Short-range Repulsive Forces Research Articles

Estimating the dopant distribution in Ca-doped α-SiAlON: statistical HAADF-STEM analysis and large-scale atomic modeling.

We investigated the dopant distribution in Ca-doped α-SiAlON by using high-angle annular dark-field scanning transmission electron microscopy and a multi-slice image simulation. Our results showed that the electron wave propagated by hopping to adjacent Si(Al) and N(O) columns. The image intensities of the Ca columns had wider dispersions than other columns. To estimate the Ca distribution in the bulk material, we performed a Monte Carlo atomic simulation of the α-SiAlON with Ca dopants. A model including a short-range Coulomb-like repulsive force between adjacent Ca atoms reproduced the dispersion of the intensity distribution of the Ca column in the experimental image.

Optical Properties of Gold Nanoclusters Functionalized with a Small Organic Compound: Modeling by an Integrated Quantum-Classical Approach.

Motivated by the growing importance of organometallic nanostructured materials and nanoparticles as microscopic devices for diagnostic and sensing applications, and by the recent considerable development in the simulation of such materials, we here choose a prototype system - para-nitroaniline (pNA) on gold nanoparticles - to demonstrate effective strategies for designing metal nanoparticles with organic conjugates from fundamental principles. We investigated the motion, adsorption mode, and physical chemistry properties of gold-pNA particles, increasing in size, through classical molecular dynamics (MD) simulations in connection with quantum chemistry (QC) calculations. We apply the quantum mechanics-capacitance molecular mechanics method [Z. Rinkevicius et al. J. Chem. Theory Comput. 2014, 10, 989] for calculations of the properties of the conjugate nanoparticles, where time dependent density functional theory is used for the QM part and a capacitance-polarizability parametrization of the MM part, where induced dipoles and charges by metallic charge transfer are considered. Dispersion and short-range repulsion forces are included as well. The scheme is applied to one- and two-photon absorption of gold-pNA clusters increasing in size toward the nanometer scale. Charge imaging of the surface introduces red-shifts both because of altered excitation energy dependence and variation of the relative intensity of the inherent states making up for the total band profile. For the smaller nanoparticles the difference in the crystal facets are important for the spectral outcome which is also influenced by the surrounding MM environment.

Effect of long-range repulsive Coulomb interactions on packing structure of adhesive particles.

The packing of charged micron-sized particles is investigated using discrete element simulations based on adhesive contact dynamic model. The formation process and the final obtained structures of ballistic packings are studied to show the effect of interparticle Coulomb force. It is found that increasing the charge on particles causes a remarkable decrease of the packing volume fraction ϕ and the average coordination number 〈Z〉, indicating a looser and chainlike structure. Force-scaling analysis shows that the long-range Coulomb interaction changes packing structures through its influence on particle inertia before they are bonded into the force networks. Once contact networks are formed, the expansion effect caused by repulsive Coulomb forces are dominated by short-range adhesion. Based on abundant results from simulations, a dimensionless adhesion parameter Ad*, which combines the effects of the particle inertia, the short-range adhesion and the long-range Coulomb interaction, is proposed and successfully scales the packing results for micron-sized particles within the latest derived adhesive loose packing (ALP) regime. The structural properties of our packings follow well the recent theoretical prediction which is described by an ensemble approach based on a coarse-grained volume function, indicating some kind of universality in the low packing density regime of the phase diagram regardless of adhesion or particle charge. Based on the comprehensive consideration of the complicated inter-particle interactions, our findings provide insight into the roles of short-range adhesion and repulsive Coulomb force during packing formation and should be useful for further design of packings.

Nonlocal Peridynamic Modeling and Simulation on Crack Propagation in Concrete Structures

An extended peridynamic approach for crack propagation analysis in concrete structures was proposed. In the peridynamic constitutive model, concrete material was described as a series of interacting particles, and the short-range repulsive force and anisotropic behavior of concrete were taken into account in the expression of the interactive bonding force, which was given in terms of classical elastic constants and peridynamic horizon. The damage of material was defined locally at the level of pairwise bond, and the critical stretch of material bond was described as a function of fracture strength in the classical concrete failure theory. The efficiency and accuracy of the proposed model and algorithms were validated by simulating the propagation of mode I and I-II mixed mode cracks in concrete slabs. Furthermore, crack propagation in a double-edge notched concrete beam subjected to four-point load was simulated, in which the experimental observations are captured naturally as a consequence of the solution.

