Refusing to twist: demonstration of a line hexatic phase in DNA liquid crystals

We report conclusive high resolution small angle x-ray scattering evidence that long DNA fragments form an untwisted line hexatic phase between the cholesteric and the crystalline phases. The line hexatic phase is a liquid-crystalline phase with long-range hexagonal bond-orientational order, long-range nematic order, but liquidlike, i.e., short-range, positional order. So far, it has not been seen in any other three dimensional system. By line-shape analysis of x-ray scattering data we found that positional order decreases when the line hexatic phase is compressed. We suggest that such anomalous behavior is a result of the chiral nature of DNA molecules.     

Two-dimensional melting transition observed in a block copolymer

We report the observation of two-dimensional melting in a monolayer film of a sphere-forming diblock copolymer. By annealing in a well-controlled temperature gradient we obtain a complete record of the transition from a low-temperature hexatic phase to a high-temperature liquid in a single experiment. We investigate the temperature dependence of the orientational and translational correlation lengths, as well as of the topological defect density. All evidence suggests that the melting transition is first-order, but correlations in the liquid phase indicate the existence of an underlying second-order transition preempted by the first-order freezing.     

Dynamics of polydisperse hard-spheres under strong confinement

We use molecular simulation to probe the connection between local structure and the unusual re-entrant dynamics observed for polydisperse hard-sphere liquids confined in thin slit pores. The local structure is characterised by calculating 2-D bond-orientational order parameters associated with square and hexatic order for particles in the layer adjacent to the confining walls. When the wall separation is commensurate with the average particle size, the particles primarily exhibit local hexatic order, whereas local square order increases in prevalence for incommensurate geometries. The relaxation time extracted from the ensemble-averaged mean-square displacement increases exponentially with the static correlation length associated with hexatic local order in strongly confined commensurate geometries, in agreement with theoretical predictions for dynamical slowing. Square order, by contrast, is not associated with a growing length scale for either commensurate or incommensurate geometries, indicating that it is strongly geometrically frustrated. Our results suggest that the influence of bond-orientational order on dynamical slowing may be altered by changing the extent of confinement.     

Shear melting of a hexagonal columnar crystal by proliferation of dislocations

A hexagonal columnar crystal undergoes a shear-melting transition above a critical shear rate or stress. We combine the analysis of the shear-thinning regime below the melting with that of synchrotron x-ray scattering data under shear and propose the melting to be due to a proliferation of dislocations, whose density is determined by both techniques to vary as a power law of the shear rate with a 2/3 exponent, as expected for a creep model of crystalline solids. Moreover, our data suggest the existence under shear of a line hexatic phase, between the columnar crystal and the liquid phase.     

Numerical Study on Depinning Dynamics of Fluids Interacting with Disorder

We investigate the depinning of two-dlmensional fluids interacting with quenched disorder, based on Langevin simulations. For weak disorder the fluids depin elastically and flow in an ordered state. A power-law scaling fit between velocity and driving force can be obtained for the onset of motion in the elastic regime. This is in good agreement with that of colloid, charge density wave, and superconducting vortex systems. With an increasing strength of the disorder, we find a sharp crossover to plastic depinning, accompanied by a substantial increase in the depinning force. The scaling fit obtained in the elastic regime becomes invalid when plastic flow occurs. In the plastic regime, the fluids flow in channels and the hexatic order decays exponentially with drives.     

Numerical Study on Depinning Dynamics of Fluids Interacting with Disorder

We investigate the depinning of two-dimensional fluids interacting with quenched disorder, based on Langevin simulations. For weak disorder the fluids depin elastically and flow in an ordered state. A power-law scaling lit between velocity and driving force can be obtained for the onset of motion in the elastic regime. This is in good agreement with that of colloid, charge density wave, and superconducting vortex systems. With an increasing strength of the disorder, we find a sharp crossover to plastic de. Pinning, accompanied by a substantial increase in the depinning force. The scaling fit obtained in the elastic regime becomes invalid when plastic flow occurs. In the plastic regime, the fluids flow in channels and the hexatic order decays exponentially with drives.     

Behavior of the layer compression elastic modulus near,above, and below a smectic C–hexatic I critical point in binary mixtures

We present the first study of the layer compression modulus B carried out near, above and below the Smectic C–Hexatic I critical point in racemic mixtures of methylbutyl phenyl octylbiphenyl-carboxylate (8SI) and the octyloxy biphenyl analog (8OSI), at frequencies ranging from 0.2 Hz to 2 ×103 Hz. The behavior of B as a function of temperature shows a progressive evolution from a first order transition in 8SI to a continuous supercritical behavior in 8OSI. The latter is characterized by an increase in B, which appears above the transition, and which is followed by a leveling off when the temperature is decreased towards the transition. It is proposed that this behavior stems from the relaxation of the hexatic domains which are frozen in the frequency range studied. For the supercritical and near-critical compounds, B exhibits a small dip near the transition temperature, which is visible in the low frequency range only, indicating that the dynamics associated with the critical point is very slow. We also report measurements in the Crystal-J phase of the pure compounds, and show that 8SI behaves mechanically as a hexatic phase and 8OSI as a soft crystal phase.     

Thermodynamics and phase transitions in two-dimensional Yukawa systems

The results of numerical simulations of strongly-coupled two-dimensional dissipative Yukawa systems are presented. The thermodynamic characteristics of these systems were studied, namely the internal energy, the specific heat and the entropy. For the first time, it is discovered that the considered characteristics have two singular points on the melting line; one of these points corresponds to the first-order phase transition from crystal to the hexatic phase, and another point corresponds to the second-order phase transition from the hexatic phase to the isotropic liquid. The obtained results are compared to the existing numerical and analytical data.     

