共查询到10条相似文献,搜索用时 171 毫秒
1.
We study the director field around a spherical particle immersed in a uniformly aligned nematic liquid crystal and assume
that the molecules prefer a homeotropic orientation at the surface of the particle. Three structures are possible: a dipole,
a Saturn-ring, and a surface-ring configuration, which we investigate by numerically minimizing the Frank free energy supplemented
by a magnetic-field and a surface term. In the dipole configuration, which is the absolutely stable structure for micron-size
particles and sufficiently strong surface anchoring, a twist transition is found and analyzed. We show that a transition from
the dipole to the Saturn ring configuration is induced by either decreasing the particle size or by applying a magnetic field.
The effect of metastability and the occurrence of hysteresis in connection with a magnetic field are discussed. The surface-ring
configuration appears when the surface-anchoring strength W is reduced. It is also favored by a large saddle-splay constant K24. A comparison with recent experiments [#!itapdb:Poulin1997!#,#!itapdb:Poulin1998!#] gives a lower bound for W, i.e., for the interface of water and pentylcyanobiphenyl (5CB) in the presence of the surfactant sodium dodecyl sulfate.
Received 2 November 1998 相似文献
2.
B. O'Shaughnessy D. Vavylonis 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(3):363-372
We study the reaction kinetics of end-functionalized polymer chains dispersed in an unreactive polymer melt. Starting from
an infinite hierarchy of coupled equations for many-chain correlation functions, a closed equation is derived for the 2nd
order rate constant k after postulating simple physical bounds. Our results generalize previous 2-chain treatments (valid in dilute reactants limit)
by Doi [#!doi:inter2!#], de Gennes [#!gennes:polreactionsiandii!#], and Friedman and O'Shaughnessy [#!ben:interdil_all_aip!#],
to arbitrary initial reactive group density n0 and local chemical reactivity Q. Simple mean field (MF) kinetics apply at short times, .For high Q, a transition occurs to diffusion-controlled (DC) kinetics with (where xt is rms monomer displacement in time t) leading to a density decay . If n0 exceeds the chain overlap threshold, this behavior is followed by a regime where during which k has the same power law dependence in time, , but possibly different numerical coefficient. For unentangled melts this gives while for entangled cases one or more of the successive regimes ,t
-3/8
and t
-3/4
may be realized depending on the magnitudes of Q and n0. Kinetics at times longer than the longest polymer relaxation time are always MF. If a DC regime has developed before then the long time rate constant is where R is the coil radius. We propose measuring the above kinetics in a model experiment where radical end groups are generated
by photolysis.
Received: 2 June 1998 / Revised: 9 July 1998 / Accepted: 10 July 1998 相似文献
3.
M. Suszynska L. Krajczyk R. Capelletti 《Applied Physics A: Materials Science & Processing》1999,68(4):471-473
2 O-rich droplets dispersed throughout the SiO2-rich matrix and nanoparticles of metallic silver. In exchanged specimens the joint effect of colloidal silver and electric
field results in partial transformation of the amorphous droplets into crystalline Na2O particles.
Received: 11 February 1998 / Accepted: 20 November 1998 / Published online: 24 February 1999 相似文献
4.
5.
B. Yahiaoui A. Gharbi M. Kléman J. Malthête 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(1):99-110
Electron microscopy observations of replicas of freeze-fractured samples of two columnar hexagonal phases of different nature (a lyotropic one, the inverse AOT in water; a thermotropic one, ) yield very different results: most defects at microscopic scales are screw dislocations in the lyotropic phase, longitudinal
edge dislocations in the thermotropic phase. A possible way to interpret these differences is as follows: in the lyotropic
the Lamé coefficients and μ and the bend modulus K3 would not display any anomaly compared to expected values; in the thermotropic the shear modulus μ would be ten times smaller
than the compressibility modulus , while K3 would still be comparable to (but larger than) the bend modulus of a small molecules liquid crystal. We present an elementary
theoretical model of the latter case which could explain the anomalous measurements of K3 and of the longitudinal compressibility (Ref. [#!ref10!#]) without contradicting more recent measurements of (Refs. [#!ref17!#,#!ref22!#]). Essentially, the hexagonal phase would be a phase with defects (longitudinal dislocations) akin to an hexatic phase but with some differences.
Re?u : 26 mai 1997 / Révisé : 20 Janvier 1998 /
Accepté : 27 avril 1998 相似文献
6.