Study on the droplet impact on hydrophobic surface in terms of van der Waals surface tension model

Research on the droplet impact on a hydrophobic surface is of important theoretical significance and engineering value, both in mesoscopic fluid mechanics and interactions between microfluid and special materials. The van der Waals (vdW) equation of state relates the pressure to the temperature and the density of the fluid, and gives the long-range attractive force and short-range repulsive force between particles. The van der Waals equation of state can be used to describe the surface tension between liquid and vapor. As a pure meshless particle method, the smoothed particle hydrodynamic (SPH) method can use the vdW equation of state written in SPH form of N-S equations to describe the surface tension. The vdW surface tension mode is validated by simulating the coalescence of two equally sized static droplets in vacuum. Repellant of the hydrophobic surface is derived from a core potential. By combining the vdW surface tension and the repulsive force of the surface, the phenomenon of a liquid droplet impact with a certain initial velocity on the hydrophobic surface is studied. The SPH model is not only capable to describe the spreading of the droplet after it contacts the surface, but also clearly reproduces the springback, bouncing and secondary impact of the droplet. During the deformation of the droplet, the inertia force impels the spreading process of the droplet whilst the springback and bouncing behavior is dominated by the surface tension. The simulated results are in good agreement with the published experimental observations and VOF simulated results, indicating that the way we treat the surface tension and the repulsive force of the hydrophobic surface is effective and applicable in droplet impact surface problems. The impact velocity and liquid viscosity are considered to be two important factors that affect the deformation of the droplet after it contacts the surface. By varying the impact velocity within a certain range it is concluded that the maximum liquid-solid contact area increases as the impact velocity grows, and the bounced droplet will leave the surface when the velocity is big enough. Another comparison between different liquid viscosities shows that the maximum contact area decreases as the liquid viscosity increases because of the viscous dissipation, and the droplet barely rebound when the liquid viscosity is big enough.

Drops on microstructured surfaces: A numerical study using many-body dissipative particle dynamics

Because of their ability of self-clean, superhydrophobic surfaces have received substantial attention for years especially in surface science field. In this paper, the drop's wettability on different rough surfaces is simulated by using many-body dissipative particle dynamics (MDPD) and a contrast with the Cassie-Baxter theory's predictions is made. A combination of short-range repulsive and long-range attractive forces is used as wall-fluid interaction to generate different wettability, and a simple but efficient numerical method is introduced to measure the contact angle. The simulation could capture the static and dynamic properties of drop on textured surfaces, it is also shown that the microstructured surfaces can pin the three-phase (solid-liquid-vapour) contact line and this phenomenon has also been observed by other researchers in their physical experiments, suggesting that people should be careful when using the Cassie-Baxter theory. An analysis was given about energy transformation between kinetic energy and surface energy. The simulated results also show that the low Φs can cause the drop to rebound easily under the same impact velocity.

An analytical determination of microstructure and stresses in a dense, sheared monolayer of non-Brownian spheres

AbstractWe propose an analytical model for the determination of the microstructure and stresses in a sheared suspension that consists of a dense monolayer of identical spheres in a viscous fluid. We calculate the anisotropy in the orientational distribution of spheres, associated with a short-range repulsive force assumed to act between the spheres, and a particle pressure and normal stress difference that result from this anisotropy. The microstructure and stresses are similar to those measured in Stokesian dynamics simulations.

Valence Electron Distributions in Ferroelectric Barium Titanate Nanopowders

Barium titanate BaTiO3 is one of the most important perovskite-type electroceramics, which undergoes the phase transition at 1300C from cubic to tetragonal, and exhibits ferroelectricity at room temperature. The phase transition depends on the particle size. BaTiO3 powders with the particle sizes less than several tens of nanometers are known to show no phase transition and hence no ferroelectricity at room temperature. The size effect of BaTiO3 is the most important issue in designing small ceramic capacitors with high capacitance. Our group has been devoted to visualizing the electron density distributions of perovskite-type oxides by analyzing the synchrotron-radiation x-ray powder diffraction (SXRD) data measured at SPring-8 using the maximum entropy method (MEM)/Rietveld method [1, 2]. In this study, the distributions of valence electrons in the outer shells of atoms are derived accurately from the SXRD data of BaTiO3 nanopowders to prove the characteristic chemical bondings which govern the ferroelectric phase transition. The powder samples used were 500 and 35 nm in particle sizes. The former showed the phase transition whereas the latter showed no phase transition. The MEM valence electron density studies at 2000C in the cubic structure revealed the clear structural variations that the Ti-O covalent bonding is found in the 500 nm sample, while all the valence electrons are localized at the O sites in the 35 nm sample exactly like an ionic crystal. Ferroelectricity originates from the balance between the long-rage Coulomb force and the short-range repulsion force. The obtained results provide direct experimental evidence that the electron orbitals hybridization on the Ti-O bonds weakens the short-range repulsion force, and causes the second-order Jahn-Teller distortion on the TiO6 octahedron in the 500 nm sample. We consider that the Ti-O bonding in the prototype structure governs the ferroelectric phase transition temperature in BaTiO3.

