Triple minima in the free energy of semiflexible polymers

We study the free energy of the worm-like-chain model, in the constant-extension ensemble, as a function of the stiffness lambda for finite chains of length L. We find that the polymer properties obtained in this ensemble are qualitatively different from those obtained using constant-force ensembles. In particular, we find that as we change the stiffness parameter, t=L/lambda, the polymer makes a transition from the flexible to the rigid phase and there is an intermediate regime of parameter values where the free energy has three minima and both phases are stable. This leads to interesting features in the force-extension curves.     

Interactions of Fullerene (C60) and its Hydroxyl Derivatives with Lipid Bilayer: A Coarse-Grained Molecular Dynamics Simulation

Coarse-grained molecular dynamic simulations were employed to study the interactions of fullerene (C₆₀) and its hydroxyl derivatives (C₆₀(OH)_n, n?=?4, 5, 6, 8, 12, and 16) with a lipid bilayer composed of dipalmitoylphosphatidylcholine molecules. It was found that the C₆₀ moves towards the center of the bilayer and laid between central and peripheral regions of the bilayer. The potential mean force was calculated to estimate free energy profile when pulling the fullerene from its initial position to the center of the bilayer using an umbrella sampling method. Results showed that the hydrophobic region of the membrane acts as a barrier to transport a nonpolar C₆₀ molecule through the bilayer. This makes a deep minimum in the free energy profile between the center and head regions of membrane. Various numbers of polar functional groups (–OH) were then used to make derivatives of fullerene and change the hydrophilic of the molecule. It was found that optimal number of hydroxyl groups to facilitate the transportation of C₆₀(OH)_n through the bilayer is 4.     

Lateral pressure profile in lipid membranes with curvature: Analytical calculation

An analytical expression is obtained for the lateral pressure profile in the hydrophobic part of a lipid bilayer of finite curvature. Calculations are carried out within a microscopic model of a lipid bilayer, according to which the energy of a lipid chain represents the energy of a flexible string of finite thickness and the interaction between lipid chains is considered as a steric (entropic) repulsion. This microscopic model allows one to obtain an expression for the distribution of lateral pressure in membranes with given curvature if one considers the bending of a membrane as a small deviation from a flat conformation and applies perturbation theory in the small parameter L ₀ J, where L ₀ is the hydrophobic thickness of a monolayer and J is the mean curvature of the lipid bilayer. The resulting pressure profile depends on the microscopic parameters of the lipid chain: the bending modulus of the lipid chain, incompressible area per lipid chain, and the thickness of a flat monolayer. The coefficient of entropic repulsion between lipids is calculated self-consistently. The analytical results obtained for the lateral pressure distribution are in qualitative agreement with molecular dynamic simulations.     

Incommensurate diffusion in confined systems

Molecular simulations corroborate the existence of the disputed window effect, i.e., an increase in diffusion rate by orders of magnitude when the alkane chain length increases so that the shape of the alkane is no longer commensurate with that of a zeolite cage. This window effect is shown to be characteristic for molecular sieves with pore openings that approach the diameter of the adsorbate. Furthermore, the physical compatibility between the adsorbate and the adsorbent has a direct effect on the heat of adsorption, the Henry coefficients, the activation energy, and the frequency factors.     

Spontaneous polarization and repolarization in crystals of the perovskite type

Based on molecular kinetic concepts, we have theoretically studied the processes of polarization and repolarization in ABO₃ crystals with a perovskite structure. We have calculated the free energy taking into account the possible displacements of the A atoms of the transition element from the equilibrium positions and the ordering of the dipole moments; we have found the dependence of the free energy for different phases of the crystal on temperature, the dipole order parameter, and the energy constants. We have obtained the thermodynamic equilibrium equation determining the temperature dependence of the degree of dipole order; we have found the Curie-Weiss transition temperature from the paraelectric to the ferroelectric state. Analysis of the equations obtained allowed us to provide a basis for the polarization and repolarization processes observed experimentally in BaTiO₃ and KNbO₃ crystals and to estimate the phase transition temperatures.Dnepropetrovsk State University. Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 5, pp. 8–14, May, 1995.     

