Orientational Order Research Articles

Adsorption Free Energy of Cellulose Nanocrystal on Water-Oil Interface.

To investigate the amphiphilicity of cellulose, a series of molecular dynamics simulations were performed with a cellulose nanocrystal and a water–octane interfacial system. Assuming that the axis of cellulose is parallel to the water–octane interface, the freedoms of motion of the nanocrystal were restricted to two, the distance from the interface and the orientation around the axis. The mean force and the mean torque on the nanocrystal were evaluated with sufficiently long simulation at each crystal configuration, and their numerical integration gave a smooth free energy surface as the potential of mean force. The cellulose sample used here was found to be much more hydrophilic than oleophilic with the free energy difference kcal/mol. Three adsorption states with local minimum of adsorption free energy are distinguished in the free energy surface—the direct contact type which is similar to previously reported one, the hydrophilic-surface/water/octane type where a thin water layer is sandwiched between the surface and the octane phase, and the oleophilic/water/octane type where a thin water layer also exists. Water molecules in these water layers contribute to stabilize the adsorption states by taking a special orientational order and slow self-diffusion.

Interplay of orientational order and roughness in simulated thin film growth of anisotropically interacting particles.

Roughness and orientational order in thin films of anisotropic particles are investigated using kinetic Monte Carlo simulations on a cubic lattice. Anisotropic next-neighbor interactions between the lattice particles were chosen to mimic the effects of shape anisotropy in the interactions of disk- or rodlike molecules with van der Waals attractions. Increasing anisotropy leads first to a preferred orientation in the film (which is close to the corresponding equilibrium transition) while the qualitative mode of roughness evolution (known from isotropic systems) does not change. At strong anisotropies, an effective step-edge (Ehrlich-Schwoebel) barrier appears and a nonequilibrium roughening effect is found, accompanied by reordering in the film which can be interpreted as the nucleation and growth of domains of lying-down disks or rods. The information on order and roughness is combined into a diagram of dynamic growth modes.

Highly Orientational Order Perovskite Induced by In situ-generated 1D Perovskitoid for Efficient and Stable Printable Photovoltaics.

Employing low-dimensional perovskite has been proven to be a promising approach to enhance the efficiency and stability of perovskite solar cells. Here, thiopheniformamidine hydrochloride is introduced into CH3 NH3 PbI3 -based printable mesoscopic perovskite solar cells, to form 1D iodide lead thiophenamidine (TFPbI3 ) in situ. This judiciously designed low-dimensional perovskite can effectively passivate the defect of perovskite and induce the perovskite crystals to grow in a direction perpendicular to the substrate. Thus, the obtained 1D@3D perovskite could suppress the charge recombination and promote the charge transfer significantly. Benefiting from its dual effect and robustness, a significantly improved power conversion efficiency of 17.42% is yielded. The authors also develop high-performance printable mesoscopic perovskite solar cells with a champion efficiency approaching 13% for aperture area about 11.8 cm2 , as well as outstanding operational stability, retaining 90% of the original power conversion efficiency after 1000 hours of continuous illumination at the maximum power point in air.

Orientational ordering and electron-phonon interaction in K3C60 superconductor

The normal state and superconducting state properties of a well characterized K3C60 superconductor are studied in terms of the combined measurements of x-ray diffraction, Raman scattering, magnetization, and resistivity by using the same sample synthesized by the wet chemical method. The temperature downshift of about 50 K is observed for the orientational ordering from pristine C60 to the synthesized sample based on the anomalies of both the resistivity and phonon frequency. When entering the superconducting state, the enhanced self-energy effects are reflected from the softening and broadening of the phonon modes. The obtained electron-phonon interaction strength of 0.63 in the superconducting state accounts well for the transition temperature of 18 K in K3C60 and the plausible Coulomb pseudopotential and expected isotope exponent.

Structure of the twist-bend nematic phase with respect to the orientational molecular order of the thioether-linked dimers.

An analysis of the IR absorbance for the segmented functional groups of liquid crystal dimers mesogen and linker enabled the orientation order to be determined and information about the dipole interactions in the nematic and twist-bend nematic phases to be obtained. The long axis orientational order increases as the temperature decreases in the nematic phase, although much more slowly than for the classical nematics, and then reverses this trend in the twist-bend nematic phase due to the tilt of the molecules. In the nematic phase, the short axis of the molecule performs an isotropic uniform rotation and has a uniaxial alignment. In the twist-bend nematic phase, however, biaxial ordering occurs and grows significantly in accordance with the helical deformation of the director. Changes in the mean absorbance in the twist-bend nematic phase were observed: a decrease for the longitudinal dipole at the nematic-twist-bend nematic phase transition, thus emphasizing the antiparallel axial interaction of the dipoles, while the absorbance of the transverse dipoles remains unchanged up to 340 K, and then the latter become parallelly correlated.

