Homogeneous ice nucleation from supercooled water was studied in the temperature range of 220-240 K through combining the forward flux sampling method (Allen et al., J. Chem. Phys., 2006, 124, 024102) with molecular dynamics simulations (FFS/MD), based on a recently developed coarse-grained water model (mW) (Molinero et al., J. Phys. Chem. B, 2009, 113, 4008). The calculated ice nucleation rates display a strong temperature dependence, ranging from 2.148 ± 0.635 × 10(25) m(-3) s(-1) at 220 K to 1.672 ± 0.970 × 10(-7) m(-3) s(-1) at 240 K. These rates can be fitted according to the classical nucleation theory, yielding an estimate of the effective ice-water interface energy γ(ls) of 31.01 ± 0.21 mJ m(-2) for the mW water model. Compared to experiments, our calculation underestimates the homogeneous ice nucleation rate by a few orders of magnitude. Possible reasons for the discrepancy are discussed. The nucleating ice embryo contains both cubic ice Ic and hexagonal ice Ih, with the fraction of each structure being roughly 50% when the critical size is reached. In particular, a novel defect structure containing nearly five-fold twin boundaries is identified in the ice clusters formed during nucleation. The way such defect structure is formed is found to be different from mechanisms proposed for the formation of the same defect in metallic nanoparticles and thin film. The quasi five-fold twin boundary structure found here is expected to occur in the crystallization of a wide range of materials with the diamond cubic structure, including ice.

Radiochromic film has become an important tool to verify dose distributions in highly conformal radiation therapy such as IMRT. Recently, a new generation of these films, EBT3, has become available. EBT3 has the same composition and thickness of the sensitive layer of the previous EBT2 films, but its symmetric layer configuration allows the user to eliminate side orientation dependence, which is reported for EBT2 films. The most important EBT3 characteristics have been investigated, such as response at high‐dose levels, sensitivity to scanner orientation and postirradiation coloration, energy and dose rate dependence, and orientation dependence with respect to film side. Additionally, different IMRT fields were measured with both EBT3 and EBT2 films and evaluated using gamma index analysis. The results obtained show that most of the characteristics of EBT3 film are similar to the EBT2 film, but the orientation dependence with respect to film side is completely eliminated in EBT3 films. The study confirms that EBT3 film can be used for clinical practice in the same way as the previous EBT2 film.PACS number: 87.56.Fc

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This work focuses on the possibility of improving performance properties of polyamide-layered\ud silicate nanocomposite films for packaging applications by using, as alternative matrix, a statistical copolymer of\ud the nylon-6 having a partially aromatic structure. Nanocomposites at different silicate loadings (commercial organomodified\ud montmorillonite) were produced by cast-film extrusion using three polyamide matrices: nylon-6 and\ud two copolyamides with similar chemical structure but different molecular weights. Oxygen barrier and mechanical\ud properties of the produced films were investigated and correlated to their nanostructure through analytical techniques\ud sensitive to different aspects of the same morphology, such as TEM, rheological, and dynamic-mechanical analyses.\ud Permeability data were interpolated on the basis of different theoretical approaches, from which a quantitative\ud indication on exfoliation and orientation levels of silicate layers in the matrices was obtained. TEM images at\ud different magnifications were used to empirically evaluate the average length-thickness ratio of silicate platelets\ud and to verify the correspondence with aspect ratios and order parameters calculated by data fitting. A strong\ud correlation between nanomorphology and properties was observed in the different nanocomposite systems and\ud all the three matrices exhibited performance improvement with increasing the silicate content. Nanocomposite\ud films based on the copolyamide with higher molecular weight displayed more exfoliated regions and a preferential\ud orientation of silicate layers, leading to the most significant oxygen barrier improvements and the best mechanical\ud propertie

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An s-polarized laser beam that impinges at small incidence angle on a homeotropically aligned nematic liquid crystal produces very interesting nonlinear phenomena. In this paper, we show that, due to the symmetry of the system, a cascade of successive homoclinic gluing bifurcations is responsible for the transition towards a stochastic regime in the experiment. We compare the experimental results with a model describing a sequence of gluing bifurcations.