On the structure and spectrum of classical two-dimensional clusters with a logarithmic interaction potential

We present a numerical study of the effect of the repulsive logarithmic inter-particle interaction on the ground state configuration and the frequency spectrum of a confined classical two-dimensional cluster containing a finite number of particles. In the case of a hard wall confinement all particles form one ring situated at the boundary of the potential. For a general r^n confinement potential, also inner rings can form and we find that all frequencies lie below the frequency of a particular mode, namely the breathing-like mode. An interesting situation arises for the parabolic confined system(i.e. n=2). In this case the frequency of the breathing mode is independent of the number of particles leading to an upper bound for all frequencies. All results can be understood from Earnshaw's theorem in two dimensions. In order to check the sensitivity of these results, the spectrum of vortices in a type II superconductor which, in the limit of large penetration depths, interact through a logarithmic potential, is investigated.Comment: 11 pages, 6 figure

Clinicopathological evidence that neovascularisation is a cause of recurrent varicose veins

Objectives:Recurrent varicose veins may result from poor initial surgical technique or progression of varicosities in collateral veins. In some cases new veins may develop at the saphenofemoral junction (neovascularisation) and cause recurrent saphenofemoral incompetence. This was a histological study of recurrent varicose veins.Design:This clinicopathological study included 20 patients (median age 55 years) who had surgery for recurrent saphenofemoral incompetence.Materials and methods:A total of 28 legs had groin re-exploration with repeat flush saphenofemoral ligation. The venous tissue block from the saphenofemoral region (including the proximal thigh varicosity) was excised and orientated for histological analysis. Evidence of neovascularisation was sought using routine histological sections and S100 immunohistochemistry.Results:At operation, thin-walled, serpentine neovascular veins were detected clinically as the principal cause of recurrence in 19 groins. In five groins recurrence was due to a residual missed vein at the saphenofemoral junction, and in four recurrence was caused by cross groin collaterals. On histological sections, evidence of neovascularisation was present in 27 of 28 groins. In eight it co-existed with the veins missed at the original operation but it was the sole identified cause of recurrent saphenofemoral incompetence in 19 (68%) groins.Conclusions:Neovascularisation was the principal cause of recurrent saphenofemoral incompetence in this series

Viscous instabilities in flowing foams: A Cellular Potts Model approach

The Cellular Potts Model (CPM) succesfully simulates drainage and shear in foams. Here we use the CPM to investigate instabilities due to the flow of a single large bubble in a dry, monodisperse two-dimensional flowing foam. As in experiments in a Hele-Shaw cell, above a threshold velocity the large bubble moves faster than the mean flow. Our simulations reproduce analytical and experimental predictions for the velocity threshold and the relative velocity of the large bubble, demonstrating the utility of the CPM in foam rheology studies.Comment: 10 pages, 3 figures. Replaced with revised version accepted for publication in JSTA

Exploring the climate of Proxima B with the Met Office Unified Model

This is the author accepted manuscript. The final version is available from EDP Sciences via the DOI in this record.The corrigendum to this article is in ORE at: http://hdl.handle.net/10871/34331We present results of simulations of the climate of the newly discovered planet Proxima Centauri B, performed using the Met Office Unified Model (UM). We examine the responses of both an ‘Earth-like’ atmosphere and simplified nitrogen and trace carbon dioxide atmosphere to the radiation likely received by Proxima Centauri B. Additionally, we explore the effects of orbital eccentricity on the planetary conditions using a range of eccentricities guided by the observational constraints. Overall, our results are in agreement with previous studies in suggesting Proxima Centauri B may well have surface temperatures conducive to the presence of liquid water. Moreover, we have expanded the parameter regime over which the planet may support liquid water to higher values of eccentricity (& 0.1) and lower incident fluxes (881.7 W m−2 ) than previous work. This increased parameter space arises because of the low sensitivity of the planet to changes in stellar flux, a consequence of the stellar spectrum and orbital configuration. However, we also find interesting differences from previous simulations, such as cooler mean surface temperatures for the tidally-locked case. Finally, we have produced high resolution planetary emission and reflectance spectra, and highlight signatures of gases vital to the evolution of complex life on Earth (oxygen, ozone and carbon dioxide).I.B., J.M. and P.E. acknowledge the support of a Met Office Academic Partnership secondment. B.D. thanks the University of Exeter for support through a Ph.D. studentship. N.J.M. and J.G.’s contributions were in part funded by a Leverhulme Trust Research Project Grant, and in part by a University of Exeter College of Engineering, Mathematics and Physical Sciences studentship. We acknowledge use of the MONSooN system, a collaborative facility supplied under the Joint Weather and Climate Research Programme, a strategic partnership between the Met Office and the Natural Environment Research Council. This work also used the University of Exeter Supercomputer, a DiRAC Facility jointly funded by STFC, the Large Facilities Capital Fund of BIS and the University of Exeter