Electrically poled vapor-deposited organic glasses for integrated electro-optics.

We introduce electrically poled small molecule assemblies that can serve as the active electro-optic material in nano-scale guided-wave circuits such as those of the silicon photonics platform. These monolithic organic materials can be vacuum-deposited to homogeneously fill nanometer-size integrated-optics structures, and electrically poled at higher temperatures to impart an orientational non-centrosymmetric order that remains stable at room temperature. An initial demonstration using the DDMEBT molecule and corona poling delivered a material with the required high optical quality, an effective glass transition temperature of the order of ∼80°C, and an electro-optic coefficient of 20 pm/V.

Effects of ion adsorption on graphene oxide films and interfacial water structure: A molecular-scale description

Graphene oxide is a promising, emerging separation material, as it is durable, dispersible in water, and has naturally forming functional groups. Bulk studies using graphene oxide flakes have demonstrated impressive metal adsorption. However, little interfacial information about water and metal organization near graphene oxide is available. A mechanistic understanding of water and ions interactions with graphene oxide films is critical toward advanced separations, including improved sorption efficiency and membrane regeneration. We study metal ion and local water organization near graphene oxide thin films formed at the air/water interface. These films are not typical membranes and allow us to determine nanoscale information about the graphene oxide-water interface. We accomplish this with x-ray reflectivity (XR), x-ray fluorescence near total reflection (XFNTR), and vibrational sum frequency generation spectroscopy (SFG). These interface-specific techniques provide the electron density profile normal to the interface, number of adsorbed ions, and information about the orientational ordering and hydrogen-bonding network of interfacial water, respectively. Via XFNTR and SFG, we find that trivalent yttrium ions preferentially adsorb to graphene oxide and affect its structure, compared to divalent strontium and monovalent cesium ions. Two different interfacial water populations can be described, based on their hydrogen bonding strength, and the adsorbed ions affect these populations differently. These results provide fundamental information about ion and water organization at the interface and help address the large computational-experimental agreement gap for graphene oxide systems. Additionally, they are relevant for improved soft-scaffold graphene oxide membranes and downstream applications.

Modulating the structural characteristics of layered nonwoven fabrics devised by fibre fineness for enhanced filtration performance

This paper reports an investigatory study on the structure and properties of nonwoven fabrics made from different fibre fineness. The structure of the nonwoven fabrics was analysed using the Image analysis technique and Lindsley’s technique. Pore channel tortuosity was also calculated to quantify the porous path formed in the fibrous media. A relationship is developed between the structure and property of the nonwoven fabrics which helps in designing a suitable nonwoven filter media. In the later part, an attempt is made to regulate the structure by layering the batts made from different linear densities of fibres in various combinations to improve the filtration performance. It was interestingly established that an inverse gradient structure of batts having decreasing order of fibre orientation in the layered structure provided the lowest pressure drop along with near highest filtration efficiency. This study provides an insight into modulating the structure of the layered nonwoven fabric using different linear density fibres to prolong the life of the filter media.

Orientation Distribution of Molecules in a Smectic Liquid Crystal with a Distorted Director Geometry.

Quantitative determination of the molecular orientation distribution function in samples of liquid crystals with a complex director geometry was performed using the numerical simulation of electron paramagnetic resonance (EPR) spectra of the spin probes in a liquid-crystalline medium. To achieve the quantitative agreement of experimental and simulated EPR spectra, the hierarchy of the orientation order was explicitly taken into account, namely, the local ordering of liquid crystal molecules by the mean-field potential of surrounding molecules, and the partial disordering of local directors within the sample. The samples under study are planar and twist LC cells with liquid crystal 8CB cooled from the nematic into the smectic A phase in the magnetic field. The presence of the magnetic field perpendicular to the cell director leads to distortion of the orientation of the liquid crystal in the cell. The spin probe technique was successfully employed for the reliable measurement of orientation distribution functions of the low nonorthorhombic symmetry. Orientation order parameters up to 12th rank were measured, including nonaxial and nonorthorhombic order parameters. It is shown that the presence of several contradicting aligning forces leads to the tilt of the preferential director toward the direction, which is a compromise between the orienting forces.