Effect of silica particle surface chemistry on the shear thickening behaviour of concentrated colloidal suspensions

We have found that heat treating silica particles can have a large effect on the shear thickening properties of suspensions comprised of 550 nm silica particles in ethylene glycol (EG). The shear thickening effect becomes stronger after heat treating the particles at the temperatures studied (120 and 220 °C) especially at the higher temperature, indicating that the interparticle interactions in these suspensions are changed by heat treatment. Having confirmed the existence of hydroxyl groups on the surface of the silica particles using differential scanning calorimetry, x-ray photoelectron spectroscopy showed that heat treatment increases the valence of the Si element on the surface, resulting in a higher density of hydroxyl groups. We believe that a denser solvation layer on the surface of heat-treated particles, formed by adsorbing EG molecules to the particle surface through hydrogen bonds between EG and hydroxyl groups, results in a higher short-range repulsive force, which is responsible for the enhanced shear thickening behaviour.

Rheology of dense suspensions of elastic capsules: normal stresses, yield stress, jamming and confinement effects.

We study the shearing rheology of dense suspensions of elastic capsules, taking aggregation-free red blood cells as a physiologically relevant example. Particles are non-Brownian and interact only via hydrodynamics and short-range repulsive forces. An analysis of the different stress mechanisms in the suspension shows that the viscosity is governed by the shear elasticity of the capsules, whereas the repulsive forces are subdominant. Evidence for a dynamic yield stress above a critical volume fraction is provided and related to the elastic properties of the capsules. The shear stress is found to follow a critical jamming scenario and is rather insensitive to the tumbling-to-tank-treading transition. The particle pressure and normal stress differences display some sensitivity to the dynamical state of the cells and exhibit a characteristic scaling, following the behavior of a single particle, in the tank-treading regime. The behavior of the viscosity in the fluid phase is rationalized in terms of effective medium models. Furthermore, the role of confinement effects, which increase the overall magnitude and enhance the shear-thinning of the viscosity, is discussed.

The equilibria of vesicles adhered to substrates by short-ranged potentials

In equilibrium, a vesicle that is adhered to a horizontal substrate by a long-range attractive, short-range repulsive force traps a thin layer of fluid beneath it. In the asymptotic limit that this layer is very thin, there are quasi-two-dimensional boundary-layer structures near the edges of the vesicle, where the membrane's shape is governed by a balance between bending and adhesive stresses. These boundary layers are analysed to obtain corrections to simpler models that instead represent the adhesive interaction by a contact potential, thereby resolving apparent discontinuities that arise when such models are used. Composite expansions of the shapes of two-dimensional vesicles are derived. When, in addition, the adhesive interaction is very strong, there is a nested boundary-layer structure for which the adhesive boundary layers match towards sharp corners where bending stresses remain important but adhesive stresses are negligible. Outside these corners, bending stresses are negligible and the vesicle's shape is given approximately by the arc of a circle. Simple composite expansions of the vesicle's shape are derived that account for the shape of the membrane inside these corners.

Subdiffusive behavior of a dilute non-Brownian suspension under shear

Shear-induced self-diffusion in a dilute suspension of non-Brownian spheres under a simple shear flow is studied in the limit of zero Reynolds number, for different volume fractions φ. Particles are simulated considering the first term in the multipolar expansion to take into account the long-range hydrodynamic interactions, and a repulsive force is added to avoid interpenetration. The final diffusive regime is established after a long time (t(diffusion)~φ(-1.5)), and the diffusion coefficient is proportional to φ(2), as expected in the presence of the short-range repulsive force. Before the diffusive regime is established, there is a rich subdiffusive behavior, particularly in the gradient direction. Each pairwise hydrodynamic interaction is reversible, not leading to streamline migration. Due to the incoherence of the different pair interactions, a plateau in the mean square displacement is first observed, lasting for a period that increases as φ is decreased. Then, a first diffusive regime is established due to three-particle interactions, with a diffusion coefficient of D((1))~φ(2.4). At longer times, a phenomenon similar to caging is observed. Particles diffuse for long times in the vicinity of some positions, and eventually large displacements are produced, moving the particle to a new position and resulting in transient large values of the kurtosis of the displacement distribution. This migration is produced by collisions through the repulsive potential. After several of those large displacements, the final diffusive regime is established.

