Nature of the transition from two- to three-dimensional ordering in a confined colloidal suspension

We report the results of extensive molecular dynamics simulations of solid-to-solid transitions in two- to six-layer colloidal suspensions confined between two smooth parallel walls. The studies are designed to elucidate the ordered particle packings that interpolate between the structures of two- and three-dimensional crystals in a confined space. At a fixed density per layer, as the wall separation increases we find a sequence of stable phases, each characterized by uniform amplitude buckling along the normal to the layer planes. The buckling is coupled to an in-plane ordering transition. The buckled phases alternate with phases whose structures contain only parallel planes of particles. The relative densities of the positively and negatively displaced particles in a buckled layer, the in-plane structures, and the behavior with respect to increasing wall separation of the split density distribution that characterizes a buckled layer, clearly identify these layers as intermediates in the reconstructive transformations ntriangle up-->(n+1) square that occur when the character of the constrained space evolves from being two dimensional to being three dimensional (triangle up denotes layers with hexagonal packing symmetry, while square denotes layers with square packing symmetry). The two transitions, ntriangle up-->n-buckled-->(n+1) square, are found to be first order.     

双层介质球体所受光作用力的分析与计算

计算了双层介质球体的光作用力.结果表明,双层结构内外层相对折秧经和相对厚度的变化都将影响光作用力的大小.特别是当内层折射率大于外层折射率时,减小内层半径可得到一较小区域,在此区域光镊系统刚度将得到有效提高,可实现对双层介质球体的稳定束缚;而当内层折射率小于外层折射率时,随内层半径的减小,介质小球可能有两个稳定平衡点出现.     

Frictional figures of merit for single layered nanostructures

We determine the frictional figures of merit for a pair of layered honeycomb nanostructures, such as graphane, fluorographene, MoS2 and WO2 moving over each other, by carrying out ab initio calculations of interlayer interaction under constant loading force. Using the Prandtl-Tomlinson model we derive the critical stiffness required to avoid stick-slip behavior. We show that these layered structures have low critical stiffness even under high loading forces due to their charged surfaces repelling each other. The intrinsic stiffness of these materials exceeds critical stiffness and thereby the materials avoid the stick-slip regime and attain nearly dissipationless continuous sliding. Remarkably, tungsten dioxide displays a much better performance relative to others and heralds a potential superlubricant. The absence of mechanical instabilities leading to conservative lateral forces is also confirmed directly by the simulations of sliding layers.     

Pattern formation during deformation of a confined viscoelastic layer: from a viscous liquid to a soft elastic solid

We study pattern formation during tensile deformation of confined viscoelastic layers. The use of a model system [poly(dimethylsiloxane) with different degrees of cross-linking] allows us to go continuously from a viscous liquid to an elastic solid. We observe two distinct regimes of fingering instabilities: a regime called "elastic" with interfacial crack propagation, where the fingering wavelength scales only with the film thickness, and a bulk regime called "viscoelastic," where the fingering instability shows a Saffman-Taylor-like behavior. We find good quantitative agreement with theory in both cases and present a reduced parameter describing the transition between the two regimes and allowing us to predict the observed patterns over the whole range of viscoelastic properties.     

Symmetry breaking of in-plane order in confined copolymer mesophases

Packing of spherical-domain block copolymer mesophases confined to a thin film is investigated as a function of the number of layers n. We find an abrupt transition from hexagonal to orthorhombic in-plane ordering of domains when n is increased from 4 to 5. As n increases further (up to 23 in this study), the symmetry of the orthorhombic phase asymptotically approaches that of the body-centered cubic (110) plane. These results are interpreted in terms of the energetics of competing packings in the bulk and at the film interfaces. Detailed structural and thermodynamic properties are obtained with self-consistent field theory.     