Window region infrared radiation transfer through clouds and the sensitivity of cloud parameters to radiative effects

The effects due to the passage of i.r. radiation through clouds are predominantly due to the window region spectrum (between 8 and 13 μm). An approximation scheme is used to identify those parameters in the cloud model that determine the heating and cooling effects and the sensitivity of these effects to the variations in the parameters. The results show that the anomalously large heating/cooling effects predicted by the exact calculations are actually a consequence of the choice of a homogeneous liquid water content profile and that an adjustment in this profile results in a lessening of these effects. The radiative effects are also found to be dependent on the size distribution in a nontrivial fashion. In particular, it is found that the use of bimodal distributions, a characteristic feature of clouds with entrainment, results in a lowering of the heating and cooling effects, even with a homogeneous vertical profile, to values more realistic than those obtained with monomodal distributions.     

The isomerization of cytosine: Intramolecular hydrogen atom transfer mediated through formic acid

The catalytic ability of H₂O and HCOOH to facilitate the tautomerism of KA to KIt isomer has been studied. It is shown that the direct tautomerism (path A) is unlikely because of the high activation free energy, whereas the presence of H₂O and HCOOH (paths B and C) significantly contributes to decreasing the activation free energy. Meanwhile, the conventional transition state theory followed by Wigner tunneling correction is applied to estimate the rate constants. The rate constant with Wigner tunneling correction for direct tautomerization is obviously smaller than that of HCOOH‐mediated tautomerization, which is the most plausible mechanism. Finally, another important finding is that the ratio of reaction rates between direct tautomerism reaction and catalyst‐induced tautomerism increases with the increase of the catalyst concentration at a given temperature. The results of the present study demonstrate the feasibility of acid catalysis for DNA bases isomerization reaction that would otherwise be forbidden.     

Influence of Fermi surface topology on kinetic properties of Fe-Co alloys in the domain close to an equiatom composition

The electron energy spectrum for the superstructure FeCo is computed by a symmetrized Green's function method. A curve of the density of the electron states N() and the Fermi surface (FS) section is constructed for the ferromagnetic state obtained by a rigid shift in conformity with the value of the atomic magnetic moment of the alloy. A FS scheme is proposed, consisting of definite sheets belonging to different energy bands emerging at the Fermi level. A semiquantitative estimate is made of the magnitude of the FS area, and the contribution of separate FS sheets to the magnitude of the anomalous Hall R_s, Nernst-Ettingshausen Q_s and thermal emf S coefficients are examined. The nature of the change in the separate FS sections as the alloy composition changes is examined within the framework of the rigid band model, and a qualitative explanation is obtained for the concentration dependence of R_s and S in Fe-Co alloys.Translated from Izvestiya Vysshikh Uchebnykh Zavedenii, Fizika, No. 12, pp. 3–9, December, 1981.     

Density of mechanisms within the flexibility window of zeolites

By treating idealized zeolite frameworks as periodic mechanical trusses, we show that the number of flexible folding mechanisms in zeolite frameworks is strongly peaked at the minimum density end of their flexibility window. 25 of the 197 known zeolite frameworks exhibit an extensive flexibility, where the number of unique mechanisms increases linearly with the volume when long wavelength mechanisms are included. Extensively flexible frameworks therefore have a maximum in configurational entropy, as large crystals, at their lowest density. Most real zeolites do not exhibit extensive flexibility, suggesting that surface and edge mechanisms are important, likely during the nucleation and growth stage. The prevalence of flexibility in real zeolites suggests that, in addition to low framework energy, it is an important criterion when searching large databases of hypothetical zeolites for potentially useful realizable structures.     

The shape of the ear's temporal window

This article examines the idea that the temporal resolution of the auditory system can be modeled using a temporal window (an intensity weighting function) analogous to the auditory filter measured in the frequency domain. To estimate the shape of the hypothetical temporal window, threshold was measured for a brief sinusoidal signal presented in a temporal gap between two bursts of noise. The duration of the gap was systematically varied and the signal was placed both symmetrically and asymmetrically within the gap. The data were analyzed by assuming that the temporal window had the form of a simple mathematical expression with a small number of free parameters. The values of the parameters were adjusted to give the best fit to the data. The analysis assumed that, for each condition, the temporal window was centered at the time giving the highest signal-to-masker ratio, and that threshold corresponded to a fixed ratio of signal energy to masker energy at the output of the window. The data were fitted well by modeling each side of the window as the sum of two rounded-exponential functions. The window was highly asymmetric, having a shallower slope for times before the center than for times after. The equivalent rectangular duration (ERD) of the window was typically about 8 ms. The ERD increased slightly when the masker level was decreased, but did not differ significantly for signal frequencies of 500 and 2000 Hz. The temporal-window model successfully accounts for the data from a variety of experiments measuring temporal resolution. However, it fails to predict certain aspects of forward masking and of the detection of amplitude modulation at high rates.     

