Crystal-like Order Research Articles

Liquid-phase characterization of molecular interactions in polyunsaturated and n-fatty acid methyl esters by (1)H low-field nuclear magnetic resonance.

BackgroundTo identify and develop the best renewable and low carbon footprint biodiesel substitutes for petroleum diesel, the properties of different biodiesel candidates should be studied and characterized with respect to molecular structures versus biodiesel liquid property relationships. In our previous paper, 1H low-field nuclear magnetic resonance (LF-NMR) relaxometry was investigated as a tool for studying the liquid-phase molecular packing interactions and morphology of fatty acid methyl esters (FAMEs). The technological potential was demonstrated with oleic acid and methyl oleate standards having similar alkyl chains but different head groups. In the present work, molecular organization versus segmental and translational movements of FAMEs in their pure liquid phase, with different alkyl chain lengths (10–20 carbons) and degrees of unsaturation (0–3 double bonds), were studied with 1H LF-NMR relaxometry and X-ray, 1H LF-NMR diffusiometry, and 13C high-field NMR.ResultsBased on density values and X-ray measurements, it was proposed that FAMEs possess a liquid crystal-like order above their melting point, consisting of random liquid crystal aggregates with void spaces between them, whose morphological properties depend on chain length and degree of unsaturation. FAMEs were also found to exhibit different degrees of rotational and translational motions, which were rationalized by chain organization within the clusters, and the degree and type of molecular interactions and temperature effects. At equivalent fixed temperature differences from melting point, saturated FAME molecules were found to have similar translational motion regardless of chain length, expressed by viscosity, self-diffusion coefficients, and spin-spin (T2) 1H LF-NMR. T2 distributions suggest increased alkyl chain rigidity, and reduced temperature response of the peaks’ relative contribution with increasing unsaturation is a direct result of the alkyl chain’s morphological packing and molecular interactions.ConclusionsBoth the peaks’ assignments for T2 distributions of FAMEs and the model for their liquid crystal-like morphology in the liquid phase were confirmed. The study of morphological structures within liquids and their response to temperature changes by 1H LF-NMR has a high value in the field of biodiesel and other research and applied disciplines in numerous physicochemical- and organizational-based properties, processes, and mechanisms of alkyl chains, molecular interactions, and morphologies.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0280-5) contains supplementary material, which is available to authorized users.

Study of liquid-phase molecular packing interactions and morphology of fatty acid methyl esters (biodiesel).

Background1H low field nuclear magnetic resonance (LF-NMR) relaxometry has been suggested as a tool to distinguish between different molecular ensembles in complex systems with differential segmental or whole molecular motion and/or different morphologies. In biodiesel applications the molecular structure versus liquid-phase packing morphologies of fatty acid methyl esters (FAMEs) influences physico-chemical characteristics of the fuel, including flow properties, operability during cold weather, blending, and more. Still, their liquid morphological structures have scarcely been studied. It was therefore the objective of this work to explore the potential of this technology for characterizing the molecular organization of FAMEs in the liquid phase. This was accomplished by using a combination of supporting advanced technologies.ResultsWe show that pure oleic acid (OA) and methyl oleate (MO) standards exhibited both similarities and differences in the 1H LF-NMR relaxation times (T2s) and peak areas, for a range of temperatures. Based on X-ray measurements, both molecules were found to possess a liquid crystal-like order, although a larger fluidity was found for MO, because as the temperature is increased, MO molecules separate both longitudinally and transversely from one another. In addition, both molecules exhibited a preferred direction of diffusion based on the apparent hydrodynamic radius. The close molecular packing arrangement and interactions were found to affect the translational and segmental motions of the molecules, as a result of dimerization of the head group in OA as opposed to weaker polar interactions in MO.ConclusionsA comprehensive model for the liquid crystal-like arrangement of FAMEs in the liquid phase is suggested. The differences in translational and segmental motions of the molecules were rationalized by the differences in the 1H LF-NMR T2 distributions of OA and MO, which was further supported by 13C high field (HF)-NMR spectra and 1H HF-NMR relaxation. The proposed assignment allows for material characterization based on parameters that contribute to properties in applications such as biodiesel fuels.

