共查询到20条相似文献,搜索用时 46 毫秒
1.
V. Kumaran L. Srivatsan 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(2):259-266
The stability of the flow of a fluid past a solid membrane of infinitesimal thickness is investigated using a linear stability
analysis. The system consists of two fluids of thicknesses R and H R and bounded by rigid walls moving with velocities and , and separated by a membrane of infinitesimal thickness which is flat in the unperturbed state. The fluids are described
by the Navier-Stokes equations, while the constitutive equation for the membrane incorporates the surface tension, and the
effect of curvature elasticity is also examined for a membrane with no surface tension. The stability of the system depends
on the dimensionless strain rates and in the two fluids, which are defined as and for a membrane with surface tension , and and for a membrane with zero surface tension and curvature elasticity K. In the absence of fluid inertia, the perturbations are always stable. In the limit , the decay rate of the perturbations is O(k
3
) smaller than the frequency of the fluctuations. The effect of fluid inertia in this limit is incorporated using a small
wave number asymptotic analysis, and it is found that there is a correction of smaller than the leading order frequency due to inertial effects. This correction causes long wave fluctuations to be unstable
for certain values of the ratio of strain rates and ratio of thicknesses H. The stability of the system at finite Reynolds number was calculated using numerical techniques for the case where the strain
rate in one of the fluids is zero. The stability depends on the Reynolds number for the fluid with the non-zero strain rate,
and the parameter , where is the surface tension of the membrane. It is found that the Reynolds number for the transition from stable to unstable modes,
, first increases with , undergoes a turning point and a further increase in the results in a decrease in . This indicates that there are unstable perturbations only in a finite domain in the plane, and perturbations are always stable outside this domain.
Received: 29 May 1997 / Revised: 9 October 1997
/ Accepted: 26 November 1997 相似文献
2.
T. Ohta M. Nonomura 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(1):57-68
We study the domain morphology in a phase separated state of diblock copolymer-homopolymer mixtures. In the situation that
one of the blocks of copolymers is incompatible with homopolymers, formation of a bilayer membrane of block copolymers is
shown to be possible in the matrix of homopolymers. Starting with the free energy functional in terms of the local volume
fractions of each monomer, we derive the bending and curvature rigidities in the Helfrich free energy for a membrane. It is
found that the curvature modulus is negative only in a limited region of the parameters, where a closed shape of a membrane
like a vesicle is possibly formed. We establish a method to calculate the rigidities without a molecular picture in a consistent
way with the field theoretic model free energy. Stability of a bilayer membrane compared with micelles is also investigated.
Received: 22 July 1997 / Received in final form: 1 December 1997 / Accepted: 22 January 1998 相似文献
3.
Using a recently developed multiscale simulation methodology, we describe the equilibrium behaviour of bilayer membranes under the influence of curvature-inducing proteins using a linearized elastic free energy model. In particular, we describe how the cooperativity associated with a multitude of protein–membrane interactions and protein diffusion on a membrane-mediated energy landscape elicits emergent behaviour in the membrane phase. Based on our model simulations, we predict that, depending on the density of membrane-bound proteins and the degree to which a single protein molecule can induce intrinsic mean curvature in the membrane, a range of membrane phase behaviour can be observed including two different modes of vesicle-bud nucleation and repressed membrane undulations. A state diagram as a function of experimentally tunable parameters to classify the underlying states is proposed. 相似文献
4.
Tamotsu Kohyama 《Physica A》2009,388(17):3334-3344
We propose a simple mathematical model for lipid bilayer membranes of flexible fluid or solid state. The model consists of interacting coarse-grained particles with two extra variables. One indicates the spontaneous curvature at the particle position, and the other indicates the vector representing the direction normal to the membrane. When the spontaneous curvature variable is allowed to fluctuate significantly, the fluctuation causes a softening of the membrane and growth of large undulations as the amplitude of the fluctuation is increased. By changing the amplitude of the fluctuation in simulations, the bending rigidity of the membrane can be easily controlled. Because the proposed model includes anisotropic interactions between the particles, multilayered vesicles can be obtained through a reversible transition by weakening the strength of the anisotropic interactions. 相似文献
5.
Using a recently developed multiscale simulation methodology, we describe the equilibrium behaviour of bilayer membranes under the influence of curvature-inducing proteins using a linearized elastic free energy model. In particular, we describe how the cooperativity associated with a multitude of protein-membrane interactions and protein diffusion on a membrane-mediated energy landscape elicits emergent behaviour in the membrane phase. Based on our model simulations, we predict that, depending on the density of membrane-bound proteins and the degree to which a single protein molecule can induce intrinsic mean curvature in the membrane, a range of membrane phase behaviour can be observed including two different modes of vesicle-bud nucleation and repressed membrane undulations. A state diagram as a function of experimentally tunable parameters to classify the underlying states is proposed. 相似文献
6.