A simple model of molecular imaging with noncontact atomic force microscopy

Using functionalized tips, the atomic resolution of a single organic molecule can be achieved by noncontact atomic force microscopy (nc-AFM) operating in the regime of short-ranged repulsive Pauli forces. To theoretically describe the atomic contrast in such AFM images, we propose a simple model in which the Pauli repulsion is assumed to follow a power law as a function of the probed charge density. As the exponent in this power law is found to be largely independent of the sample molecule, our model provides a general method for simulating atomically resolved AFM images of organic molecules. For a single perylene-tetracarboxylic-dianhydride (PTCDA) molecule imaged with a CO-terminated tip, we find excellent agreement with the experimental data. Our model eliminates the need to take into account the full tip and sample system and therefore reduces computational cost by three orders of magnitude.

Using field data to test locust migratory band collective movement models

Wingless locust nymphs can form massive migratory groups known as bands, whose coordinated movement results from local interactions. We analysed the spatial distribution of locusts within naturally occurring bands and compared them with computer simulations to infer which interaction rules are used by individuals. We found that the empirical radial distribution of neighbours around a focal individual was isotropic, indicating a tendency for locusts to interact with neighbours all around them, rather than a bias towards pursuing individuals ahead or escaping from the ones following behind. By using maps of neighbour densities and pair correlation functions, we found evidence for a short-range repulsion force, balanced by a clustering force, presumably alignment and/or attraction, at a distance of around 3 cm. These results were similar to those observed when using a 'zonal' self-propelled particles model where repulsion/alignment/attraction forces are delimited by concentric circular zones of set radii. However, the profiles obtained either by using different combinations of forces, limiting the number of neighbours involved in interactions, or by varying the range of some zones, all appeared to produce similar results, thereby limiting the ability to more precisely determine the rules underlying locust interactions.

Colloidal stability and “in vitro” antitumor targeting ability of lipid nanocapsules coated by folate–chitosan conjugates

In this study, the synthesis and characterization of lipid nanocapsules coated with folate–chitosan conjugates at varying folate concentrations are reported; these nanocapsules have a potential application as anticancer drug carriers. The main goal of this study was to evaluate (a) the colloidal stability of the particles and (b) their cell targeting. A classical colloidal characterization of the nanocapsules was carried out by analyzing size, electrokinetic charge, and stability in different saline solutions, including cell culture media. At neutral pH, the stability was improved by the presence of folate due to electrical interactions. In addition, folate modulated the hydrophilic/phobic nature of the surface, which became critical to keep the systems stable (or not) under physiological saline conditions due to the action of short-range repulsive hydration forces. The cellular uptake of our nanocapsules was evaluated by working with four tumor cell lines. Both fluorescent analyses with particles colored by Nile Red, and antitumor activity of our systems loaded with docetaxel, demonstrated that the folate-mediated internalization of the particles in the cancer cells was improved when the nanocapsules were coated by folate–chitosan conjugates.

AFM measurements of forces between silica surfaces

Interaction forces and adhesion between a silica sphere and a flat silica surface in aqueous electrolyte solutions were investigated by atomic force microscopy. The forces were measured as a function of surface separation, pH and NaCl concentration as the surfaces were approaching each other. The adhesion force was determined upon retraction with respect to pH, NaCl concentration and contact time. The magnitude of the long range repulsive force was decreasing with decreasing pH. A short range repulsive force was observed at pH = 2, but no long range repulsive forces were observed at this pH. Force measurements showed that adhesion of silica surfaces in water was obstructed by short and long range repulsive forces. Adhesion was enhanced when both the long and the short range repulsive force was mitigated. A maximum adhesion force of 7.8 mN/m was measured at pH = 12.5 when the short range force vanished and the long range repulsive force was reduced by increasing the NaCl concentration. At pH = 12.5, the work of adhesion was calculated to be 1.2 mJ/m2 according to the Derjaguin–Muller–Toporov (DMT) model. Adhesion energy was much less at pH = 2 (0.3 mJ/m2) due to persistive short range repulsion.

