Dynamical transitions and sliding friction of the phase-field-crystal model with pinning

We study the nonlinear driven response and sliding friction behavior of the phase-field-crystal (PFC) model with pinning including both thermal fluctuations and inertial effects. The model provides a continuous description of adsorbed layers on a substrate under the action of an external driving force at finite temperatures, allowing for both elastic and plastic deformations. We derive general stochastic dynamical equations for the particle and momentum densities including both thermal fluctuations and inertial effects. The resulting coupled equations for the PFC model are studied numerically. At sufficiently low temperatures we find that the velocity response of an initially pinned commensurate layer shows hysteresis with dynamical melting and freezing transitions for increasing and decreasing applied forces at different critical values. The main features of the nonlinear response in the PFC model are similar to the results obtained previously with molecular dynamics simulations of particle models for adsorbed layers.Comment: 7 pages, 8 figures, to appear in Physcial Review

Phase Diagram and Commensurate-Incommensurate Transitions in the Phase Field Crystal Model with an External Pinning Potential

We study the phase diagram and the commensurate-incommensurate transitions in a phase field model of a two-dimensional crystal lattice in the presence of an external pinning potential. The model allows for both elastic and plastic deformations and provides a continuum description of lattice systems, such as for adsorbed atomic layers or two-dimensional vortex lattices. Analytically, a mode expansion analysis is used to determine the ground states and the commensurate-incommensurate transitions in the model as a function of the strength of the pinning potential and the lattice mismatch parameter. Numerical minimization of the corresponding free energy shows good agreement with the analytical predictions and provides details on the topological defects in the transition region. We find that for small mismatch the transition is of first-order, and it remains so for the largest values of mismatch studied here. Our results are consistent with results of simulations for atomistic models of adsorbed overlayers

Ordering kinetics of stripe patterns

We study domain coarsening of two dimensional stripe patterns by numerically solving the Swift-Hohenberg model of Rayleigh-Benard convection. Near the bifurcation threshold, the evolution of disordered configurations is dominated by grain boundary motion through a background of largely immobile curved stripes. A numerical study of the distribution of local stripe curvatures, of the structure factor of the order parameter, and a finite size scaling analysis of the grain boundary perimeter, suggest that the linear scale of the structure grows as a power law of time with a craracteristic exponent z=3. We interpret theoretically the exponent z=3 from the law of grain boundary motion.Comment: 4 pages, 4 figure

Glassy phases and driven response of the phase-field-crystal model with random pinning

We study the structural correlations and the nonlinear response to a driving force of a two-dimensional phase-field-crystal model with random pinning. The model provides an effective continuous description of lattice systems in the presence of disordered external pinning centers, allowing for both elastic and plastic deformations. We find that the phase-field crystal with disorder assumes an amorphous glassy ground state, with only short-ranged positional and orientational correlations even in the limit of weak disorder. Under increasing driving force, the pinned amorphous-glass phase evolves into a moving plastic-flow phase and then finally a moving smectic phase. The transverse response of the moving smectic phase shows a vanishing transverse critical force for increasing system sizes

Of Genes and Antigens: The Inheritance of Psoriasis

Psoriasis is one of a number of autoimmune diseases that display significant HLA associations. In particular, individuals with onset of disease prior to 40 years of age display striking associations with HLA-Cw6 and are much more likely to have a positive family for psoriasis. However, only about 10% of Cw6-positive individuals develop disease, suggesting that other genetic and/or environmental factors must be involved. Several compelling lines of epidemiologic evidence indicate that psoriasis susceptibility is inherited, albeit not in a simple monogenic fashion, and that genetic, rather than environmental, factors are primarily responsible for the variability in inheritance of psoriasis. Taken together, these observations suggest that one or more loci in addition to HLA are necessary for the development of psoriasis. The number of additional loci is likely to be small, because i) the disease is very common ii) substantial excess risk of psoriasis is observed in first degree relatives, and iii) nevoid variants of psoriasis have been reported, suggestive of somatic mutation of a single gene during development. The substantial homogeneity of the psoriatic phenotype and the clear evidence for increased HLA association and heritability in juvenile onset disease indicate that despite its complexity, psoriasis is a common disease whose etiology is amenable to elucidation through the techniques of modern molecular genetics. J Invest Dermatol 103:150S-153S, 199