J. L. García-Palacios 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(2):293-308
Given two strings X and Y of N and M characters respectively, the Longest Common Subsequence (LCS) Problem asks for the longest sequence of (non-contiguous) matches
between X and Y. Using extensive Monte-Carlo simulations for this problem, we find a finite size scaling law of the form for the average LCS length of two random strings of size N over S letters. We provide precise estimates of for .We consider also a related Bernoulli Matching model where the different entries of an array are occupied with a match independently with probability 1/S. On the basis of a cavity-like analysis we find that the length of a longest sequence of matches in that case behaves as
where r=M/N and . This formula agrees very well with our numerical computations. It provides a very good approximation for the Random String
model, the approximation getting more accurate as S increases. The question of the “universality class” of the LCS problem is also considered. Our results for the Bernoulli
Matching model show very good agreement with the scaling predictions of [#!HwaLassig96_PRL!#] for Needleman-Wunsch sequence
alignment. We find however that the variance of the LCS length has a scaling different from Var in the Random String model, suggesting that long-ranged correlations among the matches are relevant in this model. We finally
study the “ground state” properties of this problem. We find that the number of solutions typically grows exponentially with N. In other words, this system does not satisfy “Nernst's principle”. This is also reflected at the level of the overlap between
two LCSs chosen at random, which is found to be self averaging and to approach a definite value q
S
<1 as .
Received: 23 April 1998 / Revised: 30 July 1998 / Accepted: 14 August 1998 相似文献
7.
M. Mosbacher H.-J. Münzer J. Zimmermann J. Solis J. Boneberg P. Leiderer 《Applied Physics A: Materials Science & Processing》2001,72(1):41-44
We report on the role of local optical field enhancement in the neighborhood of particles during dry laser cleaning (DLC)
of silicon wafer surfaces. Samples covered with spherical colloidal particles (PS, SiO2) and arbitrarily shaped Al2O3 particles with diameters from 320–1700 nm were cleaned using laser pulses with durations from 150 fs to 6.5 ns and wavelengths
ranging from 400–800 nm. Cleaned areas were investigated with scanning electron and atomic force microscopy. Holes in the
substrate with diameters of 200–400 nm and depths of 10–80 nm, depending on the irradiation conditions, were found at the
former positions of the particles. For all pulse durations analyzed (fs, ps, ns), holes are created at laser fluences as small
as the threshold fluence. Calculations of the optical field intensities in the particles’ neighbourhood by applying Mie theory
suggest that enhancement of the incident laser intensity in the near field of the particles is responsible for these effects.
DLC for sub-ns pulses seems to be governed by the local ablation of the substrate rather than by surface acceleration.
Received: 31 May 2000 / Accepted: 7 September 2000 / Published online: 22 November 2000 相似文献
8.
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
9.
A. Trovato J. van Mourik A. Maritan 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):63-73
A lattice model of a hetero-polymer with random hydrophilic-hydrophobic charges interacting with the solvent is introduced,
whose continuum counterpart has been proposed by Garel, Leibler and Orland [#!GLO!#]. The transfer matrix technique is used
to study various constrained annealed systems which approximate at various degrees of accuracy the original quenched model.
For highly hydrophobic chains an ordinary -point transition is found from a high temperature swollen phase to a low temperature compact phase. Depending on the type
of constrained averages, at very low temperatures a swollen phase or a coexistence between compact and swollen phases are
found. The results are carefully compared with the corresponding ones obtained in the continuum limit, and various improvements
in the original calculations are discussed.
Received: 10 April 1998 / Revised: 4 June 1998 / Accepted: 1st July 1998 相似文献
10.
A. Volmer S. Mukherji T. Nattermann 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):627-645
In anisotropic or layered superconductors thermal fluctuations as well as impurities induce a van der Waals (vdW) attraction
between flux lines, as has recently been shown by Blatter and Geshkenbein in the thermal case [#!BlatterGeshkenbein!#] and
by Mukherji and Nattermann in the disorder dominated case [#!NattermannMukherji!#]. This attraction together with the entropic
or disorder induced repulsion has interesting consequences for the low field phase diagram. We present two derivations of
the vdW attraction, one of which is based on an intuitive picture, the other one following from a systematic expansion of
the free energy of two interacting flux lines. Both the thermal and the disorder dominated case are considered. In the thermal
case in the absence of disorder, we use scaling arguments as well as a functional renormalization of the vortex-vortex interaction
energy to calculate the effective Gibbs free energy on the scale of the mean flux line distance. We discuss the resulting
low field phase diagram and make quantitative predictions for pure BiSCCO (Bi2Sr2CaCu2O8). In the case with impurities, the Gibbs free energy is calculated on the basis of scaling arguments, allowing for a semi-quantitative
discussion of the low-field, low-temperature phase diagram in the presence of impurities.
Received: 9 February 1998 / Accepted: 17 April 1998 相似文献