T. Charitat B. Fourcade 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):333-336
An o-ring takes spontaneously the shape of a chair when strong enough torsion is applied in its tangent plane. This state
is metastable, since work has to be done on the o-ring to return to the circular shape. We show that this metastable state
exists in a Hamiltonian where curvature and torsion are coupled via an intrinsic curvature term. If the o-ring is constrained to be planar (2d case), this metastable state displays a kink-anti-kink
pair. This state is metastable if the ratio is less than , where C and A are the torsion and the bending elastic constants [#!landau!#]. In three dimensions, our variational approach shows that
. This model can be generalized to the case where the bend is induced by a concentration field which follows the variations
of the curvature.
Received: 27 August 1997 / Revised: 23 October 1997 / Accepted: 12 November 1997 相似文献
7.
Biomembrane undulations are elementary excitations in the elastic surfaces of cells and vesicles. As such they can provide surprising insights into the mechanical processes that shape and stabilize biomembranes. We explain how naturally these undulations can be described by classical differential geometry. In particular, we apply the analytical formalism of differential-geometric calculus to the surfaces generated by a cell membrane and underlying cytoskeleton. After a short derivation of the energy due to a membrane's elasticity, we show how undulations arise as elementary excitations originating from the second derivative of an energy functional. Furthermore, we expound the efficiency of classical differential-geometric formalism to understand the effect of differential operators that characterize processes involved in membrane physics. As an introduction to concepts the paper is self-contained and rarely exceeds calculus level. 相似文献
8.
E. Akkermans J.-L. Pichard 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(2):223-227
The mobility of two interacting particles in a random potential is studied, using the sensitivity of their levels to a change
of boundary conditions. The delocalization in Hilbert space induced by the interaction of the two particle Fock states is
shown to decrease the mobility in metals and to increase it in insulators. In contrast to the single particle case, the spectral
rigidity is not directly related to the level curvature. Therefore, another curvature of topological origin is introduced,
which defines the energy scale below which the spectrum has the universal Wigner-Dyson rigidity.
Received: 26 September 1997 / Accepted: 30 October 1997 相似文献
9.
Effect of thermal undulations on the bending elasticity and spontaneous curvature of fluid membranes
We amplify previous arguments why mean curvature should be used as measure of integration in calculating the effective bending
rigidity of fluid membranes subjected to a weak background curvature. The stiffening of the membrane by its fluctuations,
recently derived for spherical shapes, is recovered for cylindrical curvature. Employing curvilinear coordinates, we then
discuss stiffening for arbitrary shapes, confirm that the elastic modulus of Gaussian curvature is not renormalized in the
presence of fluctuations, and show for the first time that any spontaneous curvature also remains unchanged.
Received 19 April 1999 and in Received in final form 7 January 2000 相似文献
10.
B. Bassetti C.A. Mantica P. Jona 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):359-369
A material surface of pure constituents with a flexible molecular chain (amphiphilics) is considered; thermodynamic behaviour
is studied in the chain length-temperature plane. The Hamiltonian of the system is modelled as the sum of a formation term
which refers to the polymer nature of the chain, and of a fluctuation term with a specific elastic form. For closed systems
the model exhibits phases with uniform curvature and conformational order/disorder or, alternatively, modulated phases; a
critical chain length is found for the existence of modulated phases; the dependence of transition temperature on energy parameters
is determined. A critical region is found for open systems, where conformational disorder drives spontaneous generation of
curvature; this lies above a characteristic chain length and around the shape transition temperature.
Received: 13 November 1996 / Revised: 9 May 1997 / Received in final form: 4
November 1997 / Accepted: 10 November 1997 相似文献
11.