The atomic-scale mechanism for the enhanced glass-forming-ability of a Cu-Zr based bulk metallic glass with minor element additions.

It is known that the glass forming-ability (GFA) of bulk metallic glasses (BMGs) can be greatly enhanced via minor element additions. However, direct evidence has been lacking to reveal its structural origin despite different theories hitherto proposed. Through the high-resolution transmission-electron-microscopy (HRTEM) analysis, here we show that the content of local crystal-like orders increases significantly in a Cu-Zr-Al BMG after a 2-at% Y addition. Contrasting the previous studies, our current results indicate that the formation of crystal-like order at the atomic scale plays an important role in enhancing the GFA of the Cu-Zr-Al base BMG.

Medium-range Order of Zr54Cu38Al8 Bulk Metallic Glass

ABSTRACTWe have previously reported, based on fluctuation electron microscopy (FEM) data, that Zr50Cu45Al5 bulk metallic glass (BMG) contains significant icosahedral and crystal-like medium-range order. Here, we report similar finding for Zr54Cu38Al8 BMG, which is a poorer glass former. Like Zr50Cu45Al5, Zr54Cu38Al8 contains icosahedral and crystal-like structures. In the as-cast state, the crystal-like peak in the FEM data is stronger than icosahedral-like peak. After annealing at 0.83Tg (573 K), the icosahedral-like peak increases, but, unlike Zr50Cu45Al5, the crystal-like peak does not decrease. This tendency toward stronger, more thermally stable crystal-like order may be associated with the poorer glass forming ability of Zr54Cu38Al8.

Importance of many-body correlations in glass transition: An example from polydisperse hard spheres

Most of the liquid-state theories, including glass-transition theories, are constructed on the basis of two-body density correlations. However, we have recently shown that many-body correlations, in particular, bond orientational correlations, play a key role in both the glass transition and the crystallization transition. Here we show, with numerical simulations of supercooled polydisperse hard spheres systems, that the length-scale associated with any two-point spatial correlation function does not increase toward the glass transition. A growing length-scale is instead revealed by considering many-body correlation functions, such as correlators of orientational order, which follows the length-scale of the dynamic heterogeneities. Despite the growing of crystal-like bond orientational order, we reveal that the stability against crystallization with increasing polydispersity is due to an increasing population of icosahedral arrangements of particles. Our results suggest that, for this type of systems, many-body correlations are a manifestation of the link between the vitrification and the crystallization phenomena. Whether a system is vitrified or crystallized can be controlled by the degree of frustration against crystallization, polydispersity in this case.

A novel particle tracking method with individual particle size measurement and its application to ordering in glassy hard sphere colloids

Particle tracking is a key to single-particle-level confocal microscopy observation of colloidal suspensions, emulsions, and granular matter. The conventional tracking method has not been able to provide accurate information on the size of individual particle. Here we propose a novel method to localise spherical particles of arbitrary relative sizes from either 2D or 3D (confocal) images either in dilute or crowded environment. Moreover this method allows us to estimate the size of each particle reliably. We use this method to analyse local bond orientational ordering in a supercooled polydisperse colloidal suspension as well as the heterogeneous crystallisation induced by a substrate. For the former, we reveal non-trivial couplings of crystal-like bond orientational order and local icosahedral order with the spatial distribution of particle sizes: Crystal-like order tends to form in regions where very small particles are depleted and the slightly smaller size of the central particle stabilizes icosahedral order. For the latter, on the other hand, we found that very small particles are expelled from crystals and accumulated on the growth front of crystals. We emphasize that such information has not been accessible by conventional tracking methods.

