Plume fragmentation by bulk interactions in turbulent Rayleigh-Bénard convection

Using compressed gases with Prandtl numbers near 0.7, we obtained flow visualizations of turbulent Rayleigh-Bénard convection in a cylindrical sample with an aspect ratio Γ≡D/L?10 (D is the diameter and L the height) by the shadowgraph method. Focusing on the plumes under the top plate, we found that their length had a log-normal distribution, suggesting a fragmentation process. Fragmentation events could be visually identified in the images and involved plume interactions with bulk fluctuations or upwelling domain walls. We found the mean spacing between plumes to vary with the Rayleigh number in proportion to the volume-averaged Kolmogorov length of the turbulent bulk fluctuations, providing further evidence for plume-bulk interactions.     

AB/CD嵌段共聚物共混体系多尺度结构的自洽场模拟

本文采用自洽场理论模拟了AB/CD嵌段共聚物共混体系的自组装. 改变组分B与D间的相互作用, 得到了不同空间尺度上的两种层状结构和只能在非对称组成情况才能得到的核壳结构. 结果表明, 这种多尺度结构的形成是因为BD间排斥作用的减弱或者吸引 作用的增强导致二者间相互融合程度的增加. 当BD间的相互融合程度与AB和CD间的相互融合程度相当时, 体系会发生宏观层状与微观层状结构间的转变. 此外, 本文还从能量角度揭示了体系发生这种结构转变的深层次原因. 关键词： 嵌段共聚物 共混 多尺度结构 自洽场     

Magnetic Phase Transition Driven by Frustrated Interactions in a Longitudinal‐Field Ising Chain

The ground‐state magnetic phase transitions in a classical spin chain with long‐range interactions in a system of trapped ions are investigated. The tunable competing interactions, mediated by both longitudinal and transverse photon modes, lead to competition among different spins, due to which magnetic frustration occurs. Various ground‐state spin configurations are separated by multiple phase transitions when the strength and sign of these interactions are tuned continuously. The spin chains are highly degenerated because of the frustration, so there is some melting of several magnetic phases.     

Phase behaviors in a binary mixture of diblock copolymers confined between two parallel walls

The phase behaviors in a binary mixture of diblock copolymers confined between two parallel walls are investigated by using a cell dynamics simulation of the time-dependent Ginzburg-Landau theory.The morphological dependence of the wall-block interaction and the distance between walls(confinement degree) has been systematically studied,and the effect of repulsive interactions between different monomers is also discussed.It is interesting that multiple novel morphological transitions are observed by changing these factors,and various multilayered sandwich structures are formed in the mixture.Furthermore,the parametric dependence and physical reasons for the microdomain growth and orientational order transitions are discussed.From the simulation,we find that much richer morphologies can form in a binary mixture of diblock copolymers than those in a pure diblock copolymer.Our results provide an insight into the phase behaviors under parallel wall confinement and may provide guidance for experimentalists.This model system can also give a simple way to realize orientational order transition in soft materials through confinement.     

Phase behaviors in binary mixture of diblock copolymers confined between two parallel walls

The phase behaviors in binary mixture of diblock copolymers confined between two parallel walls are investigated by using cell dynamics simulation of the time-dependent Ginzburg-Landau theory. The morphological dependence of the wall-block interaction and the distance between walls (confinement degree) has been systematically studied, and the effect of repulsive interactions between different monomers is also discussed. It is interesting that multiple novel morphological transitions are observed by changing these factors, and various multilayered sandwich structures are formed in the mixture. Furthermore, the parametric dependence and physical reasons for the microdomain growth and orientational order transitions are discussed. From the simulation, we find that much richer morphologies can form in binary mixture of diblock copolymers than those in pure diblock copolymer. Our results provide an insight into the phase behaviors under parallel walls confinement and may provide guidance for experimentalists. This model system can also give a simple way to realize orientational order transition in soft materials through confinement.     

Superfrustration of charge degrees of freedom

We review recent results, obtained with P. Fendley, on frustration of quantum charges in lattice models for itinerant fermions with strong repulsive interactions. A judicious tuning of kinetic and interaction terms leads to models possessing supersymmetry. In such models frustration takes the form of what we call superfrustration: an extensive degeneracy of supersymmetric ground states. We present a gallery of examples of superfrustration on a variety of 2D lattices.     

Origin of the 150-K anomaly in LaFeAsO: competing antiferromagnetic interactions, frustration, and a structural phase transition

From all-electron fixed-spin-moment calculations we show that ferromagnetic and checkerboard antiferromagnetic ordering in LaFeAsO are not stable and the stripe antiferromagnetic configuration with M(Fe)=0.48 microB is the only stable ground state. The main exchange interactions between Fe ions are large, antiferromagnetic, and frustrated. The magnetic stripe phase breaks the tetragonal symmetry, removes the frustration, and causes a structural distortion. These results successfully explain the magnetic and structural phase transitions in LaFeAsO recently observed by neutron scattering. The presence of competing strong antiferromagnetic exchange interactions suggests that magnetism and superconductivity in doped LaFeAsO may be strongly coupled, much like in the high-T(c) cuprates.     

Random copolymers at a selective interface: Many chains with excluded volume interactions

We investigate numerically, using the bond-fluctuation model, the adsorption of many random AB-copolymers with excluded volume interactions at the interface between two solvents. We find two regimes, controlled by the total number of polymers. In the first (dilute) regime, the copolymers near the interface extend parallel to it, while in the second regime they extend perpendicular to it. The density at the interface and the density in the bulk depend differently on the total number of copolymers: In the first regime the density at the interface increases more rapidly than in the bulk, whereas the opposite is true in the second regime. Received 4 March 1998 and Received in final form 22 September 1998     

Structural transition of nanogel star polymers with pH by controlling PEGMA interactions with acid or base copolymers

We use small angle X-ray scattering (SAXS) to characterise a class of star diblock polymers with a nanogel core on which the outer block arms are comprised of random copolymers of temperature sensitive PEGMA with pH sensitive basic (PDMAEMA) and acidic (PMAA) monomers. The acquired SAXS data show that many of the nanogel star polymers undergo a sharp structural transition over a narrow range of pH, but with unexpectedly large shifts in the apparent pKa with respect to that of the acidic or basic monomer unit, the linear polymer form or even an alternate star polymer with a tightly cross-linked core chemistry. We have demonstrated a distinct and quantifiable structural response for the nanogel star copolymers by altering the core or by pairing the monomers PDMAEMA–PEGMA and PMAA–PEGMA to achieve structural transitions that have typically been observed in stars through changes in arm length and number.     

Direct measurement of interfacial curvature distributions in a bicontinuous block copolymer morphology

Self-consistent field theory predicts that the complex phase behavior of block copolymers does not originate solely from the interface seeking constant mean curvature as once thought, but instead reflects competing minimization of interfacial tension and packing frustration. To test this prediction, we directly measure interfacial curvature distributions from a 3D image reconstruction of the bicontinuous gyroid morphology. Results obtained here reveal that the gyroid interface is not constant mean curvature and confirm the importance of packing frustration in the stabilization of such complex nanostructures.