共查询到10条相似文献,搜索用时 140 毫秒
1.
J.-F. Berret D.C. Roux P. Lindner 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):67-77
We have investigated the simple shear flow behavior of wormlike micelles using small-angle neutron scattering and mechanical
measurements. Ternary surfactant solutions made of cetylpyridinium chloride, hexanol and brine (0.2 M NaCl) and hereafter
abbreviated as CPCl-Hex were studied in the concentrated regime, . In a preliminary report (Berret et al. [#!ref16!#]), the discontinuity of slope observed in the shear stress versus shear rate curve was interpreted in terms of first-order phase transition between an isotropic state and a shear-induced
nematic state ( transition). At the transition rate, , the solution exhibits a macroscopic phase separation into viscous and fluid layers (inhomogeneous shear flow). Above a second
characteristic shear rate, the flow becomes homogeneous again, the sheared solution being nematic only. The neutron patterns
obtained in the two-state inhomogeneous region have been re-examined. Based on a consistent analysis of both orientational
and translational degrees of freedom related to the wormlike micelles, we emphasize new features for the transition. In the present paper, the shear rate variations of the relative proportions of each phase in the two-state region,
as well as the viscosity ratio between isotropic and nematic phases are derived. We demonstrate in addition that slightly
above the transition rate, the shear induced nematic phase is already strongly oriented, with an order parameter P
2
= 0.65. The orientational state is that of a nematic flow-oriented monodomain. Finally, from the locations of the neutron
scattering maxima for each isotropic and nematic contributions, we evaluate the concentrations for each phase and and derived a dynamical phase diagram of CPCl-Hex, in terms of the stress
versus
and . According to the classification by Schmitt et al. [#!ref22!#], the transition observed in CPCl-Hex micellar solutions could result from a positive flow-concentration coupling, in agreement
with the observed monotonically increasing shear stress in the two-phase region.
Received: 16 February 1998 / Revised: 18 February 1998 /
Accepted: 24 May 1998 相似文献
2.
R. Kemkemer D. Kling D. Kaufmann H. Gruler 《The European physical journal. E, Soft matter》2000,1(2-3):215-225
In culture migrating and interacting amoeboid cells can form nematoid arrangements in analogy to a nematic liquid crystal
phase. A nematoid arrangement is formed if the interaction has an apolar symmetry. Different cell types like human melanocytes
(= pigment cells of the skin), human fibroblasts (= connective tissue cells), human osteoblasts (= bone cells), human adipocytes
(= fat cells) etc., form a nematoid structure. Our hypothesis is that elastic properties of these nematoid structures can
be described in analogy to that of classical nematic liquid crystals. The orientational elastic energy is derived and the
orientational defects (disclination) of nematoid arrangements are investigated. The existence of half-numbered disclinations
shows that the nematoid structure has an apolar symmetry. The density- and order parameter dependence of the orientational
elastic constants and their absolute values are estimated. From the defect structure, one finds that the splay elastic constant
is smaller than the bend elastic constant (melanocytes). The core of a disclination is either a cell free space or occupied
by non oriented cells (isotropic state), by a cell with a different symmetry, or by another cell type.
Received 3 May 1999 and Received in final form 29 September 1999 相似文献
3.
V. Popa-Nita 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(1):83-90
We extend the random anisotropy nematic spin model to study nematic-isotropic transitions in porous media. A complete phase
diagram is obtained. In the limit of relative low randomness the existence of a triple point is predicted. For relatively
large randomness we have found a depression in temperature at the transition, together with a first order transition which
ends at a tricritical point, beyond which the transition becomes continuous. We use this model to investigate the motion of
the nematic-isotropic interface. We assume the system to be isothermal and initially quenched into the metastable régime of the isotropic phase. Using an appropriate form of the free energy density we obtain the domain wall solutions of the time-dependent
Ginzburg-Landau equation. We find that including a random field leads to smaller velocity of the interface and to larger interface
width.
Received 12 November 1998 and Received in final form 15 March 1999 相似文献
4.
I. Jánossy 《Pramana》2003,61(2):435-445
It is suggested that liquid crystal—polymer interfaces are coupled systems, in which the components mutually influence the
orientational state of each other. The photo-orientation process at liquid crystal-polymer interfaces provides a striking
example of such a coupling. Experiments show that the anisotropic structure generated by polarised light at a polymer surface
is strongly affected by the phase of the liquid crystal covering the polymer. Photo-orientation is significantly more efficient
when the liquid crystal is in the isotropic phase than when it exhibits orientational order. The observations are interpreted
by assuming that in the smectic and nematic phases the liquid crystal stabilises to a large extent polymer chain-segments
aligned parallel to the director, while it blocks the photo-induced formation of chain-segments in the perpendicular direction.
Other situations, in which the coupling between the liquid crystal and the polymer can be important, are also discussed briefly. 相似文献
5.
