二维情况下两组分带电囊泡形变耦合相分离的理论模拟研究

结合离散空间变分方法和耗散动力学研究了二维两组分带电囊泡的形变耦合相分离,系统地考察了囊泡带电量组分含量、带电组分的电荷密度、两组分间的相容性和温度等因素对形变耦合相分离动力学的影响.模拟结果表明电荷引入可增加不同组分间的表观相溶性.当温度较高时,静电相互作用可直接抑制囊泡相分离,避免了同种组分的团聚;当温度较低时,静电相互作用则可明显增加分相相区数目,使其呈微观相分离,从而避免了同种组分大范围的团聚.     

Correlated process of phase separation and microstructure evolution of ternary Co-Cu-Pb alloy

The phase separation and rapid solidification of liquid ternary Co₄₅Cu₄₂Pb₁₃ immiscible alloy have been investigated under both bulk undercooling and containerless processing conditions. The undercooled bulk alloy is solidified as a vertical two-layer structure, whereas the containerlessly solidified alloy droplet is characterized by core-shell structures. The dendritic growth velocity of primary α(Co) phase shows a power-law relation to undercooling and achieves a maximum of 1.52 m/s at the undercooling of 112 K. The Pb content is always enriched in Cu-rich zone and depleted in Co-rich zone. Numerical analyses indicate that the Stokes motion, solutal Marangoni convection, thermal Marangoni convection, and interfacial energy play the main roles in the correlated process of macrosegregation evolution and microstructure formation.     

单开口膜泡形状转变的研究

基于双层耦合模型,先通过求解无黏附能情况下满足给定边界条件的欧拉-拉格朗日方程组,找到了约化面积差?a稍大于1的内凹开口形状解,并发现以往Umeda和Suezaki(2005 Phys.Rev.E 71 011913)给出的杯形解是对应?a1的另一支解,该支解在?a趋于1时开口是外凸的.进而在无黏附能和有黏附能的情况下对开口膜泡的两支解进行了深入研究,发现在?a=1附近这两支解之间有一个间隙,在该间隙内不存在开口解.随着黏附半径的增大,该间隙的位置较缓慢地向右移动.在?a=1附近,在无黏附能时的闭合形状只有球形一个解,而在有黏附能的情况下,闭合形状在1附近的一个区间内都有解.在无黏附及有黏附情况下的计算结果都表明这两支开口解及闭合形状属于不同的分支,它们之间不能连续演化.在间隙右侧的这一支解随着?a的增大可以通过自交形状连续地演化到开口哑铃形.在有黏附的情况下,在?a参数空间,同一支解会发生折叠,即出现同一?a值对应多个解(形状)的情况,这在以往双层耦合模型的计算中没有出现过.讨论了?a对无黏附和有黏附开口膜泡的形状和能量的影响.     

Adhesion of binary giant vesicles containing negative spontaneous curvature lipids induced by phase separation

We report the adhesion of binary giant vesicles composed of two types of phospholipids, one has negative spontaneous curvature which tends to bend toward the head group and the other has zero spontaneous curvature. In a homogeneous one-phase region, the giant vesicles do not adhere to each other, whereas in a coexisting two-phase region, the giant vesicles show adhesion. A fluorescence microscope observation reveals that the adhesion takes place through the domains rich in phospholipids having negative spontaneous curvature. We propose a phase separation induced hemifusion model where two apposed monolayers of adjacent vesicles are hemifused in order to reduce the bending energy of monolayers with negative spontaneous curvature and the boundary energy between the domains and matrix. We provide a strong evidence for the hemifusion model by lipid transfer experiments.     

Nearly spherical vesicles: Shape fluctuations

Isolated vesicles with “insufficient” area have a finite surface tension and spherical shapes, whereas vesicles with “excess” area are necessarily aspherical. We consider the crossover behavior between both kinds of vesicles occurring as the equilibrium area increases. In the mean field approximation it is a second-order phase transition from the spherical to aspherical shape. We demonstrate the fluctuations smear the transition. The critical behavior of the amplitudes of fluctuations and of their characteristic times is investigated. Pis’ma Zh. éksp. Teor. Fiz. 63, No. 3, 203–208 (10 February 1996) Published in English in the original Russian journal. Edited by Steve Torstveit.     

