共查询到10条相似文献,搜索用时 140 毫秒
1.
H. Stark J. Stelzer R. Bernhard 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(3):515-523
Recently, it was observed that water droplets suspended in a nematic liquid crystal form linear chains [Poulin et al., Science 275, 1770 (1997)]. The chaining occurs, e.g., in a large nematic drop with homeotropic boundary conditions at all the surfaces. Between each pair of water droplets a
point defect in the liquid crystalline order was found in accordance with topological constraints. This point defect causes
a repulsion between the water droplets. In our numerical investigation we limit ourselves to a chain of two droplets. For
such a complex geometry we use the method of finite elements to minimize the Frank free energy. We confirm an experimental
observation that the distance d of the point defect from the surface of a water droplet scales with the radius r of the droplet like .When the water droplets are moved apart, we find that the point defect does not stay in the middle between the droplets,
but rather forms a dipole with one of them. This confirms a theoretical model for the chaining. Analogies to a second order
phase transition are drawn. We also find the dipole when one water droplet is suspended in a bipolar nematic drop with two
boojums, i.e., surface defects at the outer boundary. Finally, we present a configuration where two droplets repel each other without a
defect between them.
Received 11 December 1998 相似文献
2.
The Frank elasticity constants which describe splay (K
1), twist (K
2), and bend (K
3) distortion modes are investigated for 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic liquid crystal. The calculations
rest on statistical-mechanical approaches where the absolute values of K
i
(i=1,2,3) are dependent on the direct correlation function (DCF) of the corresponding nematic state. The DCF was determined
using the pair correlation function by solving the Ornstein-Zernike equation. The pair correlation function, in turn, was
obtained from molecular dynamics (MD) trajectory. Three different approaches for calculations of the elasticity constants
were employed based on different level of approximation about the orientational order and molecular correlations. The best
agreement with experimental values of elasticity constants was obtained in a model where the full orientational distribution
function was used. In addition we have investigated the approximation about spherical distribution of the intermolecular vectors
in the nematic phase, often used in derivation of various mean-field theories and employed here for the construction of the
DCF. We found that this assumption is not strictly valid, in particular a strong deviation from the isotropic distribution
is observed for short intermolecular distances.
Received 22 March 2000 and Received in final form 9 June 2000 相似文献
3.
The growth kinetics of self-assembled monolayers (SAMs) of thiophene compounds on Au(111) surfaces was revealed by Fourier-transform
infrared reflection absorption spectroscopy (FT-IR-RAS). Thiophene and terthiophene form well-ordered SAMs on Au(111) surfaces
by immersing gold substrates into their ethanol solutions for ca. 15 h. Gibbs free energies for the adsorption processes of thiophene and terthiophene were found to be identical. However,
the growth and molecular orientation of SAMs are different between two thiophene compounds. Terthiophene in SAMs orients parallel
to the surface. The SAM growth of terthiophene obeys a time-dependent Langmuir scheme. On the other hand, the thiophene SAM undergoes a two-step growth process with unique molecular orientations.
In the primary phase, thiophene assumes a parallel orientation on the Au(111) surface. In the second phase, thiophene is oriented
close to the normal of the surface. The different growth process between thiophene and terthiophene is attributable to the
topology of sulfur positions in the molecules.
Received 23 May 2001 and Received in final form 11 February 2002 相似文献
4.
L.-H. Tang P. Smilauer D.D. Vvedensky 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(3):409-412
Two types of mechanisms are proposed for mound coarsening during unstable epitaxial growth: stochastic, due to deposition
noise, and deterministic, due to mass currents driven by surface energy differences. Both yield the relation H=(RWL)2 between the typical mound height W, mound size L, and the film thickness H. An analysis of simulations and experimental data shows that the parameter R saturates to a value which discriminates sharply between stochastic () and deterministic () coarsening. We derive a scaling relation between the coarsening exponent 1/z and the mound-height exponent which, for a saturated mound slope, yields .
Received: 11 November 1997 / Revised in final form: 28 November 1997 /
Accepted: 28 November 1997 相似文献
5.
Free energy of semiflexible polymers and structure of interfaces 总被引:1,自引:0,他引:1
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 Sij 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 相似文献
6.