Nonequilibrium phase behavior during the random sequential adsorption of tethered hard disks

We simulate random sequential adsorption of tethered hard disks which undergo limited Monte Carlo surface diffusion to study kinetics and nonequilibrium phase behavior. Tethers allow the disks to move within a specified distance of their original adsorption locations, placing a control on the local entropy of each disk. As the surface coverage increases, systems with sufficiently long tethers form hexatic or crystalline lattices, while short tethers frustrate organization. Lattices form with surprisingly short tether lengths-on the order of one disk diameter.     

Existence of a hexatic phase in porous media

Molecular simulations for simple fluids in narrow slit-shaped carbon pores exhibit crystal-hexatic and hexatic-liquid transitions that are consistent with Kosterlitz-Thouless-Halperin-Nelson-Young theory. The temperature range over which the hexatic phase is stable is dramatically widened under confinement. Remarkably, the transitions, which are continuous for a single adsorbed layer, become weakly first order when the pore can accommodate two molecular layers. Nonlinear dielectric effect measurements for CCl4 and aniline in activated carbon fibers (pore width 1.4 nm) show divergence at these transitions, confirming the hexatic phase.     

Boundary effects in chiral polymer hexatics

Boundary effects in liquid-crystalline phases can be large due to long-ranged orientational correlations. We show that the chiral-hexatic phase can be locked into an apparent three-dimensional N+6 phase via such effects. Simple numerical estimates suggest that the recently discovered "polymer hexatic" may actually be this locked phase.     

On the reversibility of anode supported proton conducting solid oxide cells

Reversible proton conducting solid oxide cells (SOCs) off a highly efficient route to matching supply from intermittent, renewable resources, with power demand by consumers. The cells would store excess electrical energy as chemical fuel during times of peak production, and operate in reverse during times of peak demand. In this study we examine the operation of anode supported proton conducting SOCs in electrolysis mode. The required overpotential for a given current density decreases with increasing humidity at the anode and increasing temperature. All of the V-I curves show distinct curvature. The electrode polarization resistance increases and electrolyte ohmic resistance decreases with increasing current density. This is accompanied by a deviation below the theoretical rate of hydrogen production. We interpret these changes as resulting from deviation away from pure proton conduction in the cell with increasing polarization.     

Observation of a hexatic columnar liquid crystal of polydisperse colloidal disks

We report the observation of a new type of columnar liquid crystal phase, which is formed by thin hard colloidal disks in a dense suspension. High-resolution small-angle x-ray diffraction reveals a combination of long-range bond-orientational order and short-range translational order between the columns, the hallmark of the hexatic phase. Our results imply that geometric frustration related to the size polydispersity of the particles destroys long-range translational order and therefore promotes the formation of this novel phase.     

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.     

Highly ordered hexatic phases with large spontaneous polarization

In this article the physical properties of hexatic phases of three substances MHPNBC, FOOPP and FNHPP have been studied by differential scanning calorimetry, texture observation and dielectric spectroscopy. Experimental results are discussed from the point of view of existing theories. It is interesting that two of the substances studied, the FOOPP and FNHPP, exhibit enhanced spontaneous polarization in the highly ordered SmI* phase and show a jump of the spontaneous polarization in the vicinity of the SmC*–SmI* transition. In the SmI* phase of FOOPP a very high value of spontaneous polarization of the order of 530?nC?cm^?2 was found. Based on the results obtained the macroscopic and microscopic properties of the hexatic phases are discussed.     

似生物膜系统之六角液相

首度在二维溶致型层状液晶系统中，发现了液相与固相之间存在一“六角方向性”的新状态．这个状态显示了的六角长程有序与似液相的位置相关连特性之特征，而与熟知的二维融化理论所预期的现象有显著的差异．     

New hexatic liquid phase observed in lyotropic thin films

An intermediate surface hexatic phase between the liquid and the crystalline phases has been found for the first time in a lyotropic lamellar liquid-crystal system. This phase is highly unusual in that it has long-range sixfold bond-orientational order but liquidlike nearest-neighbor positional correlations, and could represent a significant departure from our current understanding of defect-mediated melting in two dimensions.     

气温和颗粒密度对声场中颗粒动力学影响的数值模拟

综合考虑黏性夹带力、Basset力、虚拟质量力和压力梯度力,建立颗粒在声场中的动力学模型,利用变步长四阶RungeKutta算法和二阶隐式Adams插值算法对颗粒的受力和运动进行数值模拟。将模拟和实验得到的颗粒运动特性进行对比,验证数值模拟的正确性。在此基础上,研究气温和颗粒密度对颗粒动力学的影响规律。结果表明,黏性夹带力对颗粒运动起主导作用;气温升高,压力梯度力与黏性夹带力之间的相位差减小,Basset力、虚拟质量力与黏性夹带力之间的相位差增大。研究还发现,气温较低时,颗粒密度对颗粒运动有重要影响,夹带系数随着密度的增加而迅速下降;气温较高时,颗粒密度对颗粒运动的影响较小,颗粒位移振幅和夹带系数相对低温时明显增加。      

Phase transitions between orthogonal and tilted hexatic phases

Tilt-driven phase transitions between hexatic smectic phases: SmF-HexB and reversed HexB-SmF have been studied in compounds belonging to two enaminoketone derivative homologue series. The tilt angle order parameter has been measured and its temperature dependence near the phase transition point has been described by applying mean-field model. For both phase sequences the tricritical points have been observed on phase transition lines in binary mixtures of respective materials having first and second order phase transitions between hexatic phases. Received 21 June 1999