Hardening of fcc hard-sphere crystals by introducing nanochannels: Auxetic aspects.

Tailoring the materials for a given task by modifying their elastic properties is attractive to material scientists. However, recent studies of purely geometrical atomic models with structural modifications showed that designing a particular change to achieve the desired elastic properties is complex. This work concerns the impact of nanochannel inclusions in fcc hard sphere crystal on its elastic properties, especially auxetic ones. The models containing six nanochannel arrays of spheres of another diameter, oriented along the [110]-direction and its symmetric equivalents, have been studied by Monte Carlo simulations in the isothermal-isobaric (NpT) ensemble using the Parinello-Rahman approach. The inclusions have been designed such that they do not affect the cubic symmetry of the crystal. The elastic properties of three different models containing inclusions of various sizes are investigated under four thermodynamic conditions. We find that six nanochannels filled with hard spheres of larger diameter increase system stiffness compared with the fcc crystal without nanoinclusions. The current finding contrasts the recently reported results [J.W. Narojczyk et al. Phys. Status Solidi B 259, 2200464 (2022)0370-197210.1002/pssb.202200464], where the fcc hard sphere crystal with four nanochannels shows reduced stiffness compared to the system without nanoinclusions. Moreover, the six nanochannel models preserve auxetic properties in contrast to the fcc hard sphere crystal with four nanochannel arrays, which loses auxeticity.

Role of the phonon confinement effect and boundary scattering in reducing the thermal conductivity of argon nanowire.

The thermal conductivity of model argon nanowires over a wide range of temperatures from 20 K to 70 K has been calculated using the formula obtained by solving the Boltzmann equation and independently by molecular dynamic (MD) simulations. The theoretical predictions for thermal conductivity take into account the effect of phonon confinement and boundary scattering. Two known theoretical approaches were used. The first approach is based on the solution of the Boltzmann equation with given boundary conditions and uses bulk acoustic phonon dispersion and neglects the phonon confinement effect. The second approach includes also the modification of acoustic phonon dispersion due to spatial confinement. In simulations, the square and circular shapes of wire with the transverse size of nanowires from 4.3 nm to 42.9 nm have been considered. It was found that MD simulation results match the theoretical predictions reasonably well. The obtained results showed that the phonon confinement effect influences the thermal conductivity of nanowires, but the dominant factor decreasing the thermal conductivity with the thickness of nanowires is boundary scattering. Moreover, the values of the interface specular parameter indicate that the specular phonon-boundary scattering prevails over diffuse phonon-boundary scattering.

High Partial Auxeticity Induced by Nanochannels in [111]-Direction in a Simple Model with Yukawa Interactions.

Computer simulations using Monte Carlo method in the isobaric-isothermal ensemble were used to investigate the impact of nanoinclusions in the form of very narrow channels in the [ 111 ] -direction on elastic properties of crystals, whose particles interact via Yukawa potential. The studies were performed for several selected values of Debye screening length ( ( κ σ ) - 1 ). It has been observed that introduction of the nanoinclusions into the system reduces the negative value of Poisson's ratio towards [ 110 ] [ 1 1 ¯ 0 ] , maintaining practically constant values of Poisson's ratio in the directions [ 100 ] and [ 111 ] . These studies also show that concentration of particles forming the nanoinclusions in the system has a significant effect on the value of Poisson's ratio in the [ 110 ] [ 1 1 ¯ 0 ] -direction. A strong (more than fourfold) decrease of Poisson's ratio in this direction was observed, from - 0.147 ( 3 ) (system without inclusions) to - 0.614 ( 14 ) (system with nanoinclusions) at κ σ = 10 when the inclusion particles constituted about 10 percent of all particles. The research also showed an increase in the degree of auxeticity in the system with increasing concentration of nanoinclusion particles for all the screening lengths considered.

Lévy-like movement patterns of metastatic cancer cells revealed in microfabricated systems and implicated in vivo.

Metastatic cancer cells differ from their non-metastatic counterparts not only in terms of molecular composition and genetics, but also by the very strategy they employ for locomotion. Here, we analyzed large-scale statistics for cells migrating on linear microtracks to show that metastatic cancer cells follow a qualitatively different movement strategy than their non-invasive counterparts. The trajectories of metastatic cells display clusters of small steps that are interspersed with long "flights". Such movements are characterized by heavy-tailed, truncated power law distributions of persistence times and are consistent with the Lévy walks that are also often employed by animal predators searching for scarce prey or food sources. In contrast, non-metastatic cancerous cells perform simple diffusive movements. These findings are supported by preliminary experiments with cancer cells migrating away from primary tumors in vivo. The use of chemical inhibitors targeting actin-binding proteins allows for "reprogramming" the Lévy walks into either diffusive or ballistic movements.

