during ten to100 hours of fuel cell operation. More rapid migration of cations into the electrode layers has been observed over the µm length scale of MEA thickness. The movement of cations within the MEA is governed by local water content (λ = n H2O/-SO3 -), water content gradients (Δλ) and electrical potential gradients. The observed in-plane movement of Ce3+ and other cations such as Co2+ can lead to large cation concentration gradients within a fuel cell MEA. This redistribution of cations can have a significant impact on cell performance and mitigation of chemical degradation processes. Accordingly, there is great interest in understanding and quantifying the factors that govern cation movement to optimize fuel cell efficiency and durability. We will report the in-plane isotropic diffusion coefficients of Ce3+ and Co2+ within NRE211 membrane over a wide range of uniform temperature and relative humidity values. The cation concentrations have been monitored using a variety of analytical techniques including FTIR, x-ray fluorescence and UV spectroscopy (Figure 1). The derived diffusion coefficients show a strong dependence on both temperature and relative humidity value (water content) and are consistent with the macroscale redistribution of cations seen in operating fuel cells. Quantitative structure activity relationships and cation movement under RH gradients will also be discussed. Figure 1

Roll-to-roll fabrication of hierarchical superhydrophobic surfaces

In this letter, we report a roll-to-roll fabrication method to develop a hierarchical nanopatterned superhydrophobic surface. The hierarchical pattern includes a primary micropattern with an overlayed secondary nanopattern. The primary pattern of 15–30 μm length scales was fabricated through UV nanoimprint lithography. The secondary nanopattern of 20 nm size was generated through a subsequent scalable spray coating with hydrophobic silica nanoparticles to create a nanoscale random roughness over the primary pattern. The secondary nanocoating over the primary pattern resulted in an enhanced contact angle resulting in superhydrophobicity and reduced hysteresis. Freezing droplet measurements performed have demonstrated a fivefold increase in the time for the droplet to freeze on a superhydrophobic surface compared to a primary hydrophobic structure.

Imaging Anomalous Nematic Order and Strain in Optimally Doped BaFe_{2}(As,P)_{2}.

We present the strain and temperature dependence of an anomalous nematic phase in optimally doped BaFe_{2}(As,P)_{2}. Polarized ultrafast optical measurements reveal broken fourfold rotational symmetry in a temperature range above T_{c} in which bulk probes do not detect a phase transition. Using ultrafast microscopy, we find that the magnitude and sign of this nematicity vary on a 50-100 μm length scale, and the temperature at which it onsets ranges from 40K near a domain boundary to 60K deep within a domain. Scanning Laue microdiffraction maps of local strain at room temperature indicate that the nematic order appears most strongly in regions of weak, isotropic strain. These results indicate that nematic order arises in a genuine phase transition rather than by enhancement of local anisotropy by a strong nematic susceptibility. We interpret our results in the context of a proposed surface nematic phase.

Quasicrystalline Ordering in Thin Liquid Crystal Films

Quasicrystalline ordering was first observed in synthetic multi-component metallic alloys. These solid state materials exhibit quasicrystalline atomic ordering at nanometer length scales. Softmatter systems are another class of versatile materials that can exhibit quasicrystalline ordering across supra-nanometer (&gt;10 nm) to supra-micrometer (&gt;10 μm) length scales as recently observed in materials like-supramolecular dendritic molecules, ABC star polymers, binary nanoparticle systems and block co-polymers in condensed matter systems. The underlying mechanism in most of these soft quasicrystals seems to be the presence of two or more length scales in the system. Another class of development in self-assembled quasicrystals in softmatter is being observed in low molecular weight chiral and achiral nematic liquid crystals. Liquid crystal forms an efficient matrix for self- and directed-assemblies of colloidal structures where surface and geometry-tuning the particles in nematic liquid crystals gives rise to complex inter-particle interactions while the long-range order results in self-assembled structures of higher order rotational symmetries. Furthermore, there has also been attempts to generate colloidal quasicrystalline defect structures by directing the assemblies using multiple and single beam lasing techniques. In the present article, we will review self- and assisted-assembly of quasicrystalline structures in nematic liquid crystals (both chiral and achiral) and discuss the underlying mechanisms.

(Invited) How to Deal with Electrode Heterogeneity in Cell-Level Battery Modeling

