486 research outputs found
Shape and symmetry determine two-dimensional melting transitions of hard regular polygons
The melting transition of two-dimensional (2D) systems is a fundamental
problem in condensed matter and statistical physics that has advanced
significantly through the application of computational resources and
algorithms. 2D systems present the opportunity for novel phases and phase
transition scenarios not observed in 3D systems, but these phases depend
sensitively on the system and thus predicting how any given 2D system will
behave remains a challenge. Here we report a comprehensive simulation study of
the phase behavior near the melting transition of all hard regular polygons
with vertices using massively parallel Monte Carlo simulations
of up to one million particles. By investigating this family of shapes, we show
that the melting transition depends upon both particle shape and symmetry
considerations, which together can predict which of three different melting
scenarios will occur for a given . We show that systems of polygons with as
few as seven edges behave like hard disks; they melt continuously from a solid
to a hexatic fluid and then undergo a first-order transition from the hexatic
phase to the fluid phase. We show that this behavior, which holds for all
, arises from weak entropic forces among the particles. Strong
directional entropic forces align polygons with fewer than seven edges and
impose local order in the fluid. These forces can enhance or suppress the
discontinuous character of the transition depending on whether the local order
in the fluid is compatible with the local order in the solid. As a result,
systems of triangles, squares, and hexagons exhibit a KTHNY-type continuous
transition between fluid and hexatic, tetratic, and hexatic phases,
respectively, and a continuous transition from the appropriate "x"-atic to the
solid. [abstract truncated due to arxiv length limitations]
Current status of IMRT in head and neck cancer
AbstractBackgroundIMRT provides highly conformal dose distributions creating non uniform spatial intensity using different segments in the beam.Material & Methods and ResultsDifferent retrospective studies have shown a high capability of IMRT to treat tumours close to the base of skull. Prospective studies have shown a decrease in xerostomia compared with conventional 3D conformal treatment (3DCRT). Modulation of intensity is performed by the movement of the multileaf collimator (MLC) that can deliver the radiation in different ways, such as static field segments, dynamic field segments and rotational delivery (arc therapy and tomotherapy). There are slight differences among the different techniques in terms of homogeneity, dose conformity and treatment delivery time.ConclusionsThe best method to deliver IMRT will depend on multiple factors such as deliverability, practicality, user training and plan quality
Pressure Driven Flow of Polymer Solutions in Nanoscale Slit Pores
Polymer solutions subject to pressure driven flow and in nanoscale slit pores
are systematically investigated using the dissipative particle dynamics
approach. We investigated the effect of molecular weight, polymer concentration
and flow rate on the profiles across the channel of the fluid and polymer
velocities, polymers density, and the three components of the polymers radius
of gyration. We found that the mean streaming fluid velocity decreases as the
polymer molecular weight or/and polymer concentration is increased, and that
the deviation of the velocity profile from the parabolic profile is accentuated
with increase in polymer molecular weight or concentration. We also found that
the distribution of polymers conformation is highly anisotropic and non-uniform
across the channel. The polymer density profile is also found to be
non-uniform, exhibiting a local minimum in the center-plane followed by two
symmetric peaks. We found a migration of the polymer chains either from or
towards the walls. For relatively long chains, as compared to the thickness of
the slit, a migration towards the walls is observed. However, for relatively
short chains, a migration away from the walls is observed.Comment: 11 pages, 13 figure
Strong scaling of general-purpose molecular dynamics simulations on GPUs
We describe a highly optimized implementation of MPI domain decomposition in
a GPU-enabled, general-purpose molecular dynamics code, HOOMD-blue (Anderson
and Glotzer, arXiv:1308.5587). Our approach is inspired by a traditional
CPU-based code, LAMMPS (Plimpton, J. Comp. Phys. 117, 1995), but is implemented
within a code that was designed for execution on GPUs from the start (Anderson
et al., J. Comp. Phys. 227, 2008). The software supports short-ranged pair
force and bond force fields and achieves optimal GPU performance using an
autotuning algorithm. We are able to demonstrate equivalent or superior scaling
on up to 3,375 GPUs in Lennard-Jones and dissipative particle dynamics (DPD)
simulations of up to 108 million particles. GPUDirect RDMA capabilities in
recent GPU generations provide better performance in full double precision