Hydration Force between Mica Surfaces in Aqueous KCl Electrolyte Solution

Liquid-vapor molecular dynamics simulations are performed to study the interaction forces between two mica surfaces in an aqueous KCl electrolyte solution. Strong repulsive hydration force is obtained within a distance of ~2 nm between the two mica surfaces, which cannot be explained by the continuum theory of double-layer repulsion. We find that this short-range repulsive hydration force is much stronger than the double-layer force between mica surfaces. Whereas the simulation system is much smaller than the surface force measurement system, fundamental mechanisms of repulsive hydration force are revealed. In particular, important features of the step-like force oscillatory behavior during normal compression and force hysteresis during retraction are observed. Detailed analysis of the ionic density distributions shows that the "forced adsorption" of diffusive K(+) ions onto mica surfaces during compression and the subsequent "slow desorption" of the absorbed K(+) ions from mica surfaces upon retraction are responsible for the hysteresis phenomenon. From a mechanics point of view, we attribute the load bearing capacity of the dense electrolyte to the very hard hydration shells of K(+) metal ions under confinement. We find that the hydrated K(+) ions and Cl(-) co-ions remain very diffusive in the aqueous film. Water molecules in the hydration layer are also very fluidic, in the sense that the diffusion constant of water molecules is less than its bulk value by at most 3 orders of magnitude under the extreme confinement.

High-resolution imaging of C60 molecules using tuning-fork-based non-contact atomic force microscopy

Recent advances in non-contact atomic force microscopy (nc-AFM) have led to the possibility of achieving unprecedented resolution within molecular structures, accomplished by probing short-range repulsive interaction forces. Here we investigate C60 molecules adsorbed on KBr(111) and Cu(111) by tuning-fork-based nc-AFM. First, measurements of C60 deposited on KBr(001) were conducted in cryogenic conditions revealing highly resolved nc-AFM images of the self-assembly. Using constant-frequency shift mode as well as three-dimensional spectroscopic measurements, we observe that the relatively weak molecule–substrate interaction generally leads to the disruption of molecular assembled structures when the tip is probing the short-range force regime. This particular issue hindered us in resolving the chemical structure of this molecule on the KBr surface. To obtain a better anchoring of C60 molecules, nc-AFM measurements were performed on Cu(111). Sub-molecular resolutions within the molecules was achieved which allowed a direct and unambiguous visualization of their orientations on the supporting substrate. Furthermore, three-dimensional spectroscopic measurements of simultaneous force and current have been performed above the single molecules giving information of the C60 molecular orientation as well as its local conductivity. We further discuss the different imaging modes in nc-AFM such as constant-frequency shift nc-AFM, constant-height nc-AFM and constant-current nc-AFM as well as three-dimensional spectroscopic measurement (3D-DFS) employed to achieve such resolution at the sub-molecular scale.

Phase Transition in Mixed Transition Metal Oxides under High Pressure

Mixed crystals can be categorized as the crystal in which two or more different types of atoms are present and configured. These atoms interact each other in the influence of Coulomb force of attraction and the presence of short range repulsion force. Due to this interaction they tends to be arranged in equilibrium state and forms new material. A pure crystal can be doped by impurities with proper concentration of impurity to get the desired material. This concept of interaction between the atoms are called the Three Body Interaction(TBI) model. This model is based on the concept of interaction between atoms. The presence of short range repulsion force between the atoms is very crucial, in the absence of which coulomb force of attraction will be dominant and it will lead to volume collapse. Applying of high pressure on the crystal results in the transfer of charge between the atoms. This transfer of charge causes them to rearrange in some different cofigurational state different from original configuration. This rearrangement of atoms affects the different properties of material like hardness, melting point, optical properties and mechanical properties.

Irreversibility and pinching in deterministic particle separation

We investigate the settling of spherical particles through a pinching gap created by a cylindrical obstacle and a vertical wall. These macroscopic experiments capture the essence of pinched-flow-fractionation in microfluidics and highlight its deterministic nature. In the absence of pinching, we observe asymmetric trajectories consistent with a hard-core model of particle-obstacle repulsion that leads to separative lateral displacement. Then, we show that pinching promotes the onset of these short-range repulsion forces, amplifying the relative separation in the outgoing trajectory of different-size particles. Inertia effects, however, tend to reduce such relative separation and lead to a more complex behavior.