Viscosity anomaly in core-softened liquids

The present Letter presents a molecular dynamics study of several anomalies of core-softened systems. It is well known that many core-softened liquids demonstrate diffusion anomaly. Usual intuition relates the diffusion coefficient to shear viscosity via the Stokes–Einstein relation. However, it can break down at low temperature. In this respect it is important to see if viscosity also demonstrates anomalous behavior.     

Water-like anomalies in the core-softened systems: Dependence on the trajectory in density-temperature plane

We show that the existence of the water-like anomalies in kinetic coefficients in the core-softened systems depends on the trajectory in ρ-T plane along which the kinetic coefficients are calculated. In particular, it is shown that the diffusion anomaly does exist along the isotherms, but disappears along the isochors. We analyze the applicability of the Rosenfeld entropy scaling relations to the systems with the core-softened potentials demonstrating the water-like anomalies. It is shown that the validity of the Rosenfeld scaling relation for the diffusion coefficient also depends on the trajectory in the ρ-T plane along which the kinetic coefficients and the excess entropy are calculated. In particular, it is valid along isochors, but it breaks down along isotherms.     

The effects of forcing and dissipation on phase transitions in thin granular layers

Recent experimental and computational studies of vibrated thin layers of identical spheres have shown transitions to ordered phases similar to those seen in equilibrium systems. Motivated by these results, we carry out simulations of hard inelastic spheres forced by homogenous white noise. We find a transition to an ordered state of the same symmetry as that seen in the experiments, but the clear phase separation observed in the vibrated system is absent. Simulations of purely elastic spheres also show no evidence for phase separation. We show that the energy injection in the vibrated system is dramatically different in the different phases, and suggest that this creates an effective surface tension not present in the equilibrium or randomly forced systems. We do find, however, that inelasticity suppresses the onset of the ordered phase with random forcing, as is observed in the vibrating system, and that the amount of the suppression is proportional to the degree of inelasticity. The suppression depends on the details of the energy injection mechanism, but is completely eliminated when inelastic collisions are replaced by uniform system-wide energy dissipation.     

Phase diagram of dipolar hard and soft spheres: manipulation of colloidal crystal structures by an external field

Phase diagrams of hard and soft spheres with a fixed dipole moment are determined by calculating the Helmholtz free energy using simulations. The pair potential is given by a dipole-dipole interaction plus a hard-core and a repulsive Yukawa potential for soft spheres. Our system models colloids in an external electric or magnetic field, with hard spheres corresponding to uncharged and soft spheres to charged colloids. The phase diagram of dipolar hard spheres shows fluid, face-centered-cubic (fcc), hexagonal-close-packed (hcp), and body-centered-tetragonal (bct) phases. The phase diagram of dipolar soft spheres exhibits, in addition to the above mentioned phases, a body-centered-orthorhombic (bco) phase, and it agrees well with the experimental phase diagram [Nature (London) 421, 513 (2003)]. Our results show that bulk hcp, bct, and bco crystals can be realized experimentally by applying an external field.     

Applications of the Stell–Hemmer Potential to Understanding Second Critical Points in Real Systems

We consider the novel properties of the Stell–Hemmer core-softened potentials. First we explore how the theoretically predicted second critical point for these potentials is related to the occurrence of the experimentally observed solid–solid isostructural critical point. We then discuss how this class of potentials can generate anomalies analogous to those found experimentally in liquid water.     

Interplay between structure and density anomaly for an isotropic core-softened ramp-like potential

Using molecular dynamics simulations and integral equations wee investigate the structure, the thermodynamics, and the dynamics of a system of particles interacting through a continuous core-softened ramp-like interparticle potential. We found density, dynamic and structural anomalies similar to that found in water. Analysis of the radial distribution function for several temperatures at fixed densities shows a pattern that may be related to the origin of density anomaly.     

Spatial Arrangement of Small Particles by Imaging Laser Trapping System

We describe spatial arrangement of polystyrene latex using a microscopic imaging system. A reduced photomask pattern is imaged on the particles in a water-filled cell so that polystyrene spheres align in a two-dimensional pattern. Calculating the forces exerted on the spheres by isolated line pattern, we investigated an optimum size for sphere diameter and the line width of the patterns. We are able to align spheres in real-time by this technique.     

