Blob Model Research Articles

Disagreement on the North Atlantic Cold Blob Formation Mechanisms among Climate Models

Abstract Despite global warming, the sea surface temperature (SST) in the subpolar North Atlantic has decreased since the 1900s. This local cooling, known as the North Atlantic cold blob, signifies a unique role of the subpolar North Atlantic in uptaking heat and hence impacts downstream weather and climate. However, a lack of observational records and their constraints on climate models leave the North Atlantic cold blob formation mechanism inconclusive. Using simulations from phase 6 of Coupled Model Intercomparison Project, we assess the primary processes driving the North Atlantic cold blob within individual models and whether the mechanisms are consistent across models. We show that 11 out of 32 models, which we call “Cold Blob” models, simulate the subpolar North Atlantic cooling over 1900–2014. Further analyzing the heat budget of the subpolar North Atlantic SST shows that models have distinct mechanisms of cold blob formation. While 4 of the 11 Cold Blob models indicate decreased oceanic heat transport convergence (OHTC) as the key mechanism, another four models suggest changes in radiative processes making predominant contributions. The contribution of OHTC and radiative processes is comparable in the remaining three models. Such a model disagreement on the mechanism of cold blob formation may be associated with simulated base-state Atlantic meridional overturning circulation (AMOC) strength, which explains 39% of the intermodel spread in the contribution of OHTC to the simulated cold blob. Models with a stronger base-state AMOC suggest a greater role of OHTC, whereas those with a weaker base-state AMOC indicate that radiative processes are more responsible. This model discrepancy suggests that the cold blob formation mechanism diagnosed from single model should be interpreted with caution. Significance Statement The mechanisms driving sea surface temperatures over the subpolar North Atlantic to cool since the 1900s remain uncertain due to the lack of direct observations. Here, we use a temperature change decomposition framework to dissect the historical trend of surface temperature simulated in multiple global climate models. The models diverge on whether the subpolar North Atlantic cooling is induced by reduced ocean heat transport convergence or altered radiative processes. Notably, the importance of ocean heat transport convergence is influenced by the simulated base-state strength of Atlantic meridional overturning circulation and the Irminger Sea’s mixed layer depth. This finding cautions against concluding the cooling mechanism from a single model and highlights a need for ongoing observations to constrain AMOC-related climate projection in the subpolar North Atlantic.

Flexibility of Poly(alkyl methacrylate)s Characterized by Their Persistence Length Determined through Pyrene Excimer Formation.

A series of poly(alkyl methacrylate)s and poly(oligo(ethylene glycol) methyl ether methacrylate)s labeled with 1-pyrenebutanol were referred to as the PyC4-PCnMA samples with n = 1, 4, 6, 8, 12, and 18 and the PyC4-PEGnMA samples with n = 0-5, 9, 16, and 19, respectively. Pyrene excimer formation (PEF) upon the encounter between an excited and a ground-state pyrenyl labels was employed to determine their persistence length (lp) in o-xylene. The fluorescence decays of the PyC4-PCnMA and PyC4-PEGnMA samples were acquired and analyzed with the fluorescence blob model to yield the number (Nblob) of structural units in the volume probed by an excited pyrenyl label. Nblob was found to decrease with an increasing number (NS) of non-hydrogen atoms in the side chain, reaching a plateau for the PyC4-PEGnMA samples with a longer side chain (n = 16 and 19). The Nblob values were used to determine lp. The lp values for the PyC4-PCnMA and PyC4-PEGnMA samples increased linearly with increasing NS2 as predicted theoretically, which agreed with the lp values obtained by viscometry for a series of PCnMA samples. The good agreement between the lp values retrieved by PEF and viscometry served to validate the PEF-based methodology for determining lp for linear polymers.

Colloidal adsorption in planar polymeric brushes.

The design of nano-functionalised membranes or channels, able to effectively adsorb pollutants in aqueous solutions, is a topic that is gaining a great deal of attention in the materials science community. With this work we explore, through a combination of scaling theories and molecular dynamics simulations, the adsorption of spherical non-deformable colloidal nanoparticles within planar polymeric brushes. Our strategy is twofold: first, we generalise the Alexander-de Gennes theory for planar homopolymeric brushes to the case of diblock copolymer brushes, then we map the adsorbing homopolymeric brushes onto a diblock copolymer system, where the adsorbed colloids and all interacting monomers are considered monomers in bad solvent and we apply the generalised scaling theory to this effective diblock copolymer. This allows the prediction of the average conformation of the grafted substrate, i.e. its average height, as a function of the amount of loaded particles, as well as the introduction of a continuous mapping between a homopolymeric brush, the fraction of loaded particles and the average height of the adsorbing substrate.