Auxeticity of Yukawa Systems with Nanolayers in the (111) Crystallographic Plane.

Elastic properties of model crystalline systems, in which the particles interact via the hard potential (infinite when any particles overlap and zero otherwise) and the hard-core repulsive Yukawa interaction, were determined by Monte Carlo simulations. The influence of structural modifications, in the form of periodic nanolayers being perpendicular to the crystallographic axis [111], on auxetic properties of the crystal was investigated. It has been shown that the hard sphere nanolayers introduced into Yukawa crystals allow one to control the elastic properties of the system. It has been also found that the introduction of the Yukawa monolayers to the hard sphere crystal induces auxeticity in the [ 11 1 ¯ ] [ 112 ] -direction, while maintaining the negative Poisson's ratio in the [ 110 ] [ 1 1 ¯ 0 ] -direction, thus expanding the partial auxeticity of the system to an additional important crystallographic direction.

Auxeticity enhancement due to size polydispersity in fcc crystals of hard-core repulsive Yukawa particles.

The Poisson's ratio of the fcc hard-core repulsive Yukawa crystals with size polydispersity was determined by Monte Carlo simulations in the isothermal-isobaric ensemble. The effect of size polydispersity on the auxetic properties of Yukawa crystals has been studied. It has been found that an increase of particle size polydispersity causes a decrease of the Poisson's ratio in auxetic directions as well as appearance of a negative Poisson's ratio in formerly non-auxetic directions. A measure of auxeticity was introduced to estimate quantitatively an enhancement of auxetic properties in polydisperse Yukawa crystals. The proposed measure of auxeticity can be applied to appraise the auxeticity of any studied system.

Thermal conductivity of liquid argon in nanochannels from molecular dynamics simulations.

The thermal conductivity of liquid argon in nanochannels has been calculated over a wide range of densities using two independent methods-the Green-Kubo approach in equilibrium molecular dynamics simulations and the Müller-Plathe method in non-equilibrium molecular dynamics simulations. The Lennard-Jones potential was used to model interatomic interactions. The influence of transversal size and shape of a nanochannel on the thermal conductivity of liquid argon along the length of the channel has been investigated. The transversal size of nanochannel varied from 2.25 nm to 15 nm. The simulations revealed that the thermal conductivity weakly depends on the shape (square vs circular) of channel and scales with a cross-sectional area of nanochannel. It has been observed that thermal conductivity increases with an increase of the transversal size of the channel. Also, it reaches bulk values for some characteristic size of channel that depends strongly on density. Good agreement of the computed thermal conductivities of liquid argon over a wide density range with the experimental data allowed the value of the characteristic size of channel as a function of density to be estimated. This value depends on density and varies from 5 nm to 11 nm.

Elasticity of two-dimensional crystals of polydisperse hard disks near close packing: surprising behavior of the Poisson's ratio.

The equation of state, elastic constants, and Poisson's ratio of a crystalline two-dimensional polydisperse hard disk system were determined in the close packing limit. Monte Carlo simulations in the NpT ensemble with variable shape of the periodic box reveal that the pressure and elastic constants grow with increasing polydispersity. The equation of state and the bulk modulus are well described by the free volume approximation. The latter approximation fails, however, for the shear modulus. The simulations also show that the introduction of any amount of size polydispersity in the hard disk systems causes a discontinuous "jump" of the Poisson's ratio in the close packing limit from the value ν(δ=0) = 0.1308(22), obtained for equidiameter hard disks, to ν(δ>0) ≈ 1, estimated for the polydisperse disks.

Photoconductance and inverse photoconductance in films of functionalized metal nanoparticles.

In traditional photoconductors, the impinging light generates mobile charge carriers in the valence and/or conduction bands, causing the material's conductivity to increase. Such positive photoconductance is observed in both bulk and nanostructured photoconductors. Here we describe a class of nanoparticle-based materials whose conductivity can either increase or decrease on irradiation with visible light of wavelengths close to the particles' surface plasmon resonance. The remarkable feature of these plasmonic materials is that the sign of the conductivity change and the nature of the electron transport between the nanoparticles depend on the molecules comprising the self-assembled monolayers (SAMs) stabilizing the nanoparticles. For SAMs made of electrically neutral (polar and non-polar) molecules, conductivity increases on irradiation. If, however, the SAMs contain electrically charged (either negatively or positively) groups, conductivity decreases. The optical and electrical characteristics of these previously undescribed inverse photoconductors can be engineered flexibly by adjusting the material properties of the nanoparticles and of the coating SAMs. In particular, in films comprising mixtures of different nanoparticles or nanoparticles coated with mixed SAMs, the overall photoconductance is a weighted average of the changes induced by the individual components. These and other observations can be rationalized in terms of light-induced creation of mobile charge carriers whose transport through the charged SAMs is inhibited by carrier trapping in transient polaron-like states. The nanoparticle-based photoconductors we describe could have uses in chemical sensors and/or in conjunction with flexible substrates.