Donley JP 《The European physical journal. E, Soft matter》2005,16(3):273-282
Density functional techniques are used to derive a charging expression for the non-uniform density of a molecular liquid. In the atomic limit the equation reduces to an exact form due to Fixman. The theory is simplified greatly via a physical approximation that accounts for three-body correlations beyond those included in the hypernetted chain (HNC) closure of the Ornstein-Zernike (OZ) equation. The radial distribution function is obtained as a special case. The theory is tested by examining the phase behavior of two fundamental complex fluids: the homopolymer blend and diblock copolymer melts. For the former it is found, contrary to HNC theory and its molecular generalizations, that a critical temperature Tc is predicted from the structure route. This Tc scales linearly with degree of polymerization N in agreement with Flory theory. The simplest form of the theory can be considered as a way to incorporate attractive interactions within a formalism that is very similar to that of the OZ or reference interaction site model (RISM). The relevance of the theory to charged liquids is also discussed. 相似文献
7.
T. Emig T. Nattermann 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,8(4):525-546
The simultaneous effect of both disorder and crystal-lattice pinning on the equilibrium behavior of oriented elastic objects
is studied using scaling arguments and a functional renormalization group technique. Our analysis applies to elastic manifolds,
e.g., interfaces, as well as to periodic elastic media, e.g., charge-density waves or flux-line lattices. The competition between both pinning mechanisms leads to a continuous, disorder
driven roughening transition between a flat state where the mean relative displacement saturates on large scales and a rough
state with diverging relative displacement. The transition can be approached by changing the impurity concentration or, indirectly,
by tuning the temperature since the pinning strengths of the random and crystal potential have in general a different temperature
dependence. For D dimensional elastic manifolds interacting with either random-field or random-bond disorder a transition exists for 2<D<4, and the critical exponents are obtained to lowest order in . At the transition, the manifolds show a superuniversal logarithmic roughness. Dipolar interactions render lattice effects
relevant also in the physical case of D=2. For periodic elastic media, a roughening transition exists only if the ratio p of the periodicities of the medium and the crystal lattice exceeds the critical value . For p<p
c
the medium is always flat. Critical exponents are calculated in a double expansion in and and fulfill the scaling relations of random field models.
Received 28 August 1998 相似文献
8.
Yiwu Duan J.-M. Yuan 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1999,6(3):319-326
Based on our previous work [Yiwu Duan, J.M. Yuan, C.G. Bao, Phys. Rev. A 52, 3497 (1995)], we study deeply the periodic orbits of the hydrogen molecular ion within the Born-Oppenheimer approximation
(BOA). The Thiele-Burrau's transformation is introduced to regularize the singularities associated with the Coulomb potential
terms and to transform the problem into a direct product of a pendulum and an anharmonic oscillator. This facilitates the
analysis of the bifurcation properties of the periodic orbits. Some more details are also given about the calculation of the
semiclassical density-of-state distribution using the Berry-Tabor formula.
Received: 5 February 1999 相似文献
9.
The molecular dynamics (MD) simulation, based on a realistic atom-atom interaction potential, was performed on 4-n-pentyl-4'-cyanobiphenyl (5CB) in the nematic phase. The rotational viscosity coefficients (RVCs) γ
i, (i = 1, 2) and the ratio of the RVCs λ = - γ
2/γ
1 were investigated. Furthermore, static and frequency-dependent dielectric constants and ε were calculated using parameters obtained from the MD simulation. Time correlation functions were computed and used to determine
the rotational diffusion coefficient, D
⊥. The RVCs and λ were evaluated using the existing statistical-mechanical approach (SMA), based on a rotational diffusion
model. The SMA rests on a model in which it is assumed that the reorientation of an individual molecule is a stochastic Brownian
motion in a certain potential of mean torque. According to the SMA, γ
i are dependent on the orientational order and rotational diffusion coefficients. The former was characterized using: i) orientational
distribution function (ODF), and ii) a set of order parameters, both derived from analyses of the MD trajectory. A reasonable
agreement between the calculated and experimental values of γ
i and λ was obtained.
Received 22 March 2000 and Received in final form 8 October 2000 相似文献
10.
N.J. Mottram S.J. Elston 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):277-284
We have theoretically investigated chevron formation in smectic C materials and the transformation of this chevron structure to a tilted layer structure as the cell is sheared. We find a
series of transition temperatures at which the behaviour of the cell critically changes. As the cell is cooled from the smectic
A phase past the first critical temperature there is a second order transition which forms two tilted layer states with lower energy than the smectic A bookshelf structure. Although these low energy tilted structures exist the bookshelf structure is the stable state for zero
shear. However, upon further cooling this bookshelf structure becomes unstable to the formation of a chevron state. Now when
the cell is sheared the chevron structure smoothly transforms into the tilted layer structure. As each further critical temperature
is passed an additional multiple chevron solution is formed which although a high energy, unstable state may be observed transiently. For sufficiently low
temperatures the transition from chevron to tilted layer becomes first order. This first order transition occurs as the chevron
interface merges with the surface alignment region to form the tilted layer structure.
Received 28 December 1998 and Received in final form 8 April 1999 相似文献