calculations. For a representative polymer physics application, HOOMD-blue 1.0
provides an effective GPU vs. CPU node speed-up of 12.5x.Comment: 30 pages, 14 figure
Enzymatic Cross-Linking of Alkali Extracted Arabinoxylans: Gel Rheological and Structural Characteristics
Ferulated arabinoxylans were alkali-extracted from wheat bran at different incubation times (0.0, 0.5, 1.0, 1.5 and 2.0 h). Wheat bran ferulated arabinoxylans (WBAX) arabinose-to-xylose ratio, ferulic acid content, intrinsic viscosity and viscosimetric molecular weight values decreased as the incubation time of extraction increased. WBAX enzymatic cross-linking capability was affected by incubation time while an increase in WBAX concentration from 5 to 6% (w/v) favored gelation. The WBAX gels formed presented a macroporous structure with mesh size ranging from 40 to 119 nm and hardness values varying from 1.7 to 5 N
Gelation of Arabinoxylans from Maize Wastewater — Effect of Alkaline Hydrolysis Conditions on the Gel Rheology and Microstructure
The purpose of this research was to extract arabinoxylans (AX) from maize wastewater generated under different maize nixtamalization conditions and to investigate the polysaccharide gelling capability, as well as the rheological and microstructural characteristics of the gels formed. The nixtamalization conditions were 1.5 hours of cooking and 24 hours of alkaline hydrolysis (AX1) or 30 minutes cooking and 4 hours of alkaline hydrolysis (AX2). AX1 and AX2 presented yield values of 0.9% and 0.5% (w/v), respectively. Both AX samples presented similar molecular identity (Fourier Transform Infra-Red) and molecular weight distribution but different ferulic acid (FA) content. AX1 and AX2 presented gelling capability under laccase exposure. The kinetics of gelation of both AX samples was rheologically monitored by small amplitude oscillatory shear. The gelation profiles followed a characteristic kinetics with an initial increase in the storage modulus (G\u27) and loss modulus (G") followed by a plateau region for both gels. AX1 presented higher G\u27 than AX2. In scanning electron microscopy (SEM) images, both gels present an irregular honeycomb microstructure. The lower FA content in AX2 form gels presenting minor elasticity values and a more fragmented microstructure. These results indicate that nixtamalization process conditions can modify the characteristics of AX gels
Facetted patchy particles through entropy-driven patterning of mixed ligand SAMS
We present a microscopic theory that describes the ordering of two distinct
ligands on the surface of a faceted nanoparticle. The theory predicts that when
one type of ligand is significantly bulkier than all others, the larger ligands
preferentially align themselves along the edges and vertices of the
nanoparticle. Monte Carlo simulations confirm these predictions. We show that
the intrinsic conformational entropy of the ligands stabilizes this novel
edge-aligned phase.Comment: 11 pages, 10 figure
Acemannan Gels and Aerogels
The procedures to obtain two types of acemannan (AC) physical gels and their respective aerogels are reported. The gelation was induced by the diffusion of an alkali or a non-solvent, then supercritical CO2 drying technology was used to remove the solvent out and generate the AC aerogels. Fourier-transform infrared spectroscopic analysis indicated that alkali diffusion produced extensive AC deacetylation. Conversely, the non-solvent treatment did not affect the chemical structure of AC. Both types of gels showed syneresis and the drying process induced further volume reduction. Both aerogels were mesoporous nanostructured materials with pore sizes up to 6.4 nm and specific surface areas over 370 m2/g. The AC physical gels and aerogels enable numerous possibilities of applications, joining the unique features of these materials with the functional and bioactive properties of the AC
Covalently Cross-Linked Nanoparticles Based on Ferulated Arabinoxylans Recovered from a Distiller’s Dried Grains Byproduct
The purpose of this investigation was to extract ferulated arabinoxylans (AX) from
dried distillers’ grains with solubles (DDGS) plus to investigate their capability to form covalently
cross-linked nanoparticles. AX registered 7.3 µg of ferulic acid/mg polysaccharide and molecular
weight and intrinsic viscosity of 661 kDa and 149 mL/g, correspondingly. Fourier transform infrared
spectroscopy (FTIR) was used to confirm the identity of this polysaccharide. AX formed laccase
induced covalent gels at 1% (w/v), which registered an elastic modulus of 224 Pa and a content
of FA dimers of 1.5 µg/mg polysaccharide. Scanning electron microscopy pictures of AX gels
exhibited a microstructure resembling a rough honeycomb. AX formed covalently cross-linked
nanoparticles (NAX) by coaxial electrospray. The average hydrodynamic diameter of NAX determined
by dynamic light scattering was 328 nm. NAX presented a spherical and regular shape by transmission
electron microscopy analysis. NAX may be an attractive material for pharmaceutical and biomedical
applications and an option in sustainable DDGS use
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