Additivity of the excess energy dissipation rate in a dynamically self-assembled system.

Despite its prevalence in biological systems and its promise as a route to adaptive and/or self-healing materials, dynamic self-assembly (DySA) far from thermodynamic equilibrium remains poorly understood. In this context, it is desirable to develop general thermodynamic relations describing the steady-state configurations of such dissipative assemblies. Here, numerical simulations and analytical methods are used to calculate the viscous energy dissipation rates in a prototypical, magnetohydrodynamic DySA system. In addition to the well-established criteria of mechanical equilibrium, it is shown that the naturally forming steady-state configurations/flows are characterized by a fundamentally different relation based on the viscous energy dissipation. Specifically, the total dissipation of the n-particle system may be expressed as a sum of pairwise "interactions" derived from the analogous two-particle system. This dissipation additivity holds despite the presence of many-body forces/torques between the particles and may prove useful in estimating the viscosities of colloidal suspensions.

Mechanism of the cooperative adsorption of oppositely charged nanoparticles.

Quartz crystal microbalance experiments were performed to study the kinetics of surface adsorption from solutions containing oppositely charged nanoparticles. A theoretical model was developed according to which formation of dense nanoparticle (NP) monolayers is driven by a cooperative process, in which the already-adsorbed NPs facilitate adsorption of NPs from solution. The kinetic rate constants change with the NP solution concentration and can be used to backtrack adsorption free energies. These energies agree with the predictions of a simple DLVO model.

Elastic properties of soft sphere crystal from Monte Carlo simulations.

Elastic properties of faced centered cubic (fcc) crystals composed of soft spheres, interacting through potentials of the form u(r) ~ r(-n), have been investigated by Monte Carlo (MC) simulations. It is shown that both the softness parameter (n(-1)) and temperature strongly influence the elastic properties of the studied system. The simulations show explicitly that when T > 0 the elastic constants of the hard sphere crystal can be obtained by taking the limit n --> infinity of soft spheres. When T --> 0 for any finite n, the elastic constants of the soft spheres tend to those of the static model. At all temperatures and softness parameters studied here, n, the Poisson's ratio in [110] (perpendicular direction) is negative.

Poisson's ratio of the fcc hard sphere crystal at high densities.

Elastic constants and the Poisson ratio of the fcc hard-sphere crystalline phases, free of defects and with vacancies, are determined by two Monte Carlo methods: (i) the analysis of the box fluctuations in the constant pressure ensemble with variable box shape (N-P-T) and (ii) by the free-energy differentiation with respect to deformation in the fixed box ensemble (N-V-T). Very good agreement is observed for the extrapolated to the infinitely large system limit results of both the methods. The coefficients of the leading singularities of the elastic constants near close packing are estimated; they are well described by the free volume approximation. Two mechanisms influencing the Poisson ratio are studied. (i) It is shown that at high densities particle motions decrease the Poisson ratio with respect to the static case which corresponds to zero temperature. Simulations performed for systems of soft spheres, interacting through n-inverse-power potentials, r-n, show that the elastic constants of the hard spheres can be obtained in the limit n-->infinity. When T-->0 the elastic constants of the soft spheres tend to those of the static model. (ii) It is also shown that vacancies decrease C11 and C44 and increase C12 and, hence, increase the Poisson ratio with respect to the defect-free state of the system.

Thermal conductivity of solid argon from molecular dynamics simulations.

The thermal conductivity of solid argon in the classical limit has been calculated by equilibrium molecular dynamic simulations using the Green-Kubo formalism and a Lennard-Jones interatomic potential. Contrary to previous theoretical reports, we find that the computed thermal conductivities are in good agreement with experimental data. The computed values are also in agreement with the high-temperature limit of the three-phonon scattering contribution to the thermal conductivity. We find that finite-size effects are negligible and that phonon lifetimes have two characteristic time scales, so that agreement with kinetic theory is obtained only after appropriate averaging of the calculated phonon lifetimes.

Thermal conductivity of solid argon at high pressure and high temperature: a molecular dynamics study.

The thermal conductivity of solid argon at high-pressure (up to 50 GPa) and high-temperature (up to 2000 K) has been calculated by equilibrium molecular dynamics simulations using the Green-Kubo formalism and an exponential-6 interatomic potential. A simple empirical expression is given for its pressure and temperature dependence. The results are compared with predictions based on kinetic theory. The relative change of the thermal conductivity lambda with density rho is found to be consistent with a partial differential ln lambda/ partial differential ln rho slope of approximately 6 in a wide range of pressures and temperatures, in good agreement with predictions based on kinetic theory.