As shown in Fig. 1, typical porous electrodes (in this case for a Li-ion anode) clearly have heterogeneity on the µm and smaller length scales. In addition, it has been shown that there is heterogeneity on mm length scale for even commercial quality electrodes [1]. This heterogeneity is a consequence of the particle-based manufacturing processes [2]. Furthermore, this heterogeneity is undesirable as it essentially requires that the electrodes be overdesigned to achieve adequate minimum performance. The commonly accepted Newman-type model of the full-cell sandwich essentially averages over local heterogeneities in porous electrodes in order to make the system “macrohomogeneous.” The resulting model allows for uniform pathways for electrons and ions. Homogenization of the microstructure greatly decreases the computational cost of the model while retaining the essential physics of transport and reaction within the electrodes. In this presentation I will discuss how one can measure heterogeneities as well as average values of electronic and ionic conductivities [1,3]. These can be compared to empirical correlations such as the Bruggeman equation. Our group has considerable experience in this area. With respect to modeling, I will discuss strategies for dealing with heterogeneities in the context of Newman-type models. This can include using multiple particle types [4] as well as effective medium theories that allow one to average local values of physical properties to generate an overall or effective medium property. Lastly, I will discuss what effect such averaging has on predictions of the battery model. This work was supported by the U.S. Department of Energy through the BMR program. [1] Lanterman et al., J. Electrochem. Soc. 162, A2145 (2015). [2] Forouzan et al., J. Power Sources 312, 172 (2016). [3] Zacharias et al., J. Electrochem. Soc. 160, A306 (2013). [4] Thorat et al., J. Electrochem. Soc. 158, A1185 (2011). Fig. 1 SEM/FIB cross section of Li-ion anode showing local heterogeneity. Figure 1

Macro- and microscale structure formation and aging in different arrested states of Laponite dispersions

Structure formation and aging in different arrested states of Laponite dispersions have been investigated at the macro- and microscale. Covering a wide range of solid content and salt concentrations at different pH glasses, strong and weak gels with prevailing edge-to-face (EF) or face-to-face (FF) layer contacts were formed. Mechanical shear and squeeze flow rheometry were combined with diffusing wave spectroscopy and multiple particle tracking (MPT) microrheology. Strong attractive gels form much more quickly than weak gels, and particularly repulsive glasses. Gels with preferred EF contacts are stronger and are created more quickly than gels with prevailing FF contacts. Strong gels show little aging and exhibit a weak increase of G′ ∼ tα with α = 0.11 ± 0.03, higher α values are found for weak gels, and the strongest aging is observed in glasses. MPT data reveal structural refinement at the submicrometer length scale during aging for gels but not for glasses. Strong structural heterogeneity most pronounced at pH = 8.5 occurs during gel or glass formation, but at longer times, all arrested states appear homogenous at the 0.2 μm length scale. Finally, all arrested states exhibit power law frequency dependence G″ ∼ ω0.75 at high frequencies attributed to internal bending modes of layers.Structure formation and aging in different arrested states of Laponite dispersions have been investigated at the macro- and microscale. Covering a wide range of solid content and salt concentrations at different pH glasses, strong and weak gels with prevailing edge-to-face (EF) or face-to-face (FF) layer contacts were formed. Mechanical shear and squeeze flow rheometry were combined with diffusing wave spectroscopy and multiple particle tracking (MPT) microrheology. Strong attractive gels form much more quickly than weak gels, and particularly repulsive glasses. Gels with preferred EF contacts are stronger and are created more quickly than gels with prevailing FF contacts. Strong gels show little aging and exhibit a weak increase of G′ ∼ tα with α = 0.11 ± 0.03, higher α values are found for weak gels, and the strongest aging is observed in glasses. MPT data reveal structural refinement at the submicrometer length scale during aging for gels but not for glasses. Strong structural heterogeneity most prono...

Laser-Directed Self-Assembly of Highly Aligned Lamellar and Cylindrical Block Copolymer Nanostructures: Experiment and Simulation

Laser photothermal annealing is emerging as a promising strategy for directed self-assembly of block copolymers along with its unique advantages, such as area selectivity, solvent-free ultrafast process, and highly oriented nanopattern formation without substrate prepatterning. We investigate laser-induced highly aligned lamellar and cylindrical self-assembled nanostructure formation by means of simulation as well as experiment. Self-assembled surface-perpendicular lamellar or surface-parallel cylindrical nanodomains in PS-b-PMMA thin films could be aligned by lateral steady scan of focused laser irradiation to attain excellent long-range order over 10 μm length scale. For the systematic understanding of the experimental observation, quasi-static simulation employing successive self-consistent field theory calculation has been developed. Miniaturized simulations of experimental systems could confirm a strong tendency for lamellar domains to grow in the direction of laser scanning. Cylindrical self-assembl...

Towards quantitative small-scale thermal imaging

Quantitative thermal imaging has the potential of reliable temperature measurement across an entire field-of-view. This non-invasive technique has applications in aerospace, manufacturing and process control. However, robust temperature measurement on the sub-millimetre (30 μm) length scale has yet to be demonstrated. Here, the temperature performance and size-of-source (source size) effect of a 3–5 μm thermal imaging system have been assessed. In addition a technique of quantifying thermal imager non-uniformity is described. An uncertainty budget is constructed, which describes a measurement uncertainty of 640 mK (k=2) for a target with a size of 10 mm. The results of this study provide a foundation for developing the capability for confident quantitative sub-millimetre thermal imaging.

