347 research outputs found
The electroclinic effect and modulated phases in smectic liquid crystals
We explore the possibility that the large electroclinic effect observed in
ferroelectric liquid crystals arises from the presence of an ordered array of
disclination lines and walls. If the spacing of these defects is in the
subvisible range, this modulated phase would be similar macroscopically to a
smectic A phase. The application of an electric field distorts the array,
producing a large polarization, and hence a large electroclinic effect. We show
that with suitable elastic parameters and sufficiently large chirality, the
modulated phase is favored over the smectic A and helically twisted smectic C*
phases. We propose various experimental tests of this scenario.Comment: 9 pages, 7 figures; new version includes dipolar interactions and
bend-twist couplin
How to determine local elastic properties of lipid bilayer membranes from atomic-force-microscope measurements: A theoretical analysis
Measurements with an atomic force microscope (AFM) offer a direct way to
probe elastic properties of lipid bilayer membranes locally: provided the
underlying stress-strain relation is known, material parameters such as surface
tension or bending rigidity may be deduced. In a recent experiment a
pore-spanning membrane was poked with an AFM tip, yielding a linear behavior of
the force-indentation curves. A theoretical model for this case is presented
here which describes these curves in the framework of Helfrich theory. The
linear behavior of the measurements is reproduced if one neglects the influence
of adhesion between tip and membrane. Including it via an adhesion balance
changes the situation significantly: force-distance curves cease to be linear,
hysteresis and nonzero detachment forces can show up. The characteristics of
this rich scenario are discussed in detail in this article.Comment: 14 pages, 9 figures, REVTeX4 style. New version corresponds to the
one accepted by PRE. The result section is restructured: a comparison to
experimental findings is included; the discussion on the influence of
adhesion between AFM tip and membrane is extende
Elongation of confined ferrofluid droplets under applied fields
Ferrofluids are strongly paramagnetic liquids. We study the behavior of
ferrofluid droplets confined between two parallel plates with a weak applied
field parallel to the plates. The droplets elongate under the applied field to
reduce their demagnetizing energy and reach an equilibrium shape where the
magnetic forces balance against the surface tension. This elongation varies
logarithmically with aspect ratio of droplet thickness to its original radius,
in contrast to the behavior of unconfined droplets. Experimental studies of a
ferrofluid/water/surfactant emulsion confirm this prediction.Comment: 12 pages, 7 figures. Submitted to Phys. Rev.
Elastic deformation of a fluid membrane upon colloid binding
When a colloidal particle adheres to a fluid membrane, it induces elastic
deformations in the membrane which oppose its own binding. The structural and
energetic aspects of this balance are theoretically studied within the
framework of a Helfrich Hamiltonian. Based on the full nonlinear shape
equations for the membrane profile, a line of continuous binding transitions
and a second line of discontinuous envelopment transitions are found, which
meet at an unusual triple point. The regime of low tension is studied
analytically using a small gradient expansion, while in the limit of large
tension scaling arguments are derived which quantify the asymptotic behavior of
phase boundary, degree of wrapping, and energy barrier. The maturation of
animal viruses by budding is discussed as a biological example of such
colloid-membrane interaction events.Comment: 14 pages, 9 figures, REVTeX style, follow-up on cond-mat/021242
Field-Dependent Tilt and Birefringence of Electroclinic Liquid Crystals: Theory and Experiment
An unresolved issue in the theory of liquid crystals is the molecular basis
of the electroclinic effect in the smectic-A phase. Recent x-ray scattering
experiments suggest that, in a class of siloxane-containing liquid crystals, an
electric field changes a state of disordered molecular tilt in random
directions into a state of ordered tilt in one direction. To investigate this
issue, we measure the optical tilt and birefringence of these liquid crystals
as functions of field and temperature, and we develop a theory for the
distribution of molecular orientations under a field. Comparison of theory and
experiment confirms that these materials have a disordered distribution of
molecular tilt directions that is aligned by an electric field, giving a large
electroclinic effect. It also shows that the net dipole moment of a correlated
volume of molecules, a key parameter in the theory, scales as a power law near
the smectic-A--smectic-C transition.Comment: 18 pages, including 9 postscript figures, uses REVTeX 3.0 and
epsf.st
Theory of Chiral Modulations and Fluctuations in Smectic-A Liquid Crystals Under an Electric Field
Chiral liquid crystals often exhibit periodic modulations in the molecular
director; in particular, thin films of the smectic-C* phase show a chiral
striped texture. Here, we investigate whether similar chiral modulations can
occur in the induced molecular tilt of the smectic-A phase under an applied
electric field. Using both continuum elastic theory and lattice simulations, we
find that the state of uniform induced tilt can become unstable when the system
approaches the smectic-A--smectic-C* transition, or when a high electric field
is applied. Beyond that instability point, the system develops chiral stripes
in the tilt, which induce corresponding ripples in the smectic layers. The
modulation persists up to an upper critical electric field and then disappears.