Undulated cylinders of charged diblock copolymers

We study the cylinder to sphere morphological transition of diblock copolymers in aqueous solution with a hydrophobic block and a charged block. We find a metastable undulated cylinder configuration for a range of charge and salt concentrations which, nevertheless, occurs above the threshold where spheres are thermodynamically favorable. By modeling the shape of the cylinder ends, we find that the free-energy barrier for the transition from cylinders to spheres is quite large and that this barrier falls significantly in the limit of high polymer charge and low solution salinity. This suggests that observed undulated cylinder phases are kinetically trapped structures.     

On the solid-like phase transition in crystals of polystyrene spheres in aqueous suspensions

We present results of calculations for the fractional energy difference between the bcc and fcc phases present in ordered systems of polystyrene spheres in aqueous suspensions. The theoretical model developed for those systems is an extension of the concept of a Wigner lattice. The interaction between the particles is assumed to be a screened coulomb potential. We conclude that the phase transition occurs when λr₅ = 1.05 where λ^-1 is the Debye screening length and r_s is the radius of the spherical volume of the unitary cell.     

Phyllotactic description of hard sphere packing in cylindrical channels

We develop a simple analytical theory that relates dense sphere packings in a cylinder to corresponding disk packings on its surface. It applies for ratios R=D/d (where d and D are the diameters of the hard spheres and the bounding cylinder, respectively) up to R=1+1/sin(π/5). Within this range the densest packings are such that all spheres are in contact with the cylindrical boundary. The detailed results elucidate extensive numerical simulations by ourselves and others by identifying the nature of various competing phases.     

Analytical approach to the thermodynamics and density distribution of crystalline phases of hard spheres

We extend the well-known free-volume approach to describe the thermodynamic properties and density distribution of crystalline phases of hard hypersphere systems. Despite its extreme simplicity the approach yields results which are in quantitative agreement with simulation data. The theory can, in particular, describe the properties of the body-centred cubic phase of hard spheres, for which density-functional approaches provide unphysical results, allowing for the application of perturbation theory to situations where, as is the case in some colloidal systems, the body-centred cubic is one of the most interesting phases. The theory is also tested by applying it to systems of hard discs.     

Inversion of sequence of anomalies in core-softened systems with attraction

In this paper we present a simulation study of water-like anomalies in a purely repulsive core-softened system and in a system with attraction described in our previous publications. We investigate the anomalous regions for systems with the same functional form of the potentials but with different parameters and show that the order of the region of anomalous diffusion and the region of density anomaly is inverted with increasing the width of the repulsive shoulder and the depth of the attractive well. It is shown that while the density anomaly is always inside the region of the structural anomaly, the diffusion anomaly can change its location depending on the parameters of the potential. In the presence of the attraction in the potential, the system demonstrates the silica-like behavior.     

Entropically driven colloidal crystallization on patterned surfaces

We investigate the self-assembly of colloidal spheres on periodically patterned templates. The surface potentials and the surface phases are induced entropically by the presence of dissolved, nonadsorbing polymers. A rich variety of two-dimensional fluidlike and solidlike phases was observed to form on template potentials with both one- and two-dimensional symmetry. The same methodology was then used to nucleate an oriented single fcc crystal more than 30 layers thick. The general approach provides a new route for directed self-assembly of novel mesoscopic structures.     

急冷Al-Si-Ge合金超导电性的研究

本文研究了热处理对急冷Al-8.3at%Si-8.5at%Ge合金的微观结构及超导电性的影响。结构分析表明,液态淬火的样品由两相组成:过饱和α-Al(Si,Ge)固溶体基体和Al,Si,Ge的非晶球体,后者在基体中相互连接,构成连续通路。经过100℃/50h热处理后,非晶球体尺寸缩小,不再构成连续通路,并在其中析出弥散的Si(Ge)微晶。热处理后的样品的电阻-温度及电阻-磁场转变曲线上均出现两次正常-超导转变。它可以用样品中存在两个超导相的模型予以解释。 关键词：     