We consider a bilayer membrane surrounded by small impurities, assumed to be attractive or repulsive. The purpose is a quantitative study of the effects of these impurities on the statistical properties of the supported membrane. Using the replica trick combined with a variational method, we compute the membrane mean-roughness and the height correlation function for almost-flat membranes, as functions of the primitive elastic constants of the membrane and some parameter that is proportional to the volume fraction of impurities and their interaction strength. As results, the attractive impurities increase the shape fluctuations due to the membrane undulations, while repulsive ones suppress these fluctuations. Second, we compute the equilibrium diameter of (spherical) vesicles surrounded by small random particles starting from the curvature equation. Third, the study is extended to a lamellar phase composed of two parallel fluid membranes, which are separated by a finite distance. This lamellar phase undergoes an unbinding transition. We demonstrate that the attractive impurities increase the unbinding critical temperature, while repulsive ones decrease this temperature. Finally, we say that the presence of small impurities in an aqueous medium may be a mechanism to suppress or to produce an unbinding transition, even the temperature and polarizability of the aqueous medium are fixed, in lamellar phases formed by parallel lipid bilayers. 相似文献
12.
Tilt of hydrocarbon chains of lipid molecules with respect to membrane plane is commonly considered to characterize the internal
structure of a membrane in the crystalline state. However, membranes in the liquid state can also exhibit tilt resulting from
packing constraints imposed on the lipid molecules in diverse biologically relevant structures such as intermediates of membrane
fusion, pores in lipid bilayers and others. We analyze the energetics of tilt in liquid membranes and its coupling with membrane
bending. We consider three contributions to the elastic energy: constant tilt, variation of tilt along the membrane surface
and membrane bending. The major assumption of the model is that the core of a liquid membrane has the common properties of
an elastic continuum. We show that the variation of tilt and membrane bending are additive and that their energy contributions
are determined by the same elastic coefficient: the Helfrich bending modulus, the modulus of Gaussian curvature and the spontaneous
curvature known from previous studies of pure bending. The energy of a combined deformation of bending and varying tilt is
determined by an effective tensor accounting for the two factors. In contrast, the deformation of constant tilt does not couple
with bending and its contribution to the elastic energy is determined by an independent elastic constant. While accurate determination
of this constant requires additional measurements, we estimate its value using a simplified approach. We discuss the relationships
between the obtained elastic Hamiltonian of a membrane and the previous models of membrane elasticity.
Received 10 February 2000 and Received in final form 19 June 2000 相似文献
13.
V. Kowalenko 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(2):161-168
The modes of a strongly magnetized charged Bose gas are presented for ultra-low temperatures. For longitudinal oscillations
propagating parallel to the magnetic field the dispersion relation is found to be dominated by the one-dimensional field-free
plasmon dispersion relation as found by Alexandrov, Beere and Kabanov recently in reference [1], while for propagation perpendicular
to the magnetic field they are found to be influenced by the cyclotron motion of the particles. Dispersion relations for these
modes known as Bernstein modes are given near the cyclotron frequency and its first two harmonics. The dispersion relations
for transverse modes in the system are then presented for the cases of photon propagation perpendicular and parallel to the
direction of the magnetic field.
Received: 3 July 1997 / Revised: 12 August 1997 / Accepted: 4 November 1997 相似文献
14.
We re-examine here the theoretical study of the phase separation between phospholipids and grafted long polymer chains onto
a fluid membrane. The polymer chains are assumed to be anchored to the membrane by one extremity (anchor). The anchors are big amphiphile lipid molecules. The anchors and phospholipids forming the bilayer phase separate under
the variation of a suitable parameter (temperature, pressure, membrane environment, ...). To investigate the demixtion transition,
we elaborate a new approach that takes into account the membrane undulations. We show that these undulations have the tendency
to induce additional attractive forces between anchors, and consequently, the separation transition is accentuated and occurs
at high temperature. Quantitatively, we show that the membrane undulations contribute with an extra positive segregation parameter
χm > 0 , which scales as χm
κ-2 , where κ is the bending rigidity constant. Therefore, the attraction phenomenon between species of the same kind is significant
only for those membranes of small bending rigidity constant. Finally, the study is extended to the case where the lengths
of the anchored polymer chains are randomly distributed. To achieve calculations, we choose a length distribution of fractal form. The essential conclusion is that the polydispersity increases the size of domains alternatively rich in phospholipids
and anchors. 相似文献
15.
C.M. Granzow A. Liebman G. Mahler 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,2(1):131-142
Continuous measurement models are conveniently based on master equations specified by the respective Hamiltonian and appropriate
environment operators. As demonstrated by stochastic unraveling, the latter specify the dynamical process rather than static
detection modes. We show that certain environment operators acting on a simple system may, in fact, require extended networks
for implementation: Their Hamilton parameters re-appear in the effective environment operators of the reduced model. The resulting
quantum trajectories typically involve competing paths, which may give rise to different fluctuation and noise properties
even when the corresponding ensemble behavior is practically the same.