Incommensurability of a confined system under shear

We study a chain of harmonically interacting atoms confined between two sinusoidal substrate potentials, when the top substrate is driven through an attached spring with a constant velocity. This system is characterized by three inherent length scales and closely related to physical situations with confined lubricant films. We show that, contrary to the standard Frenkel-Kontorova model, the most favorable sliding regime is achieved by choosing chain-substrate incommensurabilities belonging to the class of cubic irrational numbers (e.g., the spiral mean). At large chain stiffness, the well known golden mean incommensurability reveals a very regular time-periodic dynamics with always higher kinetic friction values with respect to the spiral mean case.     

Enhancement of the superconducting transition temperature of La2-xSrxCuO4 bilayers: role of pairing and phase stiffness

The superconducting transition temperature T(c) of bilayers comprising underdoped La(2-x)Sr(x)CuO(4) films capped by a thin heavily overdoped metallic La(1.65)Sr(0.35)CuO(4) layer, is found to increase with respect to T(c) of the bare underdoped films. The highest T(c) is achieved for x=0.12, close to the "anomalous" 1/8 doping level, and exceeds that of the optimally doped bare film. Our data suggest that the enhanced superconductivity is confined to the interface between the layers. We attribute the effect to a combination of the high pairing scale in the underdoped layer with an enhanced phase stiffness induced by the overdoped film.     

X-Ray waveguiding studies of ordering phenomena in confined fluids

We have determined the structure of a colloidal fluid confined in a gap between two walls by making use of the waveguiding properties of the gap at x-ray wavelengths. The method is based on an analysis of the coupling of waveguide modes induced by the density variations in the confined fluid. Studies on suspensions confined within gaps of a few hundred nanometers showed strongly selective mode coupling effects, indicative of an ordering of the colloidal particles in layers parallel to the confining walls.     

Nanofluidics: viscous dissipation in layered liquid films

We studied the layer-by-layer collapse of molecularly thin films of a model lubricant confined between two atomically smooth substrates. The dynamics of the consecutive expulsion of four molecular layers were found to slow down with decreasing film thickness but showed no evidence for confinement-induced solidification. Using a hydrodynamic model, we show that the sliding friction of liquid layers on top of the solid substrates is approximately 18 times higher than the mutual friction between adjacent liquid layers. The latter was independent of film thickness and in close agreement with the bulk viscosity.     

Free energy studies of freezing in slit pores: an order-parameter approach using Monte Carlo simulation

We report a molecular simulation study of freezing transitions for simple fluids in narrow slit pores. A major stumbling block in previous studies of freezing in pores has been the lack of any method for calculating the free energy difference between the confined solid and liquid phases. Conventional thermodynamic integration methods often fail for confined systems, due to the difficulty in choosing a suitable path of integration. We use a different approach that involves calculating the Landau free energy as a function of a suitable order parameter, using the grand canonical Monte Carlo simulation method. The grand free energy for each phase can then be obtained by one-dimensional integration of the Landau free energy over the order parameter. These calculations are carried out for two types of wall—fluid interaction, a hard wall and a strongly attractive wall modelled on carbon. The grand free energy results for both cases clearly indicate a first order fluid to solid transition. In the case of the attractive carbon wall, there are three phases. Phase A corresponds to all layers having a liquid-like structure; phase B corresponds to the contact layers (the layers adjacent to the two pore walls) being frozen and the rest of the layers being fluid-like; phase C corresponds to all the layers being frozen. Our results for the angular structure function in the individual molecular layers show strong evidence of a transition from a two-dimensional liquid phase to a hexatic phase. This is followed by a transition from the hexatic to a crystal phase.     

