Phase diagram for a single flexible Stockmayer polymer at zero field

The equilibrium conformations of a flexible permanent magnetic chain that consists of a sequence of linked magnetic colloidal nanoparticles with short-ranged Lennard-Jones attractive interactions (Stockmayer polymer) are thoroughly analysed via Langevin dynamics simulations. A tentative phase diagram is presented for a chain of length $N=100$. The phase diagram exhibits several unusual conformational phases when compared with the non-magnetic chains. These phases are characterised by a large degree of conformational anisotropy, and consist of closed chains, helicoidal-like states, partially collapsed states, and very compact disordered states. The phase diagram contains several interesting features like the existence of at least two 'triple points'

Fairy circle landscapes under the sea

Short-scale interactions yield large-scale vegetation patterns that, in turn, shape ecosystem function across landscapes. Fairy circles, which are circular patches bare of vegetation within otherwise continuous landscapes, are characteristic features of semiarid grasslands. We report the occurrence of submarine fairy circle seascapes in seagrass meadows and propose a simple model that reproduces the diversity of seascapes observed in these ecosystems as emerging from plant interactions within the meadow. These seascapes include two extreme cases, a continuous meadow and a bare landscape, along with intermediate states that range from the occurrence of persistent but isolated fairy circles, or solitons, to seascapes with multiple fairy circles, banded vegetation, and "leopard skin" patterns consisting of bare seascapes patterns consisting of bare seascapes dotted with plant patches. The model predicts that these intermediate seascapes extending across kilometers emerge as a consequence of local demographic imbalances along with facilitative and competitive interactions among the plants with a characteristic spatial scale of 20 to 30 m, consistent with known drivers of seagrass performance. The model, which can be extended to clonal growth plants in other landscapes showing fairy rings, reveals that the different seascapes observed hold diagnostic power as to the proximity of seagrass meadows to extinction points that can be used to identify ecosystems at risks

Kinetics of phase transformation in depletion driven colloids

PACS number(s): 82.70.Gg, 82.70.RrWe present results from a detailed numerical study of the kinetics of phase transformations in a model two-dimensional depletion-driven colloidal system. Transition from a single, dispersed phase to a two-phase coexistence of monomers and clusters is obtained as the depth of the interaction potential among the colloidal particles is changed. Increasing the well depth further, fractal clusters are observed in the simulation. These fractal clusters have a hybrid structure in the sense that they show hexagonal closed-packed crystalline ordering at short length scales and a ramified fractal nature at larger length scales. For sufficiently deep potential wells, the diffusion-limited cluster-cluster aggregation model is recovered in terms of the large-scale fractal dimension Df of the clusters, the kinetic exponent z, and the scaling form of the cluster size distribution. For shallower well depths inside the two-phase coexistence region, simulation results for the kinetics of cluster growth are compared with intermediate-stage phase separation in binary mixtures. In the single-phase region, growth kinetics agree well with a mean-field aggregation-fragmentation model of Sorensen, Zhang, and Taylor.J.J.C. and T.S. acknowledge financial support from the Spanish MCyT through Grant No. BMF2001-0341-C02-01. A.C. and C.S.were supported by NASA through Grant No. NAG 3-2360Peer reviewe

Ice polyamorphism in the minimal Mercedes-Benz model of water

We investigate ice polyamorphism in the context of the two-dimensional Mercedes-Benz model of water. We find a first-order phase transition between a crystalline phase and a high-density amorphous phase. Furthermore, we find a reversible transformation between two amorphous structures of high and low density; however, we find this to be a continuous and not an abrupt transition, as the low-density amorphous phase does not show structural stability. We discuss the origin of this behavior and its implications with regard to the minimal generic modeling of polyamorphism. © 2012 American Institute of Physics.Simulations were performed at the IFISC’s Nuredduna highthroughput computing clusters, supported by the projects GRID-CSIC and FISICOS (FIS2007-60327, funded by the Spanish MINCNN and the ERDF). J.H.E.C. acknowledges MINCINN (Spain) Project No. FIS2010-22322-528C02-02. O.P. and P.A.S. acknowledge MINCINN (Spain) Project No. FIS2010-22322-528C02-01.Peer Reviewe