Anomalous temperature dependence of surface tension and capillary waves at liquid gallium

The temperature dependence of surface tension \gamma(T) at liquid gallium is studied theoretically and experimentally using light scattering from capillary waves. The theoretical model based on the Gibbs thermodynamics relates the temperature derivative of \gamma to the surface excess entropy -\Delta S. Although capillary waves contribute to the surface entropy with a positive sign the effect of dipole layer on \Delta S is negative. Experimental data collected at a free Ga surface in the temperature range from 30 to 160 C show that the temperature derivative of the tension changes sign near 100 C.Comment: 11 pages, 1 Postscript figure, submitted to J. Phys.

Double-impulse magnetic focusing of launched cold atoms.

We have theoretically investigated three-dimensional focusing of a launched cloud of cold atoms using a pair of magnetic lens pulses (the alternate-gradient method). Individual lenses focus radially and defocus axially or vice versa. The performance of the two possible pulse sequences are compared and found to be ideal for loading both 'pancake' and 'sausage' shaped magnetic/optical microtraps. It is shown that focusing aberrations are considerably smaller for double-impulse magnetic lenses compared to single-impulse magnetic lenses. An analysis of clouds focused by the double-impulse technique is presented

Probing liquid surface waves, liquid properties and liquid films with light diffraction

Surface waves on liquids act as a dynamical phase grating for incident light. In this article, we revisit the classical method of probing such waves (wavelengths of the order of mm) as well as inherent properties of liquids and liquid films on liquids, using optical diffraction. A combination of simulation and experiment is proposed to trace out the surface wave profiles in various situations (\emph{eg.} for one or more vertical, slightly immersed, electrically driven exciters). Subsequently, the surface tension and the spatial damping coefficient (related to viscosity) of a variety of liquids are measured carefully in order to gauge the efficiency of measuring liquid properties using this optical probe. The final set of results deal with liquid films where dispersion relations, surface and interface modes, interfacial tension and related issues are investigated in some detail, both theoretically and experimentally. On the whole, our observations and analyses seem to support the claim that this simple, low--cost apparatus is capable of providing a wealth of information on liquids and liquid surface waves in a non--destructive way.Comment: 25 pages, 12 figures, to appear in Measurement Science and Technology (IOP

