Short-range Repulsive Interactions Research Articles

Effect of surface modifier on flow properties of epoxy suspensions containing model plate-like nanoparticles

The influence of inter-particle interaction on the rheology of an uncured epoxy containing model α-zirconium phosphate (ZrP) nanoplatelets with aspect ratio of 160 is reported. Epoxy suspensions containing nanoplatelets exfoliated with tetra-n-butyl ammonium hydroxide (TBA), a low molecular weight quaternary ammonium cation, show short-range repulsive potential with weak elastic response at low concentration. At semi-dilute concentrations, the suspensions are solid-like at intermediate frequency and transition to viscous flow for time scales longer than the rotary diffusion process. The weak elasticity at intermediate frequency is attributed to the effect of Brownian motion on the rotational motion of the isolated plates. Suspensions containing nanoplatelets exfoliated with hydrophilic polyetheramine oligomers show similar behaviour to the ZrP-TBA system, but shifted to lower concentration. The rheological behaviour is attributed to steric stabilization of the nanoplatelets by extended oligomer brushes with short-range repulsive interactions. For suspensions containing nanoplatelets exfoliated with hydrophobic polyetheramines with shorter length, there is evidence for elastic response on local length scales and the flow behaviour shows strong history and temperature dependence. Rheological signatures associated with equilibrium nanoplatelet dispersions with repulsive interactions are discussed.

Study of elastic moduli and thermal properties of RMnO3 (R = La, Nd) compounds

In this paper, we develop a theoretical model for quantitative analysis of temperature-dependent heat capacity calculation of the magnetoresistance compounds RMnO 3 ( R = La , Nd ). The results on heat capacity obtained by us are in good agreement with the measured values. An effective interionic interaction potential (EIoIP) with the long-range Coulomb, van der Waals (vdW) interaction and short-range repulsive interaction up to second neighbor ions within the Hafemeister and Flygare approach was formulated to estimate the Debye and Einstein temperature and was found to be consistent with the available experimental data. In addition, the properties studied are the cohesive energy, molecular force constant, Restrahlen frequency and Gruneisen parameter. After characterizing thermal properties, a systematic investigation of elastic behavior has been undertaken and it has been found that the elastic moduli are decreasing continuously with increasing temperature.

Fermionization of two-component few-fermion systems in a one-dimensional harmonic trap

The nature of strongly interacting Fermi gases and magnetism is one of the most important and studied topics in condensed-matter physics. Still, there are many open questions. A central issue is under what circumstances strong short-range repulsive interactions are enough to drive magnetic correlations. Recent progress in the field of cold atomic gases allows one to address this question in very clean systems where both particle numbers, interactions and dimensionality can be tuned. Here we study fermionic few-body systems in a one dimensional harmonic trap using a new rapidly converging effective-interaction technique, plus a novel analytical approach. This allows us to calculate the properties of a single spin-down atom interacting with a number of spin-up particles, a case of much recent experimental interest. Our findings indicate that, in the strongly interacting limit, spin-up and spin-down particles want to separate in the trap, which we interpret as a microscopic precursor of one-dimensional ferromagnetism in imbalanced systems. Our predictions are directly addressable in current experiments on ultracold atomic few-body systems.

Mechanism governing separation in microfluidic pinched flow fractionation devices

We present a computational investigation of the mechanism governing size-based particle separation in microfluidic pinched flow fractionation. We study the behavior of particles moving through a pinching gap (i.e., a constriction in the aperture of a channel) in the Stokes regime as a function of particle size. The constriction aperture is created by a plane wall and spherical obstacle, and emulates the pinching segment in pinched flow fractionation devices. The simulation results show that the distance of closest approach between the particle and obstacle surfaces (along a trajectory) decreases with increasing particle size. We then use the distance of closest approach to investigate the effect of short-range repulsive non-hydrodynamic interactions (e.g., solid-solid contact). We define a critical trajectory as the one in which the minimum particle-obstacle separation is equal to the range of the non-hydrodynamic interactions. The results further show that the initial offset of the critical trajectory (defined as the critical offset) increases with particle size. We interpret the variation of the critical offset with particle size as the basis for size-based microfluidic separation in pinched flow fractionation. We also compare the effect of different driving fields on the particle trajectories; we simulate a constant force driving the particles in a quiescent fluid as well as a freely suspended particles in a pressure-driven flow. We observe that the particles driven by a constant force approach closer to the obstacle than those suspended in a flow (for the same initial offset). On the other hand, the increment in the critical offset (as a function of particle size) is larger in the pressure-driven case than in the force-driven case. Thus, pressure-driven particle separation using pinched flow fractionation would prove more effective than its force-driven counterpart.