Comparison of MHC class I risk haplotypes in Thai and Caucasian psoriatics shows locus heterogeneity at PSORS1

Earlier studies have shown that psoriasis in Japan and Thailand is associated with two different major histocompatibility complex (MHC) haplotypes – those bearing HLA-Cw6 and those bearing HLA-Cw1 and HLA-B46. In an independent case-control sample from Thailand, we confirmed the association of psoriasis with both haplotypes. No association was seen in Thai HLA-Cw1 haplotypes lacking HLA-B46 , nor was HLA-Cw1 associated with psoriasis in a large Caucasian sample. To assess whether these risk haplotypes share a common origin, we sequenced genomic DNA from a Thai HLA-Cw1-B46 homozygote across the ∼300 kb MHC risk interval, and compared it with sequence of a HLA-Cw6-B57 risk haplotype. Three small regions of homology were found, but these regions share equivalent sequence similarity with one or more clearly non-risk haplotypes, and they contain no polymorphism alleles unique to all risk haplotypes. Differences in psoriasis phenotype were also observed, including lower risk of disease, greater nail involvement, and later age at onset in HLA-Cw1-B46 carriers compared with HLA-Cw6 carriers. These findings suggest locus heterogeneity at PSORS1 (psoriasis susceptibility 1), the major psoriasis susceptibility locus in the MHC, with HLA-Cw6 imparting risk in both Caucasians and Asians, and an allele other than HLA-Cw1 on the HLA-Cw1-B46 haplotype acting as an additional risk variant in East Asians.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/79072/1/TAN_1526_sm_tables1.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/79072/2/j.1399-0039.2010.01526.x.pd

Investigating possible ethnicity and sex bias in clinical examiners: an analysis of data from the MRCP(UK) PACES and nPACES examinations

Bias of clinical examiners against some types of candidate, based on characteristics such as sex or ethnicity, would represent a threat to the validity of an examination, since sex or ethnicity are 'construct-irrelevant' characteristics. In this paper we report a novel method for assessing sex and ethnic bias in over 2000 examiners who had taken part in the PACES and nPACES (new PACES) examinations of the MRCP(UK)

Prenatal Exclusion of Lamellar Ichthyosis Based on Identification of Two New Mutations in the Transglutaminase 1 Gene

Lamellar ichthyosis is a severe, generalized, autosomal recessive genodermatosis characterized clinically by large, parchment-like scales and histologically by acanthosis and marked hyperkeratosis. Genetic heterogeneity in lamellar ichthyosis has been recognized with reports of two linked loci (on chromosomes 14q11 and 2q33–35). In a cohort of four small families with lamellar ichthyosis we found confirmatory evidence for linkage (p ≤ 0.01) to D14S275, a microsatellite marker close to transglutaminase 1 on chromosome 14q11. We also identified two novel transglutaminase 1 mutations in an affected sibling pair from one of these families. The paternal mutation in exon 3, 1387insCAGC, causes a frameshift predicted to result in premature termination of translation within the same exon. The maternal mutation in exon 8, 4561delAC, also causes a frameshift and a premature stop codon in this exon. The mother of these siblings recently became pregnant with twins. Genotyping and direct sequencing of DNA isolated from fetal amniotic fluid cultures revealed the presence of the paternal but the absence of the maternal mutation, thus predicting a normal skin phenotype. Both twins were born with normal-appearing skin. Our findings demonstrate that mutations of both alleles of the transglutaminase 1 gene are the cause of lamellar ichthyosis in this family, and illustrate an emerging clinical application of molecular genetics in dermatology

Flame propagation in random media

We introduce a phase-field model to describe the dynamics of a self-sustaining propagating combustion front within a medium of randomly distributed reactants. Numerical simulations of this model show that a flame front exists for reactant concentration $c > c^* > 0$, while its vanishing at $c^*$ is consistent with mean-field percolation theory. For $c > c^*$, we find that the interface associated with the diffuse combustion zone exhibits kinetic roughening characteristic of the Kardar-Parisi-Zhang equation.Comment: 4, LR541