Mitotic phosphatases: no longer silent partners

Recent work has highlighted the important role played by protein phosphatase complexes in the regulation of mitosis from yeast to mammals. There have been important advances in defining the roles of the protein serine/threonine phosphatases PP1 and PP2A and the dual specificity protein tyrosine phosphatases CDC25 and Cdc14. Three independent studies defined a regulatory role for PP2A in the control of sister chromatid cohesion, involving a direct interaction with shugoshin. A chromatin targeting subunit has been identified for PP1 and the complex shown to play an essential role in chromosome segregation. Key regulatory residues within CDC25 have been mapped and its activity tied both to the initial activation of cyclin-dependent kinases at the centrosome and to DNA damage checkpoints. Novel roles have been defined for Cdc14, including regulation of rDNA and telomere segregation and participation in spindle assembly. These exciting advances show that protein phosphatases are not merely silent partners to kinases in regulating the control of cell division. Introduction The process of cell division is complex and involves multiple independent regulatory steps, most of which are controlled by reversible protein phosphorylation. In higher eukaryotes, mitosis involves condensation of chromosomes, disassembly of the nuclear lamina, breakdown of the nuclear envelope and disassembly of many forms of nuclear bodies, including nucleoli. Completion of mitosis requires alignment and proper segregation of chromosomes into daughter cells followed by reassembly of nuclei and cytokinesis. These and many other events, such as centrosome separation and spindle assembly, are tightly regulated, and several critical checkpoints occur during mitosis to ensure fidelity. Failure to complete any of the key steps can trigger apoptosis and cell death. While the important role of protein phosphorylation in regulating mitotic events has long been recognized, much of the work in this area has focused on the kinases, primarily the Cdk/Cyclin, Aurora, Polo-like and NIMA families (see This review will focus on recent advances in understanding the contributions of four major classes of protein phosphatases to the regulation of processes involved in controlling cell division, specifically the protein serine/ threonine phosphatases PP1 and PP2A and the dualspecificity protein tyrosine phosphatases (DUSPs) CDC25 and Cdc14. We will draw on examples from species as diverse as yeast, insects and mammals, reflecting the high evolutionary conservation of these regulated events. Serine/threonine phosphatases Both PP1 (termed Glc7 in budding yeast and Dis2 in fission yeast) and PP2A are serine/threonine-specific protein phosphatase catalytic subunits that form holoenzyme complexes with one or more regulatory subunits. These regulatory subunits can affect cellular location and/or substrate specificity. In contrast with most kinases, the PP1 and PP2A catalytic subunits can potentially act on a wide range of substrates and thus substrate specificity is conferred by their interaction partners. Thus, a critical step in understanding the role of PP1 and PP2A holoenzymes is to define their regulatory subunits and the mechanism by which they are targeted to their physiological substrates. Much of the literature ascribing specific roles to PP1 or PP2A has relied on differential effects of inhibitors such as okadaic acid, which in vitro blocks PP2A activity at lower concentrations than are required to inhibit PP1 PP2A PP2A plays a prominent role in the regulation of mitosis and signalling pathways. In addition to its interaction with both scaffolding and variable subunits (termed 'A' and 'B' subunits, respectively) in a trimeric complex (see Using immunoprecipitation and yeast two-hybrid studies, several groups independently identified a specific PP2A trimeric complex that interacts with Sgo1 [7 ,8 ,9 ]. On the basis of RNAi studies and analysis of a non-PP2A-binding hSgo1 mutant, Tang and colleagues [7 ] proposed that interaction with PP2A is required for centromeric localization of hSgo1 and proper chromosome segregation. Independently, the same PP2A complex was immunopurified from HEK 293T cells using Flagtagged hSgo1 [8 ]. Immunofluorescence studies by Kitajima and colleagues showed colocalization of hSgo1 and the B56 PP2A regulatory subunit at mammalian centromeres. Using RNAi in mammals, they also reported that knockdown of hSgo2, but not of hSgo1, resulted in loss of centromeric PP2A. Conversely, knockdown of PP2A led to a loss of centromeric hSgo1 [8 ]. Studies on both budding and fission yeast undergoing meiosis also showed that Sgo1 interacts with PP2A at centromeres and serves to protect the cohesin Rec8 subunit from phosphorylation and cleavage [9 ]. Interestingly, tethering of yeast PP2A at specific sites on chromosome arms preserved cohesion at these sites even after meiosis I, when arm cohesin should dissociate, showing an intrinsic ability of PP2A to protect cohesin, independent of Sgo1 [8 ,9 ]. The PP2A complex may thus work both directly at centromeres to maintain cohesion and by facilitating accumulation of Sgo1, which also acts to prevent cleavage of cohesin. Taken together, these studies point to an important new role for PP2A in the control of chromosome cohesion, mediated, at least in part, through interactions with shugoshins ( PP2A has also been implicated in regulating mitotic exit. Wang and Ng [10] provided evidence suggesting that a PP2A-Cdc55 complex dephosphorylates the mitotic exit network (MEN) activator Tem1 in budding yeast. This prevents mitotic exit by blocking release of Cdc14 from 624 Cell division, growth and death Figure 1 Role of PP2A in maintenance of chromosome cohesion. This diagram summarizes three recent studies that identified a specific PP2A trimeric complex acting with shugoshin to protect cohesin at centromeres from phosphorylation and cleavage until the metaphase-anaphase transition. In metazoan mitosis, cohesin is removed from chromosome arms at prometaphase but remains at the centromere regions, protected by shugoshin and PP2A. At the metaphase-anaphase transition, separase is activated and cleaves this residual cohesin, resulting in a loss of cohesion and separation of sister chromatids. PP1 PP1 has been shown to contribute to the regulation of multiple cellular processes including glycogen metabolism and muscle contraction, mediated by interaction of the PP1 catalytic domain with regulatory proteins termed 'targeting subunits'. Over 50 