Chiral symmetry breaking/restoration in a dyonic vacuum

We discuss the topological phenomena in the QCD-like theories with a variable number of fundamental fermions ${N}_{f}$, focusing on the temperatures at or above the critical value ${T}_{c}$ of chiral symmetry restoration. The nonzero average of the Polyakov line, or holonomy, splits instantons into (anti)self-dual dyons, and we study both the bosonic and fermionic interactions between them. The high temperature phase is a dilute gas of ``molecules'' made of $2{N}_{c}$ dyons, neutral in topological, electric, and magnetic charges. At intermediate temperatures the diluteness of the ``molecular gas'' reaches some critical level at which chiral symmetry gets restored: we comment on why it is different for the fundamental and adjoint fermions. At high density the ensemble is a strongly coupled liquid with crystal-like short range order: we speculate about its possible structure at small and large ${N}_{f}$. We finally show that certain lattice observations are in agreement with the proposed model, and suggest a number of further lattice tests.

Manifestations of electron interactions in photogalvanic effect in chiral nanotubes

Carbon nanotubes provide one of the most accessible experimental realizations of one-dimensional electron systems. In the experimentally relevant regime of low doping, the Luttinger liquid formed by electrons may be approximated by a Wigner crystal. The crystal-like electronic order suggests that nanotubes exhibit effects similar to the M\"ossbauer effect where the momentum of an emitted photon is absorbed by the whole crystal. We show that the circular photovoltaic effect in chiral nanotubes is of the same nature. We obtain the frequency dependence of the photovoltage and characterize its singularities in a broad frequency range where the electron correlations are essential. Our predictions provide a basis for using the photogalvanic effect as a new experimental probe of electron correlations in nanotubes.

Roles of icosahedral and crystal-like order in the hard spheres glass transition

A link between structural ordering and slow dynamics has recently attracted much attention from the context of the origin of glassy slow dynamics. Candidates for such structural order are icosahedral, exotic amorphous and crystal-like. Each type of order is linked to a different scenario of glass transition. Here we experimentally access local structural order in polydisperse hard spheres by particle-level confocal microscopy. We identify the key structures as icosahedral and FCC-like order, both statistically associated with slow particles. However, when approaching the glass transition, the icosahedral order does not grow in size, whereas crystal-like order grows. It is the latter that governs the dynamics and is linked to dynamic heterogeneity. This questions the direct role of the local icosahedral ordering in glassy slow dynamics and suggests that the growing length scale of structural order is essential for the slowing down of dynamics and the non-local cooperativity in particle motion.

Organic Field‐Effect Transistors based on Highly Ordered Single Polymer Fibers

Ultrahigh-mobility organic field-effect transistors (OFETs) based on a CDT-BTZ donor-acceptor copolymer are realized by reaching high molecular order and pronounced alignment in single fibers within a short OFET channel via solution processing. The macromolecules directionally self-assemble in a quasi crystal-like order in the fibers providing in this way an unhindered charge carrier pathway with mobilities as high as 5.5 cm(2) V(-1) s(-1).

Fluctuations in a ferrofluid monolayer: An integral equation study

Using integral equation theory in the reference hypernetted chain (RHNC) approximation we investigate the structure and phase behavior of a monolayer of dipolar spheres. The dipole orientations of the particles fluctuate within the plane. The resulting angle dependence of the correlation functions is treated via an expansion in two-dimensional rotational invariants. For homogeneous, isotropic states the RHNC correlation functions turn out to be in good agreement with Monte Carlo simulation data. We then use the RHNC theory combined with a stability (fluctuation) analysis to identify precursors of the low-temperature behavior. As expected, the fluctuations point to pair and cluster formation in the range of low and moderate densities. At high densities, there is no clear indication for a ferroelectric transition, contrary to what is found in three-dimensional dipolar fluids. The stability analysis rather indicates an alignment of chains supplemented by local crystal-like order.