Defect-mediated phase transitions in active soft matter.

How do topological defects affect the degree of order in active matter? To answer this question we investigate an agent-based model of self-propelled particles, which accounts for polar alignment and short-ranged repulsive interactions. For strong alignment forces we find collectively moving polycrystalline states with fluctuating networks of grain boundaries. In the regime where repulsive forces dominate, the fluctuations generated by the active system give rise to quasi-long-range transitional order, but-unlike the thermal system-without creating topological defects.

Observation of Coulomb repulsion between Cu intercalants inCuxBi2Se3

Using scanning tunneling microscopy and ab initio simulations, we have identified several configurations for Cu dopants in ${\text{Cu}}_{x}$${\text{Bi}}_{2}$${\text{Se}}_{3}$, with Cu intercalants being the most abundant. Through statistical analysis, we show strong short-range repulsive interactions between Cu intercalants. At intermediate range (g5 nm), the pair distribution function shows oscillatory structure along the $\ensuremath{\langle}10\overline{1}\ensuremath{\rangle}$ directions, which appear to be influenced by different diffusion barriers along the $\ensuremath{\langle}10\overline{1}\ensuremath{\rangle}$ and $\ensuremath{\langle}2\overline{1}\overline{1}\ensuremath{\rangle}$ directions.

Aspects of jamming in two-dimensional athermal frictionless systems

In this work we provide an overview of jamming transitions in two dimensional systems focusing on the limit of frictionless particle interactions in the absence of thermal fluctuations. We first discuss jamming in systems with short range repulsive interactions, where the onset of jamming occurs at a critical packing density and where certain quantities show a divergence indicative of critical behavior. We describe how aspects of the dynamics change as the jamming density is approached and how these dynamics can be explored using externally driven probes. Different particle shapes can produce jamming densities much lower than those observed for disk-shaped particles, and we show how jamming exhibits fragility for some shapes while for other shapes this is absent. Next we describe the effects of long range interactions and jamming behavior in systems such as charged colloids, vortices in type-II superconductors, and dislocations. We consider the effect of adding obstacles to frictionless jamming systems and discuss connections between this type of jamming and systems that exhibit depinning transitions. Finally, we discuss open questions such as whether the jamming transition in all these different systems can be described by the same or a small subset of universal behaviors, as well as future directions for studies of jamming transitions in two dimensional systems, such as jamming in self-driven or active matter systems.

Analytical calculation of four-point correlations for a simple model of cages involving numerous particles

Dynamics of a one-dimensional system of Brownian particles with short-range repulsive interaction (diameter σ) is studied with a liquid-theoretical approach. The mean square displacement, the two-particle displacement correlation, and the overlap-density-based generalized susceptibility are calculated analytically by way of the Lagrangian correlation of the interparticulate space, instead of the Eulerian correlation of density that is commonly used in the standard mode-coupling theory. In regard to the mean square displacement, the linear analysis reproduces the established result on the asymptotic subdiffusive behavior of the system. A finite-time correction is given by incorporating the effect of entropic nonlinearity with a Lagrangian version of mode-coupling theory. The notorious difficulty in derivation of the mode-coupling theory concerning violation of the fluctuation-dissipation theorem is found to disappear by virtue of the Lagrangian description. The Lagrangian description also facilitates analytical calculation of four-point correlations in the space-time, such as the two-particle displacement correlation. The two-particle displacement correlation, which is asymptotically self-similar in the space-time, illustrates how the cage effect confines each particle within a short radius on one hand and creates collective motion of numerous particles on the other hand. As the time elapses, the correlation length grows unlimitedly, and the generalized susceptibility based on the overlap density converges to a finite value which is an increasing function of the density. The distribution function behind these dynamical four-point correlations and its extension to three-dimensional cases, respecting the tensorial character of the two-particle displacement correlation, are also discussed.