Quantized spin waves in antiferromagnetic Heisenberg chains

The quantized stationary spin wave modes in one-dimensional antiferromagnetic spin chains with easy axis on-site anisotropy have been studied by means of Landau-Lifshitz-Gilbert spin dynamics. We demonstrate that the confined antiferromagnetic chains show a unique behavior having no equivalent, neither in ferromagnetism nor in acoustics. The discrete energy dispersion is split into two interpenetrating n and n' levels caused by the existence of two sublattices. The oscillations of individual sublattices as well as the standing wave pattern strongly depend on the boundary conditions. Particularly, acoustical and optical antiferromagnetic spin waves in chains with boundaries fixed (pinned) on different sublattices can be found, while an asymmetry of oscillations appears if the two pinned ends belong to the same sublattice.     

Superheating of confined Pb thin films

In this work we report, for the first time, an experimental observation of a superheating phenomenon in metal thin films. By means of cold rolling, Pb thin films of about 20 nm thick were sandwiched by Al layers, and between them semicoherent epitaxial Pb/Al interfaces were formed. In situ x-ray diffraction analysis indicated that the confined Pb thin films could be superheated for at least 6 degrees C. Thermodynamic analysis indicated that such a substantial superheating in the confined two-dimensional thin films may originate from suppression of growth of the molten droplets by the epitaxial Al/Pb/Al confinement, instead of suppression of melt nucleation for the confined particle superheating.     

Colloidal glass transition observed in confinement

We study a colloidal suspension confined between two quasiparallel walls as a model system for glass transitions in confined geometries. The suspension is a mixture of two particle sizes to prevent wall-induced crystallization. We use confocal microscopy to directly observe the motion of colloidal particles. This motion is slower in confinement, thus producing glassy behavior in a sample which is a liquid in an unconfined geometry. For higher volume fraction samples (closer to the glass transition), the onset of confinement effects occurs at larger length scales.     

Real-space visualization of intercalated water phases at the hydrophobic graphene interface with atomic force microscopy

The phase behavior of water is a topic of perpetual interest due to its reinai kable anomalous properties and importance to biology,material science,geoscience,nanoscience,etc.It is predicted confined water at interface can exist in large amounts of crystalline or amorphous states.However,the experimental evidence of coexistence of liquid water phases at interface is still insufficient.Here,a special folding few-layers graphene film was elaborate prepared to form a hydrophobic/hydrophobic interface,which can provide a suited platform to study the structure and properties of confined liquid water.The real-space visualization of intercalated water layers phases at the folding interface is obtained using advanced atomic force microscopy(AFM).The folding graphene interface displays complicated internal interfacial characteristics.The intercalated water molecules present themselves as two phases,low-density liquid(LDL,solid-like)and high-density liquid(HDL,liquid-like),according to their specific mechanical properties taken in two multifrequency-AFM(MF-AFM)modes.Furthermore,the water molecules structural evolution is demonstrated in a series of continuous MF-AFM measurements.The work preliminary confirms the existence of two liquid phases of water in real space and will inspire further experimental work to deeply understanding their liquid dynamics behavior.     

Surface-induced ordering of a molecular fluid of flat hexagonal structure in a narrow graphite slit

We systematically investigate by Monte Carlo simulations the role of the wall structure on a fluid of flat hexagonal molecules confined between two graphite walls. Our simulations show that the centers of mass of the molecules in different layers undergo an order-disorder transition as the wall separation increases, irrespective of the details of the wall structure. The wall structure thus becomes insignificant for the intervening fluid even down to a surprisingly low wall separation.     

Study of the Fe−Ni interface by conversion electron Mössbauer spectroscopy

Nickel-gamma iron interfaces obtained by growing films of Fe on Ni substrates are studied using CEMS. We have found that a) intermixing is confined to two atomic layers, b) Fe atoms deposited on structural defects of the Ni (111) surfaces are magnetically polarized, c) Fe epitaxially grown on Ni is highly susceptible at room temperature. Moreover the Ni−Fe interface shows Neel-type magnetic anisotropy.     