Dynamic analysis of horizontal axis wind turbine by thin-walled beam theory

A mixed flexible-rigid multi-body mathematical model is applied to predict the dynamic performance of a wind turbine system. Since the tower and rotor are both flexible thin-walled structures, a consistent expression for their deformations is applied, which employs a successive series of transformations to locate any point on the blade and tower relative to an inertial coordinate system. The kinetic and potential energy terms of each flexible body and rigid body are derived for use in the Lagrange approach to formulate the wind turbine system's governing equation. The mode shapes are then obtained from the free vibration solution, while the distributions of dynamic stress and displacement of the tower and rotor are computed from the forced vibration response analysis. Using this dynamic model, the influence of the tower's stiffness on the blade tip deformation is studied. From the analysis, it is evident that the proposed model not only inherits the simplicity of the traditional 1-D beam element, but also able to provide detailed information about the tower and rotor response due to the incorporation of the flexible thin-walled beam theory.     

Free energy of semiflexible polymers and structure of interfaces

The free energy of semiflexible polymers is calculated as a functional of the compositional scalar order parameter and the orientational order parameter of second-rank tensor S_ij on the basis of a microscopic model of wormlike chains with variable segment lengths. We use a density functional theory and a gradient expansion to evaluate the entropic part of the free energy, which is given in a power series of .The interaction term of the free energy is derived with a random phase approximation. For the rigid rod limit, the nematic-isotropic transition point is given by , N and w being the degree of polymerization and the anisotropic interaction parameter, respectively, and the degree of ordering at the transition point is 0.33448. We also find that the contour length of polymer chains becomes larger in a nematic phase than in an isotropic phase. Interface profiles are obtained numerically for some typical cases. In the neighborhood of isotropic-isotropic interfaces, polymer chains tend to align parallel to the interface on the polymer-rich side and perpendicular on the poor side. When an isotropic region and a nematic region coexist, orientational order parallel to the interface is preferred in the nematic region. Received: 28 May 1998 / Revised: 12 August 1998 / Accepted: 8 September 1998     

Crossover from normal diffusion to single-file diffusion of particles in a one-dimensional channel: LJ particles in zeolite zsm-22

The crossover from normal Fickian diffusion, where mean-squared displacement goes as t to single-file diffusion, where <z²> t?^0.5 is studied as function of particle size confined in zeolite zsm-22 using molecular dynamics simulations. The simulation results indicate that the crossover is smooth as the particle size increases and the diffusion reaches single file through a series of subdiffusion processes. A small number of mutual passage of particles can destroy the correlated single-file diffusion. The density dependence on single-file mobility obtained from molecular dynamics simulations are in very good agreement with theoretical predictions.     

自由旋涡气动窗口全流场的数值模拟

建立自由旋涡气动窗口全流场仿真模型,对大密封压比气动窗口的全流场展开数值研究,得到自由旋涡气动窗口的流场结构,发现大密封压比气动窗口形成的自由旋涡射流在光束输出通道内无明显的波系结构.根据模拟结果对自由旋涡气动窗口的性能进行优化,对自由旋涡喷管上壁面型线进行二次粘性修正.优化自由旋涡射流场后,激光器输出光束通道内压力分布稳定上升;增加扩压器外端壁吹气1.19MPa、内端壁吹气1.68MPa时,自由旋涡射流总能提高,气动窗口密封压力从37.5torr降低至6torr.该研究结果对自由旋涡气动窗口技术的发展具有参考意义.     

Adsorption of nicotine from aqueous solution onto hydrophobic zeolite type USY

The isothermal adsorption of nicotine from an aqueous solution onto zeolite type USY was investigated. The adsorption isotherms of nicotine onto the zeolite at different temperatures ranging from 298 to 322 K were determined. It was found that the adsorption isotherms can be described by the model of Freundlich adsorption isotherm. Based on the adsorption isotherms the changes of adsorption heat, free energy and entropy with adsorption degree were determined. The determined decrease of adsorption heat with adsorption degree can be explained by the presence of the adsorption centers of different energy and concentration on interface of zeolite-nicotine solution. It was found that the probability function of density distribution of the heat of adsorption (DDF) has exponential form. It was concluded that the possibility of fitting the adsorption isotherms of nicotine onto the zeolite by Freundlich adsorption isotherm was a direct consequence of that. The determined increase in entropy with the increase in adsorption degree can be explained with the change of phase state of adsorbed nicotine.     