Molecular Dynamics Simulation of the Motion of Colloidal Nanoparticles in a Solute Concentration Gradient and a Comparison to the Continuum Limit

Chemical-mechanical transduction mechanisms which can actuate the movement of colloids through liquids are highly sought after as engines to propel miniaturized micro- and nanobots. One mechanism involves harnessing the long-range van der Waals attractive forces between the colloid and solute molecules dissolved in the liquid around the particle. If a concentration gradient of this solute is applied across the particle, then the imbalance in the van der Waals attraction drives the particle towards the higher concentration of solute. We present a molecular dynamics simulation using Lennard-Jones interactions between molecules of the solvent, solute, and colloid cluster which include short-range repulsive and long-range attractive potentials. The simulations demonstrate that a solute gradient can propel nanosized colloids, and that the velocity decreases with the colloid size. The solute-colloid short-range repulsive interactions are observed to be restricted to a region of specifically adsorbed solutes on the particle surface which are symmetrically adsorbed and do not contribute to the motion. The size of this region provides a cutoff for a continuum level description of the motion, and with this cutoff, continuum calculations are in excellent agreement with the molecular dynamics simulation results, completing a description of the propulsion from the nano- to the microscale.

Stripe, gossamer, and glassy phases in systems with strong nonpairwise interactions

We study structure formation in systems of classical particles in two dimensions with long-range attractive short-range repulsive two-body interactions and repulsive three-body interactions. Stripe, gossamer, and glass phases are found as a result of nonpairwise interaction.

A study of polymer knots using a simple knot invariant consisting of multiple contour integrals

In this work the thermodynamic properties of short polymer knots (up to 120 segments) defined on a simple cubic lattice are studied with the help of the Wang–Landau Monte Carlo algorithm. The sampling process is performed using pivot transformations starting from a given seed conformation. Both cases of short-range attractive and repulsive interactions acting on the monomers are considered. The properties of the specific energy, heat capacity and gyration radius of the knots 31,41 and 51 are discussed. It is found that the heat capacity exhibits a sharp peak. If the interactions are attractive, similar peaks have been observed also in single open chains and have been related to the transition from a frozen crystallite state to an expanded coil state. Some other peculiarities of the behavior of the analyzed observables are presented, such as the increase or decrease of the knot specific energy at high temperatures with increasing polymer lengths depending on whether the interactions are attractive or repulsive. Besides the investigation of the thermodynamics of polymer knots, the second goal of this paper is to introduce a method for distinguishing the topology of a knot based on a topological invariant which is in the form of multiple contour integrals and explicitly depends on the physical trajectory of the knot. The chosen invariant, denoted here ϱ(C), is related to the second coefficient of the Conway polynomial. It has been first isolated from the amplitudes of a Chern–Simons field theory with gauge group SU(N). It is shown that this invariant is very reliable in distinguishing the topology of polymer knots. One of the advantages of the proposed approach is that it allows one to reduce the number of samples needed by the Wang–Landau algorithm. Some solutions to speed up the calculations of ϱ(C) by exploiting Monte Carlo integration techniques are developed.

Pressure effect on vibrational frequency and dephasing of 1-alkyl-3-methylimidazolium hexafluorophosphate ionic liquids

Raman spectra in the range of the totally symmetric stretching mode of the [PF6](-) anion, νs(PF6), have been measured for 1-alkyl-3-methylimidazolium ionic liquids [CnC1im][PF6], for n = 4, 6, and 8, as a function of pressure at room temperature. The ionic liquids [C6C1im][PF6] and [C8C1im][PF6] remain in an amorphous phase up to 3.5 GPa, in contrast to [C4C1im][PF6], which crystallizes above ~0.5 GPa. Equations of state based either on a group contribution model or Carnahan-Starling-van der Waals model have been used to estimate the densities of the ionic liquids at high pressures. The shifts of the vibrational frequency of νs(PF6) with density observed in [C6C1im][PF6] and in [C8C1im][PF6] have been calculated by a hard-sphere model of a pseudo-diatomic solute under short-range repulsive interactions with the neighboring particles. The stochastic model of Kubo for vibrational dephasing has been used to obtain the amplitude of vibrational frequency fluctuation, <Δω(2)>, and the relaxation time of frequency fluctuation, τc, as a function of density by Raman band shape analysis of the νs(PF6) mode of [C6C1im][PF6] and [C8C1im][PF6].