磁性液体中非磁性小球与磁性纳米颗粒的相互作用及磁组装

利用磁性液体与聚苯乙烯小球溶液混合得到的复合磁性液体, 研究了聚苯乙烯小球和磁性纳米颗粒在外加磁场作用下的动力学过程. 实验结果表明, 当外加磁场的方向平行于样品平面时, 聚苯乙烯小球在沿着磁场的方向上表现出相互吸引而形成链状结构, 其动力学过程可分为聚苯乙烯小球被反磁化产生相互吸引而形成短链的快过程以及短链间相互吸引形成长链的慢过程; 当外加磁场的方向垂直于样品平面时, 相邻聚苯乙烯小球表现出排斥的相互作用而形成短程有序的二维结构, 当磁场强度增加到一定的阈值时, 聚苯乙烯小球和磁性纳米颗粒形成的团簇会产生相互吸引而组装成复合式的花瓣结构. 关键词： 磁性液体 磁组装 非磁性颗粒     

Self-induced cyclic reorganization of free bodies through thermal convection

We investigate the dynamics of a thermally convecting fluid as it interacts with freely moving solid objects. This is a previously unexplored paradigm of interactions between many free bodies mediated by thermal convection, which gives rise to surprising robust oscillations between different large-scale circulations. Once begun, this process repeats cyclically, with the collection of objects (solid spheres) entrained and packed from one side of the convection cell to the other. The cyclic frequency is highest when the spheres occupy about half of the cell bottom and their size coincides with the thickness of the thermal boundary layer. Our work shows that a deformable mass stimulates a thermally convecting fluid into oscillation, a collective behavior that may be found in nature.     

Consequences of an attractive force on collective modes and dust structures in a strongly coupled dusty plasma

We present investigations of the combined effects of Debye–Hückel repulsive and overlapping Debye spheres attractive interaction potentials around charged dust particles on collective modes, phase separation and ordered structures in a strongly coupled dusty plasma. We obtain static and dynamical information via Molecular Dynamics simulations in the liquid and crystallized phases and identify the onset of an instability in the transverse mode, by using lattice summation method. The results are useful for understanding the origin of coagulation/agglomeration of charged dust particles and the formation of ordered dust structures in low-temperature laboratory and space plasmas.     

Phase behavior of dispersions of hard spherical particles

A series of experiments on concentrated dispersions of hard colloidal spheres is discussed. The observed phase behavior is analogous to that of simple atomic systems: colloidal fluid, crystal and glass phases are found. The structure of the crystals, revealed by light diffraction, is a strongly faulted stacking of hexagonally-packed layers of particles. Dynamic light scattering confirms that the concentration of the metastable fluid phase for which long-ranged particle diffusion ceases coincides with the concentration where the glass transition is observed macroscopically. In studies of a binary mixture of colloidal spheres with a size ratio 0.61 eutectics, glass formation and the AB₁₃ type alloy structure have been identified.     

Spontaneous chirality in simple systems

Two simple examples of spontaneous chiral symmetry breaking are presented. The first is close-packed cylindrically confined spheres. As the cylinder diameter is varied, one obtains a variety of chiral phases. The second example involves unconfined dipolar particles with an isotropic attraction, which also exhibits chiral ground states. We speculate that a dilute magnetorheological fluid film, with the addition of smaller particles to provide an attractive entropic interaction, will exhibit a chiral columnar ground state.     

Equilibrium phase behavior of polydisperse hard spheres

We calculate the phase behavior of hard spheres with size polydispersity, using accurate free energies for the fluid and solid phases. Cloud and shadow curves are found exactly by the moment free energy method, but we also compute the complete phase diagram, taking full account of fractionation. In contrast to earlier, simplified treatments we find no point of equal concentration between fluid and solid or reentrant melting at higher densities. Rather, the fluid cloud curve continues to the largest polydispersity that we study (14%); from the equilibrium phase behavior a terminal polydispersity can thus be defined only for the solid, where we find it to be around 7%. At sufficiently large polydispersity, fractionation into several solid phases can occur, consistent with previous approximate calculations; we find, in addition, that coexistence of several solids with a fluid phase is also possible.