基于绝对节点坐标法的大变形柔性梁几种动力学模型研究

对在平面内大范围转动的大变形柔性梁动力学进行了研究, 基于绝对节点坐标法建立了一种新的大变形柔性梁的非线性动力学模型. 该动力学模型中考虑了柔性梁的轴向拉伸变形和横向弯曲变形, 利用Green-Lagrangian应变张量计算柔性梁的轴向应变及应变能, 利用曲率的精确表达式计算柔性梁的横向弯曲变形能. 运用拉格朗日恒等式给出了柔性梁横向弯曲变形能新的表达式, 该变形能表达式更加简洁, 通过新的变形能表达式得到了新的弹性力模型, 由此得到的动力学方程可以精确地描述柔性梁的几何大变形问题. 通过与高次耦合模型以及ANSYS中BEAM188非线性梁单元模型的比较, 验证了本模型在计算大变形时的正确性以及高次耦合模型在处理大变形问题时的不足. 进一步研究发现, 新的广义弹性力模型可以适当地简化, 给出了两种简化模型, 根据不同模型的计算效率以及计算精度的比较确定了不同模型的适用范围.     

Wooden windows: Sound insulation evaluation by means of artificial neural networks

Windows are the weakest part of a façade in terms of acoustic performance: the weighted sound insulation index (R_w), measured according to ISO 140-3, is the fundamental parameter to evaluate the façade acoustic insulation.The paper aims at developing an artificial neural network (ANN) model to estimate the R_w value of wooden windows based on a limited number of windows parameters; this is a new approach because acoustic phenomena are non-linear and affected by a plurality of factors and, therefore, usually investigated through experimentation.Data set is taken from experimental campaigns carried out at the Laboratory of Acoustics, University of Perugia. A multilayer feed-forward approach was chosen and the model was implemented in MATLAB. On the basis of the results obtained by means of a preliminary training and test campaign of several ANN architectures, five main parameters were selected as network inputs: window typology, frame and shutters thickness, number of gaskets, R_w of glazing; R_w value of the window is the network output. Different ANN configurations were trained and a root mean-square error less than 3% was obtained, comparable to measurement uncertainty.This approach allows to develop a model which, with input parameters varying within appropriate ranges, can easily estimate the acoustic performance of wooden windows without experimental campaign on prototypes, saving both money and time. If the training data set is large enough, the presented approach could be very useful for design and optimization of acoustic performance of new products.     

Energy model for the Zr-based metallic glass alloy melt with clusters

An energy model for the melt of bulk metallic glass (BMG) with clusters was established, the Gibbs free energy and interfacial energy for the Zr-Al-Ni ternary alloy melt with Zr2Ni clusters were calculated, and the effects of the clusters on the Gibbs free energy, interfacial energy and nucleation rate were analyzed. The results showed that the existence of the clusters in the Zr-Al-Ni ternary alloy melt enables the Gibbs free energy to decrease in the composition range where bulk metallic glass forms easily, makes the interfacial energy increase and changes the distribution of the interfacial energy with the alloy composition. Because of the clusters in the melt, the Gibbs free energy of the Zr66Al8Ni26 alloy melt decreases about 0.3-1 kJ/mol and the interfacial energy between the melt and crystal nucleus increases about 0.016 J/m2. The nucleation rate of the undercooled Zr66Al8Ni26 alloy melt decreases evidently under the influence of the clusters on Gibbs free energy and the interfacial energy, and the maximum of the nucleation rate in the melt with the Zr2Ni clusters is only about 107 mol-1·s-1.     

A new theoretical approach to the encapsulation of small molecules in zeolites

We present a new theoretical method to study the encapsulation of small molecules such as H₂, O₂, N₂, Ar and CH₄ in the Cs₃Na₉-A zeolite. To study the properties of encapsulated molecules, we used the Fermi-Dirac like statistics. The density of states, the distribution function, average binding energy and the average activation energy of encapsulated molecules are calculated. As the number of encapsulated molecules in zeolite cavities increases, the higher energy states in the cavities are gradually filled and, consequently, the activation energy for decapsulation is lowered. We also calculated the fraction of molecules with higher energy than their activation energy, revealing that the activation energy for decapsulation depends not only on the temperature but also on the number of the encapsulated molecules.     

A local approach to the computation of correlation energies of molecules

A new variational approach is introduced for the calculation of correlation energies of molecules. It is based on the local character of the correlated electron motion. Based on the approach we calculate correlation energies for simple systems. These include various Hubbard models as well as a model of the H₆-ring for which exact results are available. Our finding is that the proposed approach appears to be simpler and more economical for numerical work than conventional CI-methods. More than 90% of the correlation energy is obtained in all cases considered.