Depletion effect and biomembrane budding.

Depletion effects are well known to lead to phase separation in microsystems consisting of large and small particles with short-range repulsive interactions that act over macromolecular length scales. The equilibrium mechanics between an enveloped colloidal particle and a biomembrane caused by entropy is investigated by using a continuum model. We show that the favorable contact energy stems from entropy, which is sufficient to drive engulfment of the colloidal particle, and deformation of the biomembrane determines the resistance to the engulfment of the colloidal particle. The engulfment process depends on the ratio of the radii of the larger particle and smaller particles and the bending rigidity. The results show insights into the effects of depletion on biomembrane budding and nanoparticle transportation by a vesicle.

Using FT-IR Spectroscopy to Measure Charge Organization in Ionic Liquids

A major goal in the field of ionic liquids is correlating transport property trends with the underlying liquid structure of the compounds, such as the degree of charge organization among the constituent ions. Traditional techniques for experimentally assessing charge organization are specialized and not readily available for routine measurements. This represents a significant roadblock in elucidating these correlations. We use a combination of transmission and polarized-ATR infrared spectroscopy to measure the degree of charge organization for ionic liquids. The technique is illustrated with a family of 1-alkyl-3-methylimidazolium trifluoromethanesulfonate ionic liquids at 30 °C. As expected, the amount of charge organization decreases as the alkyl side chain is lengthened, highlighting the important role of short-range repulsive interactions in defining quasilattice structure. Inherent limitations of the method are identified and discussed. The quantitative measurements of charge organization are then correlated with trends in the transport properties of the compounds to highlight the relationship between charge and momentum transport and the underlying liquid structure. Most research laboratories possess infrared spectrometers capable of conducting these measurements; thus, the proposed method may represent a cost-effective solution for routinely measuring charge organization in ionic liquids.

High-pressure phase transformation and elastic behavior of XC (X = Si, Ge, Sn and Pt) compounds

Pressure-induced structural aspects of ZnS-type (B3) to NaCl-type (B1) structure in XC (X = Si, Ge, Sn and Pt) compounds are presented. An effective interionic interaction potential with long-range Coulomb, van der Waals (vdW) interaction and the short-range repulsive interaction up to second-neighbor ions within the Hafemeister and Flygare approach with modified ionic charge is developed. Particular attention is devoted to evaluate the vdW coefficients following the Slater–Kirkwood variational method, as both the ions are polarizable. Our results on vast volume discontinuity in pressure volume phase diagram identify the structural phase transition from B3 to B1 structure. The estimated value of the phase transition pressure (Pt) and the magnitude of the discontinuity in volume at the transition pressure are consistent as compared to the reported data. The variations of elastic constants with pressure follow a systematic trend identical to that observed in other compounds of ZnS and NaCl-type structure family and the Born relative stability criteria are valid in XC compounds.

High pressure phase transition and elastic behavior of europium monochalcogenides

A study of high-pressure phase transition and elastic properties of europium monochalcogenides with NaCl structure has been carried out in this paper using improved interaction potential model (IIPM). This model incorporates long-range Coulomb effect, three body interaction, polarization effect and short range repulsive interaction extended up to next nearest neighbors. These compounds exhibit first order structural phase transition from B1 (NaCl) to B2 (CsCl) structure at pressures 39.3, 21.1, 14.4 and 11.6GPa with sudden collapses in volume 10.76%, 11.35%, 13.34% and 12.12% respectively for EuO, EuS, EuSe and EuTe. The results obtained from present potential model show a generally good agreement with available experimental data. The elastic constants, their combinations and pressure derivatives are also reported. Thermodynamical properties like the ratio of thermal expansion coefficient to specific heat (αv/Cv), Gruneisen parameter are also studied.