Effects of Extrusion and Rolling Processes on the Microstructure and Mechanical Properties of Zn-Li-Ag Alloys

In this work, a novel Zn-0.5%Li-0.1%Ag alloy was cast and extruded into rods, which were rolled into a plate, and the effects of extrusion and rolling on the microstructure and mechanical properties of the Zn-0.5%Li-0.1%Ag alloy were evaluated. The results show that grain strengthening occurs in all of the alloys because of the presence of nano-LiZn4 precipitates. The extrusion and rolling processes promote grain size refinement and orientation order, and the microstructure and mechanical properties of the Zn-0.5%Li-0.1%Ag alloy can be significantly improved by secondary processing. The elastic modulus and tensile strength of the processed alloy increased to 83.1 GPa and 251.6 MPa, respectively, compared to 75.6 GPa and 185.8 MPa, respectively, for the as-cast Zn-0.5%Li-0.1%Ag alloy. More importantly, elongation was greatly improved, from 16.9% to 92.6%, which is an increase of up to 448%, and there were transgranular cleavage planes and intergranular cleavage planes in the fracture surfaces. The intergranular cleavage planes were dominant, and they showed ductile fracture characteristics.

Bond Orientational Order Parameters for Classifying Solid-like Clusters in a Lennard-Jones System near Liquid-Solid Transition and at Solid States.

In this paper, we introduced an order parameter, named the local structure similarity (LSS), to measure the resemblance of a cluster structure in a liquid with respect to a perfect crystal. The LSS is based on a dot product of two bond orientational order complex vectors, with one vector associated with a particle in a liquid and the other vector with a particle in a crystal. The calculation of the LSS should scan the entire space of the Euler angles determined by the two coordinate frames describing individually the liquid and the crystal. The effectiveness of the LSS was examined by solid-like clusters in a Lennard-Jones (LJ) system near its liquid-solid phase transition and at solid states below its melting point, where the thermodynamic states of the LJ system were obtained by simulation annealing. The LSS measure was utilized to scrutinize the fcc-like, hcp-like, and bcc-like clusters classified by criteria based on W4 and W6 order parameters. As indicated by our results, the two ways of classification are consistent for fcc-like and hcp-like clusters, which are in a close resemblance to their crystalline counterparts. However, the classification with positive W6 for bcc-like clusters is inconsistent with the results of the LSS measure, which was confirmed by clusters in a LJ system confined between two parallel slabs of particles in the bcc structure arrangement.

A Molecular Dynamics Study of Monomer Melt Properties of Cyanate Ester Monomer Melt Properties.

The objective of this work was to computationally predict the melting temperature and melt properties of thermosetting monomers used in aerospace applications. In this study, we applied an existing voids method by Solca. to examine four cyanate ester monomers with a wide range of melting temperatures. Voids were introduced into some simulations by removal of molecules from lattice positions to lower the free-energy barrier to melting to directly simulate the transition from a stable crystal to amorphous solid and capture the melting temperature. We validated model predictions by comparing melting temperature against previously reported literature values. Additionally, the torsion and orientational order parameters were used to examine the monomers’ freedom of motion to investigate structure–property relationships. Ultimately, the voids method provided reasonable estimates of melting temperature while the torsion and order parameter analysis provided insight into sources of the differing melt properties between the thermosetting monomers. As a whole, the results shed light on how freedom of molecular motions in the monomer melt state may affect melting temperature and can be utilized to inspire the development of thermosetting monomers with optimal monomer melt properties for demanding applications.

How Ethanolic Disinfectants Disintegrate Coronavirus Model Membranes: A Dissipative Particle Dynamics Simulation Study.

We have developed dissipative particle dynamics models for pure dipalmitoylphosphatidylcholine (DPPC), dioleoylphosphatidylcholine (DOPC), and dimyristoylphosphatidylcholine (DMPC) as well as their binary and ternary mixed membranes, as coronavirus model membranes. The stabilities of pure and mixed membranes, surrounded by aqueous solutions containing up to 70 mol % ethanol (alcoholic disinfectants), have been investigated at room temperature. We found that aqueous solutions containing 5–10 mol % ethanol already have a significant weakening effect on the pure and mixed membranes. The magnitude of the effect depends on the membrane composition and the ethanol concentration. Ethanol permeabilizes the membrane, causing its lateral swelling and thickness shrinking and reducing the orientational order of the hydrocarbon tail of the bilayer. The free energy barrier for the permeation of ethanol in the bilayers is considerably reduced by the ethanol uptake. The rupture-critical ethanol concentrations causing the membrane failure are 20.7, 27.5, and 31.7 mol % in the aqueous phase surrounding pure DMPC, DOPC, and DPPC membranes, respectively. Characterizing the failure of lipid membranes by a machine-learning neural network framework, we found that all mixed binary and/or ternary membranes disrupt when immersed in an aqueous solution containing a rupture-critical ethanol concentration, ranging from 20.7 to 31.7 mol %, depending on the composition of the membrane; the DPPC-rich membranes are more intact, while the DMPC-rich membranes are least intact. Due to the tight packing of long, saturated hydrocarbon tails in DPPC, increasing the DPPC content of the mixed membrane increases its stability against the disinfectant. At high DPPC concentrations, where the DOPC and DMPC molecules are confined between the DPPC lipids, the ordered hydrocarbon tails of DPPC also induce order in the DOPC and DMPC molecules and, hence, stabilize the membrane more. Our simulations on pure and mixed membranes of a diversity of compositions reveal that a maximum ethanol concentration of 32 mol % (55 wt %) in the alcohol-based disinfectants is enough to disintegrate any membrane composed of these three lipids.