V. Fourmaux-Demange A. Brûlet F. Boué P. Davidson P. Keller J.P. Cotton 《The European physical journal. E, Soft matter》2000,1(4):301-317
We have studied the rheology and the conformation of stretched comb-like liquid-crystalline polymers. Both the influence of
the comb-like structure and the specific effect of the nematic interaction on the dynamics are investigated. For this purpose,
two isomers of a comb-like polymetacrylate polymer, of well-defined molecular weights, were synthesized: one displays a nematic
phase over a wide range of temperature, the other one has only an isotropic phase. Even with high degrees of polymerization
N, between 40 and 1000, the polymer chains studied were not entangled. The stress-strain curves during the stretching and relaxation
processes show differences between the isotropic and nematic comb-like polymers. They suggest that, in the nematic phase,
the chain dynamics is more cooperative than for a usual linear polymer. Small-angle neutron scattering has been used in order
to determine the evolution of the chain conformation after stretching, as a function of the duration of relaxation t
r. The conformation can be described with two parameters only: , the global deformation of the polymer chain, and p, the number of statistical units of locally relaxed sub-chains. For the comb-like polymer, the chain deformation is pseudo-affine:
is always smaller than (the deformation ratio of the whole sample). In the isotropic phase, has a constant value, while pincreases as tr. This latter behavior is not that expected for non-entangled chains, in which p varies as t
r
1/2 (Rouse model). In the nematic phase, decreases as a stretched exponential function of t
r, while p remains constant. The dynamics of the comb-like polymers is discussed in terms of living clusters from which junctions are
produced by interactions between side chains. The nematic interaction increases the lifetime of these junctions and, strikingly,
the relaxation is the same at all scales of the whole polymer chain.
Received 5 May 1999 and Received in final form 18 October 1999 相似文献
6.
Y.O. Popov A.N. Semenov 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(2):245-256
Nematic ordering in anisotropic non-Gaussian elastomers is considered theoretically using mean field approximation. We focus on the effect of anisotropy during network
cross-linking on the system elasticity and, in particular, on the so-called soft deformation mode. As the main result, we
calculate the dependence of the elastomer free energy on the angle between the axis of “frozen” anisotropy and the nematic
director. The dependence of the isotropic-nematic transition point on the orientational field acting on the monomers during
the cross-linking process is also calculated.
Received: 5 November 1997 / Revised and Accepted: 29 June 1998 相似文献
7.
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
8.
A. Milchev J.P. Wittmer D.P. Landau 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(2):241-251
We use an off-lattice microscopic model for solutions of equilibrium polymers (EP) in a lamellar shear flow generated by means
of a self-consistent external field between parallel hard walls. The individual conformations of the chains are found to elongate
in flow direction and shrink perpendicular to it while the average polymer length decreases with increasing shear rate. The
Molecular Weight Distribution of the chain lengths retains largely its exponential form in dense solutions whereas in dilute
solutions it changes from a power-exponential Schwartz distribution to a purely exponential one upon an increase of the shear
rate. With growing shear rate the system becomes increasingly inhomogeneous so that a characteristic variation of the total
monomer density, the diffusion coefficient, and the center-of-mass distribution of polymer chains of different contour length
with the velocity of flow is observed. At higher temperature, as the average chain length decreases significantly, the system is shown to undergo an order-disorder transition
into a state of nematic liquid crystalline order with an easy direction parallel to the hard walls. The influence of shear
flow on this state is briefly examined.
Received 22 October 1998 and Received in final form 12 April 1999 相似文献
9.
Refractive index and density measurements have been carried out on three nO.m liquid crystalline compounds, namely, 4O.2, 4O.3 and 1O.10 belonging to the N-(p-n-alkoxybenzylidene)-p-n-alkylaniline series. From the data, the orientational order parameter has been estimated using Vuks and Neugebauer local field models. Furthermore, the orientational order parameter has been calculated directly from refractive index data employing the Vuks scaling factor method, Neugebauer f(B) parameter, effective geometry parameter and a method proposed by Kuczyński et al. It is observed that order parameter values estimated from different methods agree well near the nematic–isotropic transition and diverge as the nematic phase attains equilibrium. The temperature gradient of refractive indices and the nematic crossover temperatures have also been estimated for these compounds. The results obtained are compared and discussed. 相似文献
10.
Depletion interaction in a suspension of rodlike colloids with added non-adsorbing polymer coils is theoretically studied.
We calculate an overlap volume of depletion zone between two rodlike colloids, based on the second virial approximation. We
examine nematic-isotropic phase transition (NIT) and two-phase coexistence between an isotropic and a nematic phase at low
polymer concentrations. We find that the depletion interaction is dependent on the orientational order parameter of rodlike
colloids and leads to a decrease in the NIT concentration on the addition of polymer. The coexistence curves have a leaning-chimney
shape and are shifted to lower rod concentrations on increasing the polymer concentration.
Received 23 May 2001 and Received in final form 18 July 2001 相似文献