Shear-induced deformation of surfactant multilamellar vesicles

Surfactant multilamellar vesicles (SMLVs) play a key role in the formulation of many industrial products, such as detergents, foodstuff, and cosmetics. In this Letter, we present the first quantitative investigation of the flow behavior of single SMLVs in a shearing parallel plate apparatus. We found that SMLVs are deformed and oriented by the action of shear flow while keeping constant volume and exhibit complex dynamic modes (i.e., tumbling, breathing, and tank treading). This behavior can be explained in terms of an excess area (as compared to a sphere of the same volume) and of microstructural defects, which were observed by 3D shape reconstruction through confocal microscopy. Furthermore, the deformation and orientation of SMLVs scale with radius R in analogy with emulsion droplets and elastic capsules (instead of R(3), such as in unilamellar vesicles). A possible application of the physical insight provided by this Letter is in the rationale design of processing methods of surfactant-based systems.     

Shape diagram of vesicles in Poiseuille flow

Soft bodies flowing in a channel often exhibit parachutelike shapes usually attributed to an increase of hydrodynamic constraint (viscous stress and/or confinement). We show that the presence of a fluid membrane leads to the reverse phenomenon and build a phase diagram of shapes-which are classified as bullet, croissant, and parachute-in channels of varying aspect ratio. Unexpectedly, shapes are relatively wider in the narrowest direction of the channel. We highlight the role of flow patterns on the membrane in this response to the asymmetry of stress distribution.     

液态三元Fe-Sn-Si/Ge偏晶合金相分离过程的实验和模拟研究

本文采用自由落体实验技术和格子玻尔兹曼计算方法研究了低重力条件下液态Fe-Sn-Si/Ge合金的相分离过程. 实验发现, 二种合金液滴在自由下落过程中均发生显著的液相分离, 形成了壳核和弥散组织. 当Fe-Sn-Si合金中的Si元素被等量的Ge元素替换后, 壳核组织中富Fe区和富Sn区的分布次序会发生反转. 计算表明, 在液相分离过程中冷却速率、Marangoni对流和表面偏析对壳核构型的选择和弥散组织的形成起决定性作用.     

Relation Between Shape Equations for Axisymmetric vesicles

In deriving the shape equation for axisymmetric vesicles from the Helfrich free energy the variation must be taken with respect to the contour. It is pointed out that the widely used Helfiich shape equation and the other one given by Peterson were derived by improper variations, hence both equations need to be examined. The validity of the two equations is discussed based on analysis of their relation revealed by two derived first integrals. A newly found exact solution for the general shape equation is studied and the relation of this exact solution of spherical topology with that of the Helfrich shape equation is discussed.     

三区域膜泡相分离模式之间转变的研究

基于Helfrich弹性曲率模型,结合实验参数,用直接极小化方法对三区域膜泡的两种相分离模式的稳定形状,及其之间的相变进行了计算,得出相变点为φ_o^*=0.49,与实验结果φ_o^*=0.5很接近.用直接极小化方法还研究了三区域膜泡发生发芽的必要条件,指出只有当约化线张力系数足够大,并且膜泡内外溶剂能自由渗透时,才可能发生发芽形变,并对渗透的可能机制进行了探讨.     

Shape instabilities in vesicles: A phase-field model

A phase field model for dealing with shape instabilities in fluid membrane vesicles is presented. This model takes into account the Canham-Helfrich bending energy with spontaneous curvature. A dynamic equation for the phase-field is also derived. With this model it is possible to see the vesicle shape deformation dynamically, when some external agent instabilizes the membrane, for instance, inducing an inhomogeneous spontaneous curvature. The numerical scheme used is detailed and some stationary shapes are shown together with a shape diagram for vesicles of spherical topology and no spontaneous curvature, in agreement with known results.     

Shape fluctuation phase transition of elastic membranes with fluidity

It is confirmed by Monte Carlo (MC) simulation that the elastic membrane corresponding to discrete Polyakov–Kleinert string undergoes a second-order phase transition at finite bending rigidity. This phase transition is considered to be associated with a non-trivial fixed point of the β-function for the inverse bending rigidity.     