High-Pressure, High-Temperature Phase Diagram of Calcium Fluoride from Classical Atomistic Simulations

We study the phase diagram of calcium fluoride (CaF2) under pressure using classical molecular dynamics simulations performed with a reliable pairwise interatomic potential of the Born−Mayer−Huggins form. Our results obtained under conditions 0 ≤ P ≲ 20 GPa and 0 ≤ T ≲ 4000 K reveal a rich variety of multiphase boundaries involving different crystal, superionic, and liquid phases, for all of which we provide an accurate parametrization. Interestingly, we predict the existence of three special triple points (i.e., solid−solid−superionic, solid−superionic−superionic, and superionic−superionic−liquid coexisting states) within a narrow and experimentally accessible thermodynamic range of 6 ≤ P ≤ 8 GPa and 1500 ≤ T ≤ 2750 K. In addition, we examine the role of short-ranged repulsive (SR) and long-ranged van der Waals attractive (LA) interactions in the prediction of melting lines with the finding that SR Ca−F and LA F−F contributions are most decisive.

NONINTEGRAL FORM OF THE GROSS–PITAEVSKII EQUATION FOR POLARIZED MOLECULES

The Gross–Pitaevskii equation for polarized molecules is an integro-differential equation, consequently it is complicated for solving. We find a possibility to represent it as a nonintegral nonlinear Schrödinger equation, but this equation should be coupled with two linear equations describing the electric field. These two equations are the Maxwell equations. We recapture the dispersion of collective excitations in the three-dimensional electrically polarized BEC with no evolution of the electric dipole moment directions. We trace the contribution of the electric dipole moment. We explicitly consider the contribution of the electric dipole moment in the interaction constant for the short-range interaction. We show that the spectrum of dipolar BEC does not reveal instability at repulsive short-range interaction. Nonlinear excitations are also considered. We present dependence of the bright soliton characteristics on the electric dipole moment.

Impurity, electron–phonon and electron–electron contributions to electrical resistivity of single-crystal Mg1−xAlxB2

The electrical resistivity of Mg1−xAlxB2 superconductor is analysed from the generalised Bloch–Gruneisen equation and the Debye and Einstein temperatures are estimated by formulating an effective interaction with the long-range Coulomb, van der Waals interaction and the short-range repulsive interaction within the Hafemeister and Flygare approach. Due to inherent two energy gaps, the elastic scatterings of electron from impurities have first been estimated and within a two-band picture, the impurity-limited resistivity due to π band carrier's ρ0π is larger as compared to the contribution from σ band carriers. The inelastic scattering of σ band carriers with acoustic phonons dominates over the π band carriers with optical phonons much below T≈θD/2. An investigation exhibiting the mechanism of Mg1−xAlxB2 (x=0.0, 0.1, 0.2) was accomplished by comparing the resistivity estimated by considering both phonons with that of the reported metallic resistivity; accordingly ρdiff=[ρexp−{ρ0+ρe–ph (=ρac+ρop)}] has been analysed through electron–electron scattering. The quadratic temperature dependence of ρdiff=[ρexp−{ρ0+ρe–ph(=ρσe–ph+ρπe–ph)}] is understood in terms of inelastic electron–electron scattering.

Many-particle dynamics of bosons and fermions in quasi-one-dimensional flat-band lattices

The difference between boson and fermion dynamics in quasi-one-dimensional lattices is studied with exact simulations of particle motion and by calculating the persistent current in small quantum rings. We consider three different lattices which in the tight binding model exhibit flat bands. The physical realization is considered to be an optical lattice with bosonic or fermionic atoms. The atoms are assumed to interact with a repulsive short range interaction. The different statistics of bosons and fermions causes different dynamics. Spinless fermions are easily trapped in the flat band states due to the Pauli exclusion principle, which prevents them from interacting, while boson are able to push each other out from the flat band states.