Received: 21 July 1997 / Received in final form: 10 November 1997 / Accepted: 27 October 1997 相似文献
16.
The classical treatment of quasi-spherical vesicle undulations has, in the present work, been reviewed and extended to systems,
which are affected by a gravitational field caused by a density difference across the membrane. The effects have been studied
by the use of perturbation theory leading to corrections to the mean shape and the fluctuation correlation matrix. These corrections
have been included in an analytical expression for the flicker spectrum to probe how the experimentally accessible spectrum
changes with gravity. The results are represented in terms of the gravitational parameter, g
0 = ΔρgR
4/κ. The contributions from gravity are in most experimental situations small and thus negligible, but for values of g0 above a certain limit, the perturbational corrections must be included. Expressions for the relative error on the flicker
spectrum have been worked out, so that it is possible to define the regime where gravity is negligible. An upper limit of
g0 has also been identified, where the error in all modes of the flicker spectrum is significant due to distortion of the mean
shape.
Received 9 July 2002 and Received in final form 15 November 2002
RID="a"
ID="a"e-mail: jonas@kemi.dtu.dk
RID="b"
ID="b"e-mail: ipsen@memphys.sdu.dk 相似文献
17.
G. Barbero L.R. Evangelista N.V Madhusudana 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,1(3):327-331
We analyse the influence of adsorbed ions and the resulting surface electric field and its gradient on the anchoring properties
of nematics with ionic conductivity. We take into account two physical mechanisms for the coupling of the nematic director
with the surface electric field: (i) the dielectric anisotropy and (ii) the coupling of the quadrupolar component of the flexoelectric
coefficient with the field gradient. It is shown that for sufficiently large fields near saturated coverage of the adsorbed
ions, there can be a spontaneous curvature distortion in the cell even when the anchoring energy is infinitely strong. We
also discuss the director distortion when the anchoring energy of the surface is finite.
Received: 29 September 1997 / Received in final from: 10 November 1997 / Accepted: 18 November 1997 相似文献
18.
P. Snabre F. Magnifotcham 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,4(3):369-377
The continuous emission of gas bubbles from a single ejection orifice immersed in a viscous fluid is considered. We first
present a semi empirical model of spherical bubble growth under constant flow conditions to predict the bubble volume at the
detachment stage. In a second part, we propose a physical model to describe the rise velocity of in-line interacting bubbles
and we derive an expression for the net viscous force acting on the surrounding fluid. Experimental results for air/water-glycerol
systems are presented for a wide range of fluid viscosity and compared with theoretical predictions. An imagery technique was used to determine the bubble size and rise velocity.
The effects of fluid viscosity, gas flow rate, orifice diameter and liquid depth on the bubble stream dynamic were analyzed.
We have further studied the effect of large scale recirculation flow and the influence of a neighbouring bubble stream on
the bubble growth and rising velocity.
Received: 23 July 1997 / Revised: 16 December 1997 / Accepted: 11 May 1998 相似文献
19.
A. Banerjee M.K. Harbola 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》1998,1(3):265-269
Using density based perturbation theory [M.K. Harbola, A. Banerjee, Phys. Lett. A 222, 315 (1996)], we calculate the static hyperpolarizabilty for spherical atoms and ions from their ground-state densities. Since densities are being employed, calculations are performed
using approximate functionals for the kinetic and the exchange-correlation energies. Use of densities - instead of the wavefunctions
or Kohn-Sham orbitals - reduces the computational effort substantially. The results obtained are within of those calculated from the corresponding orbital-based calculations.
Received: 23 June 1997 / Revised: 25 September 1997 / Accepted: 24 December 1997 相似文献
20.
B. Sundarakkannan R. Kesavamoorthy 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,3(2):179-183
Raman scattering from oriented single crystals of BaFCl was recorded at various temperatures from 20 to 1073 K for the first
time. The Raman spectra, corrected for phonon population, were fitted to the sum of four Lorentzian peaks. The peak frequencies
and full width at half maximum (FWHM) of the peaks were obtained from the fit. The FWHM is accounted for by cubic and quartic
anharmonic processes. The quartic anharmonicity of the mode increases with the mode frequency. The quartic anharmonicity of
the fluorine mode is exceptionally high. The peak frequencies decrease linearly with the increasing temperature. Fluorine
mode frequencies decrease more than the internal mode frequencies do. The LO-TO splitting of the fluorine modes and that of
the internal modes increases with temperature indicating the increase of the ionic bonding character. The results are discussed.
Received: 12 May 1997 / Accepted: 25 July 1997 相似文献