Variational Monte Carlo Study of a Spinless Fermion t–V Model on a Triangular Lattice: Formation of a Pinball Liquid

We analyze a model of spinless fermions on a triangular lattice at half-filling interacting via strong nearest-neighbor repulsive interactions, V , using the variational Monte Carlo simulation technique. The comparative analysis of two different types of variational wave functions with and without translational symmetry breaking reveals that the system undergoes a phase transition at \(V_{\text{c}}/t \gtrsim 12\), accompanied by the reconstruction of the Fermi surface. The existence of three-sublattice long-range order in the strong coupling region is confirmed by the finite-size scaling analysis. The ordered phase shows characteristics expected for a so called “pinball liquid” state, which has the spontaneous separation of fermionic degrees of freedom into coexisting Wigner crystal-like charge order (pin) and a metal (ball). The pins are fixed in order to maximize the kinetic energy gain of balls which move almost freely. We also find the charge-density-wave-like state only for the symmetry broken wave functions at V < V c , which, however, possibly remains as a short-range order in the bulk limit, and is distinguished from the pinball liquid characterized by the large value of V c / t .

Micromechanical Behavior of Adhesive Granular Silica Layers: Structure Deformation

We studied the mechanical behavior of packed layers of 1-mum-sized silica particles immersed in liquids, upon indentation with a 10-mum glass sphere, attached to the cantilever of an atomic force microscope (AFM). Simultaneously, a confocal scanning laser microscope (CSLM) was used to study the deformations in the material. Our liquids consisted of (nearly) refractive-index-matching water-DMSO mixtures. Particle layers were formed by sedimentation in normal gravity. In the absence of (added) electrolyte, the collective behavior of the layer is reminiscent of that of a simple liquid. Crystal-like structures were observed, with the individual particles showing positional fluctuations. Carefully adding 2 wt % LiCl to this system leads to the formation of a weakly aggregated network, in which the crystal-like order gets lost and the particles lose their mobility. On indenting into these aggregated layers, the CSLM recordings showed imprints that closely resembled the size and shape of the indenter. A more accurate inspection of the structural changes was allowed after localizing all silica particles in three dimensions. Calculated local concentrations and coordination numbers showed that even at the level of these highly local quantities, no deformation gradients could be observed in the vicinity of the probe. Particle image velocimetry analysis suggested that deformation occurs mostly in the lateral directions. On pulling the indenter out, adhesion between the silica particles and the glass indenter became manifest via a distortion of the initially spherical dent and lower coordination numbers under the dent. Together all these behaviors indicate that the aggregated layers behave like yield-stress materials, which are solidlike up to a critical stress and liquidlike above it. The results of this study also illustrate the potential of the AFM-CSLM combination to study the detailed 3D deformation in other types of systems, like granular packings or more open particle networks.

Ordered Mesoporous Fe2O3 with Crystalline Walls

Alpha-Fe(2)O(3) has been synthesized with an ordered mesoporous structure and crystalline walls that exhibit a near-single crystal-like order. The unique magnetic behavior of the material, distinct from bulk nanoparticles of alpha-Fe(2)O(3) or mesoporous Fe(2)O(3) with disordered walls, has been established. Magnetic susceptibility, Mössbauer, and neutron diffraction data show that the material possesses the same long-range magnetic order as bulk alpha-Fe(2)O(3), despite the wall thickness being less than the 8 nm limit below which magnetic ordering breaks down in nanoparticulate alpha-Fe(2)O(3), yet the Morin transition of bulk alpha-Fe(2)O(3) is absent. It is also shown by TEM, PXRD, and EXAFS that alpha-Fe(2)O(3) with the same ordered mesoporous structure but disordered walls contains small crystalline domains. Mössbauer and magnetic susceptibility data demonstrate that this material exhibits no long-range magnetic order but superparamagnetic behavior.

Cluster Structure and Corralling Effect Driven by Interaction Mismatch in Two Dimensional Mixtures

The formation of clusters with crystal-like order in two dimensions is studied using Monte Carlo simulations. We find that the necessary conditions to induce the formation of clusters of one component is to add a second component that has a larger size and that has a long-range soft repulsive interaction with its own kind but not with the other one. The clusters of the small particles are found to be surrounded by a network of quasi-one-dimensional arrangements of the larger particles. The degree of order of the clusters is found to depend on the concentration of the larger particles. The findings explain recent experimental observations on lipid-poloxamer mixtures and they provide guidelines for how to form ordered clusters of nanoparticles in two dimensions.

