39 research outputs found
Statistical mechanics of columnar DNA assemblies
Many physical systems can be mapped onto solved or "solvable" models of
magnetism. In this work, we have mapped the statistical mechanics of columnar
phases of ideally helical rigid DNA -- subject to the earlier found unusual,
frustrated pair potential [A. A. Kornyshev and S. Leikin, J. Chem. Phys. 107,
3656 (1997)] -- onto an exotic, unknown variant of the XY model on a fixed or
restructurable lattice. Here the role of the 'spin' is played by the azimuthal
orientation of the molecules. We have solved this model using a Hartree-Fock
approximation, ground state calculations, and finite temperature Monte Carlo
simulations. We have found peculiar spin order transitions, which may also be
accompanied by positional restructuring, from hexagonal to rhombohedric
lattices. Some of these have been experimentally observed in dense columnar
aggregates. Note that DNA columnar phases are of great interest in biophysical
research, not only because they are a useful in vitro tool for the study of DNA
condensation, but also since these structures have been detected in living
matter. Within the approximations made, our study provides insight into the
statistical mechanics of these systems.Comment: 19 pages, 18 figure
Melting of columnar hexagonal DNA liquid crystals
The persistence length DNA hexagonal-cholesteric phase transition upon dilution
and/or increase in solvent ionic strength is investigated with polarized light microscopy. The ionic
strength dependence of the transition follows Lindemann criterion , i.e., the
hexagonal lattice melts when the root-mean-square fluctuations in transverse order exceed 10%
of the interaxial spacing. The spacings are derived from density and the fluctuations are estimated
with a theory of undulation enhanced electrostatic interactions. Additional support for this theory
is given by the DNA equation of state and anisotropic neutron radiation scattering from
magnetically aligned cholesteric samples just below the phase transition
Liquid crystal phase transitions in suspensions of polydisperse plate-like particles
Colloidal suspensions that form periodic self-assembling structures
on sub-micrometre scales are of potential technological
interest; for example, three-dimensional arrangements of spheres
in colloidal crystals might serve as photonic materials, intended
to manipulate light. Colloidal particles with non-spherical shapes
(such as rods and plates) are of particular interest because of their
ability to form liquid crystals. Nematic liquid crystals possess
orientational order; smectic and columnar liquid crystals additionally
exhibit positional order (in one or two dimensions
respectively). However, such positional ordering may be inhibited
in polydisperse colloidal suspensions. Here we describe a
suspension of plate-like colloids that shows isotropic, nematic and
columnar phases on increasing the particle concentration.We find
that the columnar two-dimensional crystal persists for a polydispersity
of up to 25%, with a cross-over to smectic-like ordering
at very high particle concentrations. Our results imply that liquid
crystalline order in synthetic mesoscopic materials may be easier
to achieve than previously thought
Cobalt-chromium alloys in fixed prosthodontics in Sweden
Aim: The aim of this study was to compile the usage of Co-Cr alloys in fixed prosthodontics (FP) among dental laboratories in Sweden. Methods: From March to October 2015, questionnaires were sent to 542 registered dental laboratories in Sweden. The questionnaires were divided in two parts, one for fixed dental-supported prosthodontics (FDP) and one for fixed implant-supported prosthodontics (FIP). Reminders were sent three times. Results: In total of 542 dental laboratories, 55% answered the questionnaires. Most dental laboratories use Co-Cr in FP, 134 (74%) in FDP and 89(66%) in FIP. The laboratories used Co-Cr alloys of various compositions in the prostheses, 35 for FDP and 30 for FIP. The most commonly used Co-Cr alloys for tooth-supported FDPs were (a) Wirobond® 280, (b) Cara SLM and (c) Wirobond® C. For implant-supported frameworks the frequently used alloys were: (a) Cara SLM, (b) Cara Milled and (c) Wirobond® 280. Except for the difference in composition of these alloys, they were also manufactured with various techniques. In tooth-supported prostheses the dominating technique was the cast technique while newer techniques as laser-sintering and milling were more commonly reported for implant-supported constructions. A fourth technique; the ‘pre-state’ milling was reported in FDP. Conclusion: More than 30 different Co-Cr alloys were reported as being used in FP. Thus, there is a need for studies exploring the mechanical and physical behavior and the biological response to the most commonly used Co-Cr alloys
Effect of Spatial Inhomogeneity in Dielectric Permittivity on DNA Double Layer Formation
Effects of ionic strength on the supramolecular structure in liquid crystalline solutions of persistent length DNA fragments
Cobalt-chromium alloys fabricated with four different techniques : Ion release, toxicity of released elements and surface roughness
Objective. To investigate the metal ion release, surface roughness and cytoxicity for Co-Cr alloys produced by different manufacturing techniques before and after heat treatment. In addition, to evaluate if the combination of materials affects the ion release. Methods. Five Co-Cr alloys were included, based on four manufacturing techniques. Commercially pure titanium, CpTi grade 4 and a titanium alloy were included for comparison. The ion release tests involved both Inductive Coupled Plasma Optical Emission Spectrometry and Inductive Coupled Plasma Mass Spectrometry analyses. The surface analysis was conducted with optical interferometry. Cells were indirectly exposed to the materials and cell viability was evaluated with the MTT (3-(4.5-dimethylthiazol-2-yl)-2.5-diphenyltetrazolium bromide) method. Results. All alloys showed a decrease of the total ion release when CpTi grade 4 was present. The total ion release decreased over time for all specimens and the highest ion release was observed from the cast and milled Co-Cr alloy in acidic conditions. The cast and laser-melted Co-Cr alloy and the titanium alloy became rougher after heat treatment. All materials were within the limits of cell viability according to standards. Significance. The ion release from Co-Cr alloys is influenced by the combination of materials, pH and time. Surface roughness is influenced by heat treatment. Furthermore, both ion release and surface roughness are influenced by the manufacturing technique and the alloy type. The clinical implication needs to be further investigated. (C) 2020 The Academy of Dental Materials. Published by Elsevier Inc
High-resolution small-angle X-ray diffraction study of long-range order in hard-sphere colloidal crystals
The long-range order parameters in single crystals of hard colloidal spheres grown in sediments of colloid-polymer mixtures are determined using synchrotron small-angle x-ray diffraction with a resolution of 10(-6) of the wave vector. The interplanar positional order derived from the width of lattice reflections extends over at least 500 lattice planes. The lattice planes are orientationally correlated within approximately 0.1 degrees throughout the crystals, whereas the stacking of hexagonal planes remains random