Electric field driven fast fabrication of 3D ordered Nano-micro hierarchical ammonium oxofluorotitanates towards mesocrystalline anatase TiO2

In this work, 3D ordered nano-micro hierarchical ammonium oxofluorotitanates are synthesized by in-situ dissolution of anodically grown TiO2 nanotubes and subsequent hydrolysis of (NH4)2TiF6 in an electrochemical cell via a nonclassical crystallization process by external electric field driven oriented particle attachment. The obtained ammonium oxofluorotitanate products are of mesocrystalline structure consisting of nanocrystalline building units oriented aligned at atomic level. Scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) characterizations are applied to reveal the morphology, structure and crystal phase of grown samples. By controlling the current density or/and content of ammonium fluoride, it is able to grow well-defined (NH4)3TiOF5 octahedrons and truncated octahedrons, (NH4)2TiOF4 nanorods and NH4TiOF3 disks. The transformation from (NH4)3TiOF5 octahedrons to truncated octahedrons is ascribed to the different ratios of growth rate for {111} planes and {100} planes. While the decrease of concentration of ammonium fluoride accounts for the crystal phase changes from (NH4)3TiOF5 to (NH4)2TiOF4 and finally to NH4TiOF3 disks. Upon thermal annealing, all these ammonium oxofluorotitanates could be converted to mesoporous hierarchical anatase TiO2 retaining their exterior morphology and keeping the orientation ordering of subunit nanoparticles, which could exhibit excellent photocatalytic activity when acting as photocatalyst in the photo-degradation of methylene blue.

Large-scale orientational order in bacterial colonies during inward growth.

During colony growth, complex interactions regulate the bacterial orientation, leading to the formation of large-scale ordered structures, including topological defects, microdomains, and branches. These structures may benefit bacterial strains, providing invasive advantages during colonization. Active matter dynamics of growing colonies drives the emergence of these ordered structures. However, additional biomechanical factors also play a significant role during this process. Here, we show that the velocity profile of growing colonies creates strong radial orientation during inward growth when crowded populations invade a closed area. During this process, growth geometry sets virtual confinement and dictates the velocity profile. Herein, flow-induced alignment and torque balance on the rod-shaped bacteria result in a new stable orientational equilibrium in the radial direction. Our analysis revealed that the dynamics of these radially oriented structures, also known as aster defects, depend on bacterial length and can promote the survival of the longest bacteria around localized nutritional hotspots. The present results indicate a new mechanism underlying structural order and provide mechanistic insights into the dynamics of bacterial growth on complex surfaces.

Shell effect on microstructure and diffusion in interface region of nanoencapsulated phase change material: A molecular dynamics simulation

A new simplified molecular dynamics model of nanoencapsulated phase change material (NEPCM) is constructed to investigate the shell effect in the shell-core interface. The amorphous silica wall with different thicknesses were built as the nanocapsule shell and paraffin (octadecane) was dispersed as the nanocapsule core. By MD method, the atom distribution was revealed by the linear density. The linear density of paraffin was reduced close to the shell and yet augmented far away from the shell compared with the pure paraffin density. Orientation order parameter (S) was greatly enhanced close to the silica shell, revealing that molecules near the surface were more orderly and packed perpendicularly to the shell. End-to-end distance (re) of each paraffin molecule was calculated and the distribution was revealed. The results showed the percentages of molecules fully stretched were more increased with the increasing shell thicknesses. S and re both showed the changes of the internal structures in the cases with the silica shell. The results of diffusion properties showed the addition of the silica shell broke the symmetry of the systems and the PCM diffusion was significantly weakened. Overall, the work presents the microstructures of paraffin and provides explanations for the thermophysical properties from paraffin morphology.