Freezing and melting of binary mixtures confined in a nanopore

This paper reports on a Grand Canonical Monte Carlo study of the freezing and melting of Lennard–Jones Ar/Kr mixtures confined in a slit pore composed of two strongly attractive structureless walls. For all molar compositions and temperatures, the pore, which has a width of 1.44?nm, accommodates two contact layers and one inner layer. Different wall/fluid interactions are considered, corresponding to pore walls that have a larger affinity for either Ar or Kr. The solid/liquid phase diagram of the confined mixture is determined and results compared with data for the bulk mixture. The structure of the confined mixture is studied using 2D order parameters and both positional g(r) and bond orientational G₆(r) pair correlation functions. It is found that in the confined solid phase, both the contact and inner layers have a hexagonal crystal structure. It is shown that the freezing temperature of the Ar/Kr confined mixture is higher than the bulk freezing point for all molar compositions. Also, it is found that the freezing temperature becomes larger as the ratio α of the wall/fluid to the fluid/fluid interactions increases, in agreement with previous simulation studies on pure substances confined in nanopores. In the case of pore walls having a stronger affinity for Kr atoms (ε _Ar/W<ε _Kr/W), it is observed that both the contact and inner layers of the confined mixture undergo, at the same temperature, a transition from the liquid phase to the crystal phase. The freezing of Ar/Kr mixtures confined between the walls having a stronger affinity for Ar (ε _Ar/W?>?ε _Kr/W) is more complex: for Kr molar concentration lower than 0.35, we observe the presence of an intermediate state between all layers being 2D hexagonal crystals and all the layers being liquid. This intermediate state consists of a crystalline contact layer and a liquid-like inner layer. It is also shown that the qualitative variations of the increase of freezing temperature with the molar composition depend on the affinity of the pore wall for the different components. These results confirm that, in addition to the parameter α the ratio of the wall/fluid interactions for the two species, η=?_Ar/W/?_Kr/W, is a key variable in determining the freezing and melting behaviour of the confined mixture.     

Some remarks on the chain segment dynamics in confined polymers

Recently, the chain dynamics of molten polymers in confined geometries has been investigated, using NMR. Some of these experiments have been performed on poydimethylsiloxane (PDMS) confined in planar nanolayers involving non-adsorbant solid surfaces. Here the data obtained on various PDMS systems are compared. The common property to these systems is that the local dynamics is anisotropic and the chain segments are undergoing uniaxial fluctuations around the normal n to the layers. Our data clearly show that the sign of the degree of orientational order S ( i.e. the direction of the fluctuations around n) and the broadening of the order distribution P( S) both result from the relative influence of the impenetrable interfaces and anchoring junctions on the segment behaviour.     

Transmission properties of photonic quantum well composed of dispersive materials

The transmission properties of one-dimensional photonic quantum well structure produced by inserting defect layers into dispersive photonic crystals have been investigated. These photonic crystals are stacked alternatively with two kinds of dispersive material layers. The inserted layers structure is another kind of dispersive photonic crystal. It has been turned out that the confined states are quantized.     

微纳间隙受限液体的界面黏着机理研究

利用球-盘接触黏着试验仪对微量受限液体的界面黏着行为的研究发现：临界体积范围内（皮升到纳升）的受限液滴达到临界厚度后将出现自动铺展和瞬时收缩行为（分别对应球-盘趋进和分离过程）,并同时提供一定幅值的法向黏着力及伴随出现力的突变;该界面黏着力与液滴体积、球的直径等相关,并很好地揭示了昆虫或树蛙爪垫与光滑表面间的湿黏着行为.在进一步试验的基础上,利用球—面接触模型的毛细黏着力公式对所观察到的试验现象进行了机理揭示,认为临界体积受限液滴出现的自动铺展和瞬时收缩行为与接触副的刚度相关,最大法向黏着力随液体体积的减小而减小,与接触副之间受限液体的中心区厚度有关.结合生物黏着爪垫进行了受限液体界面黏着控制机理的分析,可以指导仿生黏着爪垫的设计与控制. 关键词： 生物黏着爪垫 毛细黏着力 受限液体