Particle Aggregation in a turbulent Keplerian flow

In the problem of planetary formation one seeks a mechanism to gather small solid particles together into larger accumulations of solid matter. Here we describe a scenario in which turbulence mediates this process by aggregating particles into anticyclonic regions. If, as our simulations suggest, anticyclonic vortices form as long-lived coherent structures, the process becomes more powerful because such vortices trap particles effectively. Even if the turbulence is decaying, following the upheaval that formed the disk, there is enough time to make the dust distribution quite lumpy.Comment: 16 pages, 9 figure

Spiral Evolution in a Confined Geometry

Supported nanoscale lead crystallites with a step emerging from a non-centered screw dislocation on the circular top facet were prepared by rapid cooling from just above the melting temperature. STM observations of the top facet show a nonuniform rotation rate and shape of the spiral step as the crystallite relaxes. These features can be accurately modeled using curvature driven dynamics, as in classical models of spiral growth, with boundary conditions fixing the dislocation core and regions of the step lying along the outer facet edge.Comment: 4 pages, 3 figures, to be published in Physical Review Letter

Detecting Topology in a Nearly Flat Spherical Universe

When the density parameter is close to unity, the universe has a large curvature radius independently of its being hyperbolic, flat, or spherical. Whatever the curvature, the universe may have either a simply connected or a multiply connected topology. In the flat case, the topology scale is arbitrary, and there is no a priori reason for this scale to be of the same order as the size of the observable universe. In the hyperbolic case any nontrivial topology would almost surely be on a length scale too large to detect. In the spherical case, by contrast, the topology could easily occur on a detectable scale. The present paper shows how, in the spherical case, the assumption of a nearly flat universe simplifies the algorithms for detecting a multiply connected topology, but also reduces the amount of topology that can be seen. This is of primary importance for the upcoming cosmic microwave background data analysis. This article shows that for spherical spaces one may restrict the search to diametrically opposite pairs of circles in the circles-in-the-sky method and still detect the cyclic factor in the standard factorization of the holonomy group. This vastly decreases the algorithm's run time. If the search is widened to include pairs of candidate circles whose centers are almost opposite and whose relative twist varies slightly, then the cyclic factor along with a cyclic subgroup of the general factor may also be detected. Unfortunately the full holonomy group is, in general, unobservable in a nearly flat spherical universe, and so a full 6-parameter search is unnecessary. Crystallographic methods could also potentially detect the cyclic factor and a cyclic subgroup of the general factor, but nothing else.Comment: 16 pages, 7 figure

What do very nearly flat detectable cosmic topologies look like?

Recent studies of the detectability of cosmic topology of nearly flat universes have often concentrated on the range of values of $\Omega_{0}$ given by current observations. Here we study the consequences of taking the bounds on $\Omega_{0}$ given by inflationary models, i.e. $|\Omega_0 - 1| \ll 1$. We show that in this limit, a generic detectable non-flat manifold is locally indistinguishable from either a cylindrical ($R^2 \times S^1$) or toroidal ($R \times T^2$) manifold, irrespective of its global shape, with the former being more likely. Importantly this is compatible with some recent indications based on the analysis of high resolution CMB data. It also implies that in this limit an observer would not be able to distinguish topologically whether the universe is spherical, hyperbolic or flat. By severely restricting the expected topological signatures of detectable isometries, our results provide an effective theoretical framework for interpreting cosmological observations, and can be used to confine any parameter space which realistic search strategies, such as the `circles in the sky' method, need to concentrate on. This is particularly important in the inflationary limit, where the precise nature of cosmic topology becomes undecidable.Comment: 8 pages, LaTex2

Constraints on the Detectability of Cosmic Topology from Observational Uncertainties

Recent observational results suggest that our universe is nearly flat and well modelled within a $\Lambda$CDM framework. The observed values of $\Omega_{m}$ and $\Omega_{\Lambda}$ inevitably involve uncertainties. Motivated by this, we make a systematic study of the necessary and sufficient conditions for undetectability as well as detectability (in principle) of cosmic topology (using pattern repetition) in presence of such uncertainties. We do this by developing two complementary methods to determine detectability for nearly flat universes. Using the first method we derive analytical conditions for undetectability for infinite redshift, the accuracy of which is then confirmed by the second method. Estimates based on WMAP data together with other measurements of the density parameters are used to illustrate both methods, which are shown to provide very similar results for high redshifts.Comment: 16 pages, 1 figure, LaTeX2

Ab initio study of the vapour-liquid critical point of a symmetrical binary fluid mixture