Furthermore, even in the uniform state, the system shows chiral fluctuations,
including both incipient chiral stripes and localized chiral vortices. We
compare these predictions with observed chiral modulations and fluctuations in
smectic-A liquid crystals.Comment: 11 pages, including 9 postscript figures, uses REVTeX 3.0 and
epsf.st
Shear induced instabilities in layered liquids
Motivated by the experimentally observed shear-induced destabilization and
reorientation of smectic A like systems, we consider an extended formulation of
smectic A hydrodynamics. We include both, the smectic layering (via the layer
displacement u and the layer normal p) and the director n of the underlying
nematic order in our macroscopic hydrodynamic description and allow both
directions to differ in non equilibrium situations. In an homeotropically
aligned sample the nematic director does couple to an applied simple shear,
whereas the smectic layering stays unchanged. This difference leads to a finite
(but usually small) angle between n and p, which we find to be equivalent to an
effective dilatation of the layers. This effective dilatation leads, above a
certain threshold, to an undulation instability of the layers. We generalize
our earlier approach [Rheol. Acta, vol.39(3), 15] and include the cross
couplings with the velocity field and the order parameters for orientational
and positional order and show how the order parameters interact with the
undulation instability. We explore the influence of various material parameters
on the instability. Comparing our results to recent experiments and molecular
dynamic simulations, we find a good qualitative agreement.Comment: 15 pages, 12 figures, accepted for publication in PR
Studies on the heterogeneity of human serum Lp lipoproteins and on the occurrence of double Lp lipoprotein variants
Lp lipoproteins have been prepared by a mild method from the serum of a large number of individuals. Approximately 25% of the individuals tested showed the presence of a double Lp peak in analytical ultracentrifuge diagrams. These double peaks were designated Lp(a)-1 and Lp(a)-2 to distinguish them from the single Lp(a) peak. The mean viscosity-corrected sedimentation coefficient, S 1.004, 20 C and density of the single Lp(a) peak were 15.8±1.8 s ( n =32) and 1.076±0.01 g/ml, of the Lp(a)-1 peak were 13.5±1.1 s ( n =14) and 1.064±0.007 g/ml, and of the Lp(a)-2 peak were 16.8±1.7 s ( n =14) and 1.074±0.009 g/ml. Absorption tests using a double and single Lp preparation showed that both Lp peaks in the double variants possess Lp(a) specificity. Evidence is lacking as yet for individual specificities for either Lp(a)-1 or Lp(a)-2. Interand intra-individual heterogeneity among Lp lipoproteins is discussed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44186/1/10528_2004_Article_BF00485737.pd
Effect of the molecular structure of the polymer and nucleation on the optical properties of polypropylene homo- and copolymers.
Two soluble nucleating agents were used to modify the optical properties of nine PP homo- and random copolymers. The ethylene content of the polymers changed between 0 and 5.3 wt%. Chain regularity was characterized by the stepwise isothermal segregation technique (SIST), while optical properties by the measurement of the haze of injection molded samples. Crystallization and melting characteristics were determined by differential scanning calorimetry (DSC). The analysis of the results proved that lamella thickness and change in crystallinity influence haze only slightly. A model was introduced which describes quantitatively the dependence of nucleation efficiency and haze on the concentration of the nucleating agent. The model assumes that the same factors influence the peak temperature of crystallization and optical properties. The analysis of the results proved that the assumption is valid under the same crystallization conditions. The parameters of the model depend on the molecular architecture of the polymer. Chain regularity determines supermolecular structure and thus the dependence of optical properties on nucleation
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