396 research outputs found
Van der Waals interactions across stratified media
Working at the Lifshitz level, we investigate the van der Waals interactions
across a series of layers with a periodic motif. We derive the complete form of
the van der Waals interaction as an explicit function of the number of periodic
layers. We then compare our result with an approximation based on an
anisotropic-continuum representation of the stratified medium. Satisfactory
agreement between discrete-layer and continuum models is reached only for
thicknesses of ten or more layers.Comment: 9 pages and 4 figure
Positional, Reorientational and Bond Orientational Order in DNA Mesophases
We investigate the orientational order of transverse polarization vectors of
long, stiff polymer molecules and their coupling to bond orientational and
positional order in high density mesophases. Homogeneous ordering of transverse
polarization vector promotes distortions in the hexatic phase, whereas
inhomogeneous ordering precipitates crystalization of the 2D sections with
different orientations of the transverse polarization vector on each molecule
in the unit cell. We propose possible scenarios for going from the hexatic
phase, through the distorted hexatic phase to the crystalline phase with an
orthorhombic unit cell observed experimentally for the case of DNA.Comment: 4 pages, 2 figure
Osmotic properties of polyethyleneglycols: quantitative features of brush and bulk scaling laws
From glycosylated cell surfaces to sterically stabilized liposomes, polymers
attached to membranes attract biological and therapeutic interest. Can the
scaling laws of polymer "brushes" describe the physical properties of these
coats? We delineate conditions where the Alexander - de Gennes theory of
polymer brushes successfully describes the intermembrane distance vs. applied
osmotic stress data of Kenworthy et al. for PEG-grafted multilamellar liposomes
[Biophys. J. (1995) 68:1921]. We establish that the polymer density and size in
the brush must be high enough that, in a bulk solution of equivalent density,
the polymer osmotic pressure is independent of polymer molecular weight (the
des Cloizeaux semi-dilute regime of bulk polymer solutions). The condition that
attached polymers behave as semi-dilute bulk solutions offers a rigorous
criterion for brush scaling-law behavior. There is a deep connection between
the behaviors of polymer solutions in bulk and polymers grafted to a surface at
a density such that neighbors pack to form a uniform brush. In this regime,
two-parameter unconstrained fits of the Alexander - de Gennes brush scaling
laws yield effective monomer lengths of 3.3 to 3.5 AA, which agree with
structural predictions. The fitted distances between grafting sites are larger
than expected from the nominal content of PEG-lipids; the chains apparently
saturate the surface. Osmotic stress measurements can be used to estimate the
actual densities of membrane-grafted polymers.Comment: 26 pages with figure
Coupling between Smectic and Twist Modes in Polymer Intercalated Smectics
We analyse the elastic energy of an intercalated smectic where
orientationally ordered polymers with an average orientation varying from layer
to layer are intercalated between smectic planes. The lowest order terms in the
coupling between polymer director and smectic layer curvature are added to the
smectic elastic energy. Integration over the smectic degrees of freedom leaves
an effective polymer twist energy that has to be included into the total
polymer elastic energy leading to a fluctuational renormalization of the
intercalated polymer twist modulus. If the polymers are chiral this in its turn
leads to a renormalization of the cholesteric pitch.Comment: 8 pages, 1 fig in ps available from [email protected] Replaced
version also contains title and abstract in the main tex
Electrostatic interactions in the presence of surface charge regulation: exact results
We study the problem of charge regulation and its effects on electrostatic
interactions between dissociable charge groups immersed in a univalent
electrolyte, within a family of one dimensional exactly solvable models. We
consider the case of both charge regulated plates, but also the interaction of
pairs of finite size dielectric "particles". Using the transfer matrix
formalism we are able to determine the disjoining pressure as well as the
correlations between the charge and the dipole moments of the objects as a
function of their separation and electrolyte concentrationComment: 6 pages, 3 figure
Bond Orientational Order, Molecular Motion and Free Energy of High Density DNA Mesophases
By equilibrating condensed DNA arrays against reservoirs of known osmotic
stress and examining them with several structural probes, it has been possible
to achieve a detailed thermodynamic and structural characterization of the
change between two distinct regions on the liquid crystalline phase digram: a
higher-density hexagonally packed region with long-range bond orientational
order in the plane perpendicular to the average molecular direction; and a
lower-density cholesteric region with fluid-like positional order. X-rays
scattering on highly ordered DNA arrays at high density and with the helical
axis oriented parallel to the incoming beam showed a six-fold azimuthal
modulation of the first order diffraction peak that reflects the macroscopic
bond-orientational order. Transition to the less-dense cholesteric phase
through osmotically controlled swelling shows the loss of this bond
orientational order that had been expected from the change in optical
birefringence patterns and that is consistent with a rapid onset of molecular
positional disorder. This change in motion was previously inferred from
intermolecular force measurements and is now confirmed by NMR.
Controlled reversible swelling and compaction under osmotic stress, spanning a
range of densities between mg/ml to mg/ml, allows
measurement of the free energy changes throughout each phase and at the phase
transition, essential information for theories of liquid-crystalline states.Comment: 14 pages, 3 figures in gif format available at
http://abulafia.mgsl.dcrt.nih.gov/pics.html E-mail: [email protected]
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