Discussion on data correction for Polarization Analysis with a 3He spin filter analyzer

Fully polarized neutron reflectometry and grazing incidence small angle neutron scattering are effective methods to explore magnetic structures on the nm to μm length scales. This paper is an outline of how to fully correct for the polarization analysis (PA) inefficiencies of such an instrument and to determine the error contributions of the neutron polarizer and analyzer. This discussion considers the exact case of the polarization analysis instrumentation used on the MARIA neutron reflectometer at the MLZ or for a general polarized neutron scattering instrument using at least one 3He neutron spin filter that has the capability for adiabatic fast passage nuclear magnetic resonance flipping of the 3He polarization. This paper will work to build a conceptual understanding of how the inefficiencies of neutron polarization elements affect measured data in order to stress and encourage the application of PA corrections and to help perform successful measurements. Then, using data from a fully polarized neutron reflectometer test measurement we show how it is possible to recover signals on the order of, or even smaller than, the inefficiencies, or bleed-through, of the neutron polarization devices used.

Hybrid approach combining multiple characterization techniques and simulations for microstructural analysis of proton exchange membrane fuel cell electrodes

Abstract The cost and performance of proton exchange membrane fuel cells strongly depend on the cathode electrode due to usage of expensive platinum (Pt) group metal catalyst and sluggish reaction kinetics. Development of low Pt content high performance cathodes requires comprehensive understanding of the electrode microstructure. In this study, a new approach is presented to characterize the detailed cathode electrode microstructure from nm to μm length scales by combining information from different experimental techniques. In this context, nano-scale X-ray computed tomography (nano-CT) is performed to extract the secondary pore space of the electrode. Transmission electron microscopy (TEM) is employed to determine primary C particle and Pt particle size distributions. X-ray scattering, with its ability to provide size distributions of orders of magnitude more particles than TEM, is used to confirm the TEM-determined size distributions. The number of primary pores that cannot be resolved by nano-CT is approximated using mercury intrusion porosimetry. An algorithm is developed to incorporate all these experimental data in one geometric representation. Upon validation of pore size distribution against gas adsorption and mercury intrusion porosimetry data, reconstructed ionomer size distribution is reported. In addition, transport related characteristics and effective properties are computed by performing simulations on the hybrid microstructure.

Directed Growth of Orthorhombic Crystals in a Micropillar Array.

We report directed growth of orthorhombic crystals of potassium permanganate in spatial confinement of a micropillar array. The solution is introduced by spontaneous wicking to give a well-defined film (thickness 10-15 μm; volume ∼600 nL) and is connected to a reservoir (several microliters) that continuously "feeds" the evaporating film. When the film is supersaturated, crystals nucleate and preferentially grow in specific directions guided by one of several possible linear paths through the pillar lattice. Crystals that do not initially conform are stopped at an obstructing pillar, branch into another permitted direction, or spontaneously rotate to align with a path and continue to grow. Microspectroscopy is able to track the concentration of solute in a small region of interest (70 × 100 μm2) near to growing crystals, revealing that the solute concentration initially increases linearly beyond the solubility limit. Crystal growth near the region of interest resulted in a sharp decrease in the local solute concentration (which rapidly returns the concentration to the solubility limit), consistent with estimated diffusion time scales (<1 s for a 50 μm length scale). The ability to simultaneously track solute concentration and control crystal orientation in nanoliter samples will provide new insight into microscale dynamics of microscale crystallization.

Multi-scale simulations of apatite–collagen composites: from molecules to materials

We review scale-bridging simulation studies for the exploration of atomicto-meso scale processes that account for the unique structure and mechanic properties of apatite-protein composites. As the atomic structure and composition of such complex biocomposites only partially is known, the first part (i) of our modelling studies is dedicated to realistic crystal nucleation scenarios of inorganic-organic composites. Starting from the association of single ions, recent insights range from the mechanisms of motif formation, ripening reactions and the self-organization of nanocrystals, including their interplay with growth-controlling molecular moieties. On this basis, (ii) reliable building rules for unprejudiced scale-up models can be derived to model bulk materials. This is exemplified for (enamel-like) apatite-protein composites, encompassing up to 106 atom models to provide a realistic account of the 10 nm length scale, whilst model coarsening is used to reach μm length scales. On this basis, a series of deformation and fracture simulation studies were performed and helped to rationalize biocomposite hardness, plasticity, toughness, self-healing and fracture mechanisms. Complementing experimental work, these modelling studies provide particularly detailed insights into the relation of hierarchical composite structure and favorable mechanical properties.

Search for Screened Interactions Associated with Dark Energy below the 100 μm Length Scale.

We present the results of a search for unknown interactions that couple to mass between an optically levitated microsphere and a gold-coated silicon cantilever. The scale and geometry of the apparatus enable a search for new forces that appear at distances below 100 μm and which would have evaded previous searches due to screening mechanisms. The data are consistent with electrostatic backgrounds and place upper limits on the strength of new interactions at <0.1 fN in the geometry tested. For the specific example of a chameleon interaction with an inverse power law potential, these results exclude matter couplings β>5.6×10^{4} in the region of parameter space where the self-coupling Λ≳5 meV and the microspheres are not fully screened.