A microscopic approach to the investigation of the behaviour of a symmetrical binary fluid mixture in the vicinity of the vapour-liquid critical point is proposed. It is shown that the problem can be reduced to the calculation of the partition function of a 3D Ising model in an external field. For a square-well symmetrical binary mixture we calculate the parameters of the critical point as functions of the microscopic parameter r measuring the relative strength of interactions between the particles of dissimilar and similar species. The calculations are performed at intermediate ($\lambda=1.5$) and moderately long ($\lambda=2.0$) intermolecular potential ranges. The obtained results agree well with the ones of computer simulations.Comment: 14 pages, Latex2e, 5 eps-figures included, submitted to J.Phys:Cond.Ma

Intrinsic profiles and capillary waves at homopolymer interfaces: a Monte Carlo study

A popular concept which describes the structure of polymer interfaces by ``intrinsic profiles'' centered around a two dimensional surface, the ``local interface position'', is tested by extensive Monte Carlo simulations of interfaces between demixed homopolymer phases in symmetric binary (AB) homopolymer blends, using the bond fluctuation model. The simulations are done in an LxLxD geometry. The interface is forced to run parallel to the LxL planes by imposing periodic boundary conditions in these directions and fixed boundary conditions in the D direction, with one side favoring A and the other side favoring B. Intrinsic profiles are calculated as a function of the ``coarse graining length'' B by splitting the system into columns of size BxBxD and averaging in each column over profiles relative to the local interface position. The results are compared to predictions of the self-consistent field theory. It is shown that the coarse graining length can be chosen such that the interfacial width matches that of the self-consistent field profiles, and that for this choice of B the ``intrinsic'' profiles compare well with the theoretical predictions.Comment: to appear in Phys. Rev.

Mode-coupling theory for multiple-time correlation functions of tagged particle densities and dynamical filters designed for glassy systems

The theoretical framework for higher-order correlation functions involving multiple times and multiple points in a classical, many-body system developed by Van Zon and Schofield [Phys. Rev. E 65, 011106 (2002)] is extended here to include tagged particle densities. Such densities have found an intriguing application as proposed measures of dynamical heterogeneities in structural glasses. The theoretical formalism is based upon projection operator techniques which are used to isolate the slow time evolution of dynamical variables by expanding the slowly-evolving component of arbitrary variables in an infinite basis composed of the products of slow variables of the system. The resulting formally exact mode-coupling expressions for multiple-point and multiple-time correlation functions are made tractable by applying the so-called N-ordering method. This theory is used to derive for moderate densities the leading mode coupling expressions for indicators of relaxation type and domain relaxation, which use dynamical filters that lead to multiple-time correlations of a tagged particle density. The mode coupling expressions for higher order correlation functions are also succesfully tested against simulations of a hard sphere fluid at relatively low density.Comment: 15 pages, 2 figure

Charged domain walls as quantum strings living on a lattice

A generic lattice cut-off model is introduced describing the quantum meandering of a single cuprate stripe. The fixed point dynamics is derived, showing besides free string behavior a variety of partially quantum disordered phases, bearing relationships both with quantum spin-chains and surface statistical physics.Comment: 22 page, 17 figure

Nebuliser therapy in the intensive care unit

The relationship between identity, lived experience, sexual practices and the language through which these are conveyed has been widely debated in sexuality literature. For example, ‘coming out’ has famously been conceptualised as a ‘speech act’ (Sedgwick 1990) and as a collective narrative (Plummer 1995), while a growing concern for individuals’ diverse identifications in relations to their sexual and gender practices has produced interesting research focusing on linguistic practices among LGBT-identified individuals (Leap 1995; Kulick 2000; Cameron and Kulick 2006; Farqhar 2000). While an explicit focus on language remains marginal to literature on sexualities (Kulick 2000), issue of language use and translation are seldom explicitly addressed in the growing literature on intersectionality. Yet intersectional perspectives ‘reject the separability of analytical and identity categories’ (McCall 2005:1771), and therefore have an implicit stake in the ‘vernacular’ language of the researched, in the ‘scientific’ language of the researcher and in the relationship of continuity between the two. Drawing on literature within gay and lesbian/queer studies and cross-cultural studies, this chapter revisits debates on sexuality, language and intersectionality. I argue for the importance of giving careful consideration to the language we choose to use as researchers to collectively define the people whose experiences we try to capture. I also propose that language itself can be investigated as a productive way to foreground how individual and collective identifications are discursively constructed, and to unpack the diversity of lived experience. I address intersectional complexity as a methodological issue, where methodology is understood not only as the methods and practicalities of doing research, but more broadly as ‘a coherent set of ideas about the philosophy, methods and data that underlie the research process and the production of knowledge’ (McCall 2005:1774). My points are illustrated with examples drawn from my ethnographic study on ‘lesbian’ identity in urban Russia, interspersed with insights from existing literature. In particular, I aim to show that an explicit focus on language can be a productive way to explore the intersections between the global, the national and the local in cross-cultural research on sexuality, while also addressing issues of positionality and accountability to the communities researched