Scientific monitoring program of the Research Station “Jaume Ferrer”: Temporal evolution of the cover of potentially invasive alien algae

4 págs, 1 fig.[EN] We analyze the changes in the percentage of cover of five species of introduced marine algae during more than a decade in different marine habitats around the island of Menorca. Acrothamnion preissii is abundant in the rhizomes of Posidonia oceanica but its cover has been decreasing during the last ten years. Womersleyella setacea shows persistent high abundances in coralligenous outcrops and sciaphilic algal communities. Asparagopsis taxiformis has colonized almost all habitats but its cover has been low during all the study period, never displaying an invasive behavior. Lophocladia lallemandii has also colonized almost all kind of habitats but it has never reached high coverages during the study period. Finally, Caulerpa cylindracea, a very ubiquitous species, has shown strong interannual cover variability, but it is not currently performing as an invasive species.[ES] En este trabajo se analizan los cambios en el porcentaje de cobertura de cinco especies de algas introducidas desde hace más de una década en distintos hábitats marinos de la isla de Menorca. Acrothamnion preissii es abundante en hábitats de Posidonia oceanica pero su cobertura ha ido disminuyendo a lo largo de los últimos diez años. Womersleyella setacea continua siendo muy abundante en los hábitats de coralígeno y de algas esciáfilas. Asparagopsis taxiformis ha colonizado casi todos los hábitats pero ha presentado una cobertura muy baja durante todo el periodo de estudio sin presentar nunca un carácter invasor. Lophocladia lallemandii llegó a colonizar casi todos los hábitats pero nunca tuvo una cobertura importante. Finalmente, Caulerpa cylindracea, muy ubicua, ha sufrido variaciones interanuales en su cobertura pero actualmente tampoco presenta un carácter invasor.Las actividades de Seguimiento científico de la Estación de Investigación "Jaume Ferrer" están cofinanciadas por la Dirección General de Políticas Universitarias e Investigación del Gobierno de Baleares y el Centro Oceanográfico de Baleares (IEO-CSIC). Queremos hacer llegar nuestro agradecimiento a la Dirección General de Pesca y al Parque Natural de s’Albufera des Grau, para facilitarnos las autorizaciones pertinentes que nos han permitido realizar los trabajos en los espacios marinos protegidos de Menorca. También a Joan Moranta por su ayuda en los primeros muestreos.Peer reviewe

Complex dynamics in clonal organisms

Trabajo presentado en el Mini-Symposium: Clonal growth rates and marine landscapes: from seagrass to coral reefs (Tarek Ahmed Juffali Research Chair in Red Sea Ecology Mini-Symposium, 2021-12-08)

Translational diffusion of flexible lipid chains in a Langmuir monolayer: A dynamic Monte Carlo study

A computer simulation study of the lateral diffusion of conformationally-disordered lipid molecules in a monolayer structure is reported. The simulations were carried out with dynamic Monte Carlo methods, employing two different representations of the internal motions of the lipid chains. The results indicate that the dependence of the lateral diffusion coefficients on the density (area-per-molecule) in the monolayer is determined by the conformational behavior of the lipid chains. The classical Cohen-Turnbull theory is found to provide a good description of the simulated lateral diffusion coefficients at moderate densities. The substantial deviations found at low densities are attributed to the small density fluctuations creating the free volume needed for the lateral diffusion process.Financial support to T.S. from DGICYT ~Spain! under Project No. PB94-1167 is gratefully acknowledged.Peer Reviewe

Modeling non-linear seagrass clonal growth: Assessing the efficiency of space occupation across the seagrass flora