have been described to date, and they have the potential to regulate both the localization and the catalytic activity of PP1 (see [14] for review). Most targeting subunits share a common 'RVXF' motif that mediates direct binding to PP1 An elegant series of experiments has described a role for PP1 in controlling nuclear envelope assembly at the end of mitosis [24][25][26]. When cells enter mitosis, nuclear lamina disassembly is promoted by phosphorylation of B-type lamins. AKAP149, an ER and nuclear membrane protein, was shown to target PP1 (via an RVXF motif) to dephosphorylate B-type lamins at telophase, enabling their polymerization and thus lamina reassembly. A short peptide from AKAP149 containing the RVXF motif can displace PP1 and induce mislocalization of B-type lamins to the cytoplasm. Although the cells were able to complete mitosis, they died by apoptosis within six hours, suggesting that disruption of lamin assembly may directly trigger apoptosis. The association of PP1 isoforms with centrosomes, kinetochores and the cellular cortex and midbody region (see Pinsky et al. [29 ] took advantage of the regulation of Glc7 by targeting subunits to explore its interaction with Ipl1 (Aurora B) in budding yeast. Glc7 is known to antagonize Ipl1 activity, but it was unclear whether it dephosphorylates its substrates or regulates the kinase directly. Although the targeting subunit has not been identified, titratation of Glc7 away from Ipl1 by overexpression of Glc7 binding proteins that do not play roles in chromosome segregation led to increased phosphorylation of Ipl1 substrates, leading the authors to propose that Glc7 acts to ensure accurate chromosome segregation by dephosphorylating Ipl1 targets. CDC25 CDC25 was first identified in fission yeast 20 years ago as a factor required for entry into mitosis [30]. It is now known to activate cyclin-dependent kinases (Cdks) by removing inhibitory phosphates, which leads to Cdk phosphorylation of multiple substrates that drive the cell division process forward (see Three mammalian genes were identified that complement the yeast cdc25 knockout strain. The proteins encoded by these genes, termed CDC25A, CDC25B and CDC25C, are 60% identical in their C-terminal regions, which include the catalytic core containing the CX 5 R motif common to all protein tyrosine phosphatases. In contrast to the reasonably high homology of their catalytic domains, they are only 20-25% identical in their N-terminal regulatory domains, which contain sites for various post-translational modifications and sitespecific protein interactions, including phosphorylation of key serine and threonine residues, ubiquitination, phosphorylation-dependent binding of 14-3-3 proteins and Pin1-dependent prolyl isomerization (see There is a dramatic hyperphosphorylation of CDC25 during the transition from interphase to mitosis, and several mitotic phosphorylation sites have been mapped (see While all three mammalian CDC25 phosphatases activate their Cdk substrates in the same manner, they appear to have distinct roles in regulating cell cycle transitions (see Mitotic phosphatases: no longer silent partners Trinkle-Mulcahy and Lamond 627 have not yet been ruled out. (b) The G 2 /M transition is regulated in a similar way, with CDC25 activating Cdk1/Cyclin B by dephosphorylating critical residues. All three mammalian CDC25 isoforms have been implicated in regulation of this pathway. (c) The initial activation of Cdk1/Cyclin B has been shown to occur at centrosomes as they begin to separate during prophase, and involves the phosphorylation and activation of CDC25B by the Ajuba-Aurora A complex. The divergent N-terminal regulatory domains of the three mammalian CDC25 isoforms contain a variety of regulatory sites, including phosphorylation sites, 14-3-3 binding sites, domains that regulate degradation and nuclear import and export signals. Several of these known and recently described regulatory sites have been summarized here for (d) CDC25A, (e) CDC25B and (f) CDC25C. Cdc14 While Cdc25 is a key regulator of initiation of mitosis (and hence DNA damage checkpoint control), Cdc14 is a key regulator of late mitotic events, coordinating the temporal and spatial control of chromosome segregation with mitotic spindle disassembly and cytokinesis. In the budding yeast S. cerevisiae, Cdc14p plays a key role in exit from mitosis by dephosphorylating Cdk targets (reviewed in FEAR-controlled release of Cdc14p in budding yeast is also important for division of nucleoli and resolution of highly repetitive rDNA and telomere regions, as demonstrated in two recent studies. These regions separate at mid-anaphase, long after cohesin is cleaved. D'Amours and colleagues 628 Cell division, growth and death Figure 4 Cross-species comparison of Cdc14 localization and function. Cdc14 homologues from four different eukaryotes are listed, showing their localization during interphase and throughout mitosis. Nuclei are shown in green, spindle pole bodies (centrosomes) in red, microtubules in pink and chromosomes in blue. The localization of Cdc14 at these sites is shown in yellow. Known mitotic functions for these homologues are also listed

Automatic classification of field-collected dinoflagellates by artificial neural network

Automatic taxonomic categorisation of 23 species of dinoflagellates was demonstrated using field-collected specimens. These dinoflagellates have been responsible for the majority of toxic and noxious phytoplankton blooms which have occurred in the coastal waters of the European Union in recent years and make severe impact on the aquaculture industry. The performance by human 'expert' ecologists/taxonomists in identifying these species was compared to that achieved by 2 artificial neural network classifiers (multilayer perceptron and radial basis function networks) and 2 other statistical techniques, k-Nearest Neighbour and Quadratic Discriminant Analysis. The neural network classifiers outperform the classical statistical techniques. Over extended trials, the human experts averaged 85% while the radial basis network achieved a best performance of 83%, the multilayer perceptron 66%, k-Nearest Neighbour 60%, and the Quadratic Discriminant Analysis 56%

Membranes in the two-Higgs standard model

We present some non-topological static wall solutions in two-Higgs extensions of the standard model. They are classically stable in a large region of parameter space, compatible with perturbative unitarity and with present phenomenological bounds.Comment: 7 pages, latex, 3 figures available upon reques