Pincus blob elasticity in an intrinsically disordered protein.

Understanding the dynamic structure of intrinsically disordered proteins (IDPs) is important to deciphering their biological functions. Here, we exploit precision entropic elasticity measurements to infer the conformational behavior of a model IDP construct formed from the disordered tail of the neurofilament low molecular weight protein. The IDP construct notably displays a low-force power-law elastic regime, consistent with the Pincus blob model, which allows direct extraction of the Flory exponent, [Formula: see text], from the force-extension relationship. We find [Formula: see text] increases with added denaturant, transitioning from a nearly ideal chain to a swollen chain in a manner quantitatively consistent with measurements of IDP dimensions from other experimental techniques. We suggest that measurements of entropic elasticity could be broadly useful in the study of IDP structure.

Persistence Length of PEGMA Bottle Brushes Determined by Pyrene Excimer Fluorescence.

Seven pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s (PyEG5-PEGnMAs) were prepared with n = 0, 3, 4, 5, 7, 9, and 19 ethylene glycol units by copolymerizing a small amount of penta(ethylene glycol) 1-pyrenemethyl ether methacrylate with an EGnMA monomer. The conformation of the PyEG5-PEGnMA polymers evolved from a random coil for PyEG5-PEG0MA or poly(methyl methacrylate) to a polymeric bottle brush (PBB) architecture with increasing side chain length. The fluorescence decays of the PyEG5-PEGnMA samples were fitted according to the fluorescence blob model (FBM) whose parameters were used, in combination with the Kratky-Porod equation, to calculate the persistence length of these polymers. The persistence lengths obtained from the PEF experiments were found to increase with the square of the number (NS) of non-hydrogen atoms in the side chain as expected theoretically. The persistence lengths found with the PyEG5-PEGnMA samples in DMF also matched those found earlier for another series of PEGnMA samples labeled with 1-pyrenebutanol. The good agreement found between the persistence lengths obtained with the PEGnMA samples labeled with two different pyrene derivatives illustrates the robustness of the method and its applicability for measuring the unknown persistence length of polydisperse polymer samples.

Nonmonotonous Translocation Time of Polymers across Pores.

Polymers confined in corrugated channels, i.e., channels of varying amplitude, display multiple local maxima and minima of the diffusion coefficient upon increasing their degree of polymerization N. We propose a theoretical effective free energy for linear polymers based on a Fick-Jacobs approach. We validate the predictions against numerical data, obtaining quantitative agreement for the effective free energy, the diffusion coefficient, and the mean first passage time. Finally, we employ the effective free energy to compute the polymer lengths N_{min} at which the diffusion coefficient presents a minimum: we find a scaling expression that we rationalize with a blob model. Our results could be useful to design porous adsorbers, that separate polymers of different sizes without the action of an external flow.

Persistence Length and Encounter Frequency Determination from Fluorescence Studies of Pyrene-Labeled Poly(oligo(ethylene glycol) methyl ether methacrylate)s

A series of nine pyrene-labeled poly(oligo(ethylene glycol) methyl ether methacrylate)s (Py-PEGnMAs) with n equal to 0–5, 9, 16, and 19 were prepared by random radical copolymerization of 1-pyrenebutyl methacrylate and nine different EGnMA monomers. The process of pyrene excimer formation (PEF) in the Py-PEGnMA samples was characterized in acetone, THF, toluene, N,N-dimethylformamide (DMF), dioxane, and dimethyl sulfoxide (DMSO). The fluorescence decays of all Py-PEGnMA samples were acquired in the six solvents and analyzed with the fluorescence blob model (FBM) to yield the number Nblob of structural units (SU) in the subvolume of the polymer coil probed by an excited pyrene and referred to as a blob, and the rate constant kblob describing the encounters between two SU bearing an excited and a ground-state pyrenyl label located inside a same blob. Nblob and kblob remained constant with pyrene content for a same series of Py-PEGnMA samples. After averaging Nblob and kblob over all pyrene contents for a same Py-PEGnMA series, ⟨Nblob⟩ and the product ⟨kblob × Nblob⟩ were found to decrease with increasing side chain length reflecting a progressive stiffening and decrease in the internal dynamics of the polymethacrylate backbone, respectively. ⟨Nblob⟩ and ⟨kblob × Nblob⟩ could be parametrized as a function of the molecular weight of an SU and the solvent viscosity. The parametrized form of ⟨Nblob⟩ was applied to determine the persistence length (lp) of the PEGnMA samples using the Kratky–Porod equation. lp was found to increase linearly with the square of the side chain length of the PEGnMA samples