9 pages.-- Final full-text version of the paper available at: http://dx.doi.org/10.1007/BF02784700.The clonal growth of 9 seagrass species was modeled using a simulation model based on observed clonal growth rules (i.e., spacer length, rhizome elongation rates, branching rates, branching angle) and shoot mortality rates for seagrass species. The results of the model confirmed the occurrence of complex, nonlinear growth of seagrass clones derived from internal dynamics of space occupation. The modeled clones progressed from a diffuse-limited aggregation (DLA), dendritic growth, identified with a guerrilla strategy of space occupation, to a compact (Eden) growth, comparable to the phalanx strategy of space occupation, once internal recolonization of gaps, left by dead shoots within the clone, begins. The time at which seagrass clones shifted from diffuse limited to compact growth was predictable from the branching angle and frequency of the species and varied from 1 yr to several decades among species. As a consequence the growth behavior and the apparent growth strategy of the species changes with the development of the clones. The results of the model demonstrate that the emergent complexity of seagrass clonal growth is contained within the simple set of growth rules that can be used to represent clonal growth.This research is supported by the projects EVK3-CT-2000-00044 and LIFE 2000/NAT/E/7303 funded by the European Commission, and the projects REN2000–2123-E and BFM2001-034-C02-01 funded by the Spanish Ministry of Science and Technology.This research is supported by the projects EVK3-CT-2000-00044 and LIFE 2000/NAT/E/7303 funded by the European Commission, and the projects REN2000–2123-E and BFM2001-034-C02-01 funded by the Spanish Ministry of Science and Technolog

Spherical brushes within spherical cavities: A self-consistent field and Monte Carlo study

8 pages, 8 figures.We present an extensive numerical study on the behavior of spherical brushes confined into a spherical cavity. Self-consistent field (SCF) and off-lattice Monte Carlo (MC) techniques are used in order to determine the monomer and end-chain density profiles and the cavity pressure as a function of the brush properties. A comparison of the results obtained via SCF, MC, and the Flory theory for polymer solutions reveals SCF calculations to be a valuable alternative to MC simulations in the case of free and softly compressed brushes, while the Flory’s theory accounts remarkably well for the pressure in the strongly compressed regime. In the range of high compressions, we have found the cavity pressure P to follow a scale relationship with the monomer volume fraction v, P ∼ vα. SCF calculations give α = 2.15±0.05, whereas MC simulations lead to α = 2.73±0.04. The underestimation of α by the SCF method is explained in terms of the inappropriate account of the monomer density correlations when a mean field approach is used.Financial support from MCyT (Spain) and FEDER(EU)(Grant No. FIS2007-60327) is acknowledged. J.J.C. also wants to thank the financial support of MEC (Spain), postdoctoral grant (Grant No. EXP2006-0931).Peer reviewe

Adsorption of semiflexible block copolymers on homogeneous surfaces

We present the results of extensive numerical off-lattice Monte Carlo simulations of semiflexible block-copolymer chains adsorbed onto flat homogeneous surfaces. We have compared the behavior of several chain structures, such as homopolymers, diblocks, (AαBα) block copolymers, and random heteropolymers. In all the cases studied, we have found the adsorption process to be favored with an increase of the chain rigidity. Particularly, the adsorption of diblock structures becomes a two-step process characterized by two different adsorbing temperatures that depend on the chain stiffness κ, the chain length N, and the adsorbing energies ϵA and ϵB. This twofold adsorbing process changes to a single one for copolymers of reduced block size α. Each block of the stiff copolymer chain is found to satisfy the classical scaling laws for flexible chains, however, we found the scaling exponent ϕ to depend on the chain stiffness. The measurement of the radius of gyration exhibits a typical behavior of a polymer chain composed of N/lp blobs whose persistence length follows lp ∼ (κ/kBT)0.5 for large stiff chains.Financial support from the Spanish MEC Grant Nos. FIS2004-00953 and FIS2004-05073-C04-03 is acknowledged. KS thanks Professor Straube for encouragement and MLU, Halle for financial support.Peer reviewe