Phase transition from nematic to high-density disordered phase in a system of hard rods on a lattice

A system of hard rigid rods of length k on hypercubic lattices is known to undergo two phase transitions when chemical potential is increased: from a low density isotropic phase to an intermediate density nematic phase, and on further increase to a high-density phase with no orientational order. In this paper, we argue that, for large k, the second phase transition is a first-order transition with a discontinuity in density in all dimensions greater than 1. We show that the chemical potential at the transition is ≈kln[k/lnk] for large k, and that the density of uncovered sites drops from a value ≈(lnk)/k^{2} to a value of order exp(-ak), where a is some constant, across the transition. We conjecture that these results are asymptotically exact, in all dimensions d≥2. We also present evidence of coexistence of nematic and disordered phases from Monte Carlo simulations for rods of length 9 on the square lattice.

The N-terminal helices of amphiphysin and endophilin have different capabilities of membrane remodeling

Amphiphysin and endophilin are two members of the N-BAR protein family. We have reported membrane interactions of the helix 0 of endophilin (H0-Endo). Here we investigate membrane modulations caused by the helix 0 of amphiphysin (H0-Amph). Electron paramagnetic resonance (EPR) spectroscopy was used to explore membrane properties. H0-Amph was found to reduce lipid mobility, make the membrane interior more polar, and decrease lipid chain orientational order. The EPR data also showed that for anionic membranes, H0-Endo acted as a more potent modulator. For instance, at peptide-to-lipid (P/L) ratio of 1/20, the peak-to-peak splitting was increased by 0.27 G and 1.89 G by H0-Amph and H0-Endo, respectively. Similarly, H0-Endo caused a larger change in the bilayer polarity than H0-Amph (30% versus 12% at P/L = 1/20). At P/L = 1/50, the chain orientational order was decreased by 26% and 66% by H0-Amph and H0-Endo, respectively. The different capabilities were explained by considering hydrophobicity score distributions. We employed atomic force microscopy to investigate membrane structural changes. Both peptides caused the formation of micron-sized holes. Interestingly, only H0-Amph induced membrane fusion as evidenced by the formation of high-rise regions. Lastly, experiments of giant unilamellar vesicles showed that H0-Amph and H0-Endo generated thin tubules and miniscule vesicles, respectively. Together, our studies showed that both helices are effective in altering membrane properties; the observed changes might be important for membrane curvature induction. Importantly, comparisons between the two peptides revealed that the degree of membrane remodeling is dependent on the sequence of the N-terminal helix of the N-BAR protein family.

Appreciating the role of cell shape changes in the mechanobiology of epithelial tissues.

The wide range of epithelial cell shapes reveals the complexity and diversity of the intracellular mechanisms that serve to construct their morphology and regulate their functions. Using mechanosensitive steps, epithelial cells can sense a variety of different mechanochemical stimuli and adapt their behavior by reshaping their morphology. These changes of cell shape rely on a structural reorganization in space and time that generates modifications of the tensional state and activates biochemical cascades. Recent studies have started to unveil how the cell shape maintenance is involved in mechanical homeostatic tasks to sustain epithelial tissue folding, identity, and self-renewal. Here, we review relevant works that integrated mechanobiology to elucidate some of the core principles of how cell shape may be conveyed into spatial information to guide collective processes such as epithelial morphogenesis. Among many other parameters, we show that the regulation of the cell shape can be understood as the result of the interplay between two counteracting mechanisms: actomyosin contractility and intercellular adhesions, and that both do not act independently but are functionally integrated to operate on molecular, cellular, and tissue scales. We highlight the role of cadherin-based adhesions in force-sensing and mechanotransduction, and we report recent developments that exploit physics of liquid crystals to connect cell shape changes to orientational order in cell aggregates. Finally, we emphasize that the further intermingling of different disciplines to develop new mechanobiology assays will lead the way toward a unified picture of the contribution of cell shape to the pathophysiological behavior of epithelial tissues.

Submicrosecond electro-optical switching of one-dimensional soft photonic crystals

Soft photonic crystals are appealing due to their self-assembly ability, wide tunability, and multistimuli-responsiveness. However, their response time is relatively slow, ranging from milliseconds to minutes. Here, we report submicrosecond switching of chiral liquid crystals (LCs) with 1D photonic microstructures, where electric fields modify the orientational order of molecules and quench their fluctuations, rather than altering the orientation. Thus, the adjusted refractive indices result in a fast shift of the photonic bandgap, on the order of 100 ns, which is four orders of magnitude faster than conventional electro-optic switching in cholesterics. This work offers tremendous opportunities for soft photonic applications.