Aluminum nanoscale order in amorphous Al92Sm8 measured by fluctuation electron microscopy

Fluctuation electron microscopy (FEM) measurements and simulations have identified nanoscale aluminum-like medium-range order in rapidly quenched amorphous Al92Sm8 which devitrifies by primary Al crystallization. Al92Sm8 amorphized by plastic deformation shows neither Al nanoscale order, nor primary crystallization. Annealing the rapidly quenched material below the primary crystallization temperature reduces the degree of nanoscale Al order measured by FEM. The FEM measurements suggest that 10–20Å diameter regions with Al crystal-like order are associated with primary crystallization in amorphous Al92Sm8, which is consistent with the quenched-in cluster model of primary crystallization.

Microscopic origin of polarity in quasiamorphousBaTiO3

The recent observation of pyroelectricity in quasiamorphous thin films of $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ introduced a previously unreported type of polar ionic solid where the appearance of a macroscopic dipole moment is not accompanied by long-range crystal-like order. This poses a question regarding the mechanism of polarity in noncrystalline ionic systems and the nature of their local dipoles. By combining x-ray diffraction and x-ray-absorption fine-structure spectroscopy techniques we have identified the local dipoles as stable but distorted $\mathrm{Ti}{\mathrm{O}}_{6}$ octahedra. The magnitude of the off-center displacement of the Ti ion and the concomitant dipole moment in both quasiamorphous (polar) and amorphous (nonpolar) $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ were found to be nearly twice as large as those in bulk $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$. We propose that the mechanism of macroscopic polarity in quasiamorphous $\mathrm{Ba}\mathrm{Ti}{\mathrm{O}}_{3}$ is in a weak orientational ordering of the $\mathrm{Ti}{\mathrm{O}}_{6}$ bonding units. In this view, one may expect that other amorphous ionic oxides containing stable local bonding units, for example $\mathrm{Nb}{\mathrm{O}}_{6}$, $\mathrm{Ti}{\mathrm{O}}_{6}$, or $\mathrm{V}{\mathrm{O}}_{6}$, may also form noncrystalline polar phases.

The effect of the translational symmetry of crystalline silicon on the structure of amorphous germanium in the interfacial region

The structure of an amorphous Ge layer near an interface with a Si(111) crystal was studied by quantitative high-resolution electron microscopy. It was found that the translational symmetry of a Si crystal leads to the crystal-like order in the positions of Ge atoms in the interfacial region, the width of which is about 1.4 nm. In this region, the average orientation of interatomic bonds tilted with respect to the interface compensates for the difference in the bond lengths in crystalline Si and amorphous Ge and is responsible for the tetragonal distortion of the most likely atomic positions.

Three-particle correlations in simple liquids.

We use videomicroscopy to follow the phase-space trajectory of a two-dimensional colloidal model liquid and calculate three-point correlation functions from the measured particle configurations. Approaching the fluid-solid transition by increasing the strength of the pair-interaction potential, one observes the gradual formation of a crystal-like local order due to triplet correlations, while being still deep inside the fluid phase. Furthermore, we show that in a strongly interacting system the Born-Green equation can be satisfied only with the full triplet correlation function but not with three-body distribution functions obtained from superposing pair correlations (Kirkwood superposition approximation).

Residual order in amorphous dry films of polymer latices: indications of an influence of particle interaction

We report small angle X-ray scattering and atomic force microscopy measurements on macroscopically thick, dry films of polymer latex particles. While the surface of dried droplets has long range order due to layering effects, the overall bulk structure is amorphous. This holds for both low charge polymethylmetacrylate particles and for highly charged polystyrene particles with additional stabilisation with sodiumdodecylsurfate. In the latter case, however, considerable amounts of residual crystal-like order is observed.