Coulomb-frustrated phase separation phase diagram in systems with short-range negative compressibility

Using numerical techniques and asymptotic expansions we obtain the phase diagram of a paradigmatic model of Coulomb-frustrated phase separation in systems with negative short-range compressibility. The transition from the homogeneous phase to the inhomogeneous phase is generically first order in isotropic three-dimensional systems except for a critical point. Close to the critical point, inhomogeneities are predicted to form a bcc lattice with subsequent transitions to a triangular lattice of rods and a layered structure. Inclusion of a strong anisotropy allows for second- and first-order transition lines joined by a tricritical point.     

Shape invariance in phase space

Shape invariance is a powerful solvability condition, that allows for complete knowledge of the energy spectrum, and eigenfunctions of a system. After a short introduction into the deformation quantization formalism, this paper explores the implications of the supersymmetric quantum mechanics and shape invariance techniques to the phase space formalism. We show that shape invariance induces a new set of relations between the Wigner functions of the system, that allows for their direct calculation, once we know one of them. The simple harmonic oscillator and the Morse potential are solved as examples.     

Ferromagnetic order in silicon-manganese alloys with phase separation

A phenomenological model of high-temperature ferromagnetism in silicon-manganese alloys has been proposed taking into account phase separation in these alloys, where manganese-rich particles of the secondary phase (precipitate MnSi_{2 − z} with z ≈ 0.25–0.30) are formed inside a manganese-depleted matrix of almost pure silicon. Precipitate MnSi_{2 − z} is considered as the silicide MnSi_1.7 containing a certain number of magnetic defects whose origin is due to the presence of weakly hybridized 3d orbitals of manganese. The silicide MnSi_1.7 is a weak band ferromagnet in which strong fluctuations of the spin density (paramagnons) are present at a temperature much higher than its Curie temperature. It has been shown that the ferromagnetic exchange interactions between the magnetic moments of defects in precipitate exists due to thermal excitations of the spin density and the ferromagnetic order can appear at a temperature much higher than the Curie temperature of the silicide. The spatial structures and characteristics of this order have been described in the framework of the proposed approach for both homogeneous bulk precipitate and precipitate particles of various shapes and sizes. The short-range magnetic order near the bulk phase transition has been analyzed taking into account inhomogeneities in the distribution of magnetic defects in precipitate. The experimental data on the magnetic properties of silicon-manganese alloys have been interpreted in terms of the theoretical results obtained in this work.     

Shear-induced phase transitions in ternary polymer blends

We present a study of flow-induced phase transitions in microemulsion phases of ternary polymer blends. The results match qualitatively with the recent experimental observations on such systems but differ from the behavior expected and observed in the analogous system of surfactants. We rationalize this contrast from a molecular viewpoint suggesting that the interplay between polymer chain conformations and their flow deformations can lead to novel flow effects upon the phase, structural, and rheological behavior of multicomponent polymer systems.     

Determination and calculation of high-pressure ternary phase diagrams

Abstract

By the improved high-pressure DTA, we have determined the high-pressure vertical phase diagrams of the Cd-Pb-Sn ternary system up to 2.5 GPa.

A complete method of calculating high-pressure phase diagrams has been devised, in which the special attention has been paid to the effecs of high prssure on the partial molar free energy of each component in equilibrium phases. The high-pressure phase diagrams of Cd-Pb-Sn and Cd-Sn-Zn ternary systems up to 2.5 GPa were calculated successfully.

The agreement between the observed and the calculated resuts is satisfactory.     

Representation of phase equilibria in ternary oxide systems

The P-T-x diagram of the Yb-Mn-O system has been constructed by the static method for study of heterogeneous equilibria and the isothermal annealing method. The problems of representation of such diagrams of ternary oxide systems with one volatile component are discussed. The values of the thermodynamic functions are calculated for the reactions of dissociation of the YbMn₂O₅ and YbMnO₃ compounds and their formation from elements.     

Shape deformation and viscosity in deeply-inelastic heavy-ion collisions

Time scales and energy losses involved in tangential and radial friction as well as in viscuous and non-viscuous shape deformations are estimated. It is suggested that nuclear deformations are important only for reactions involving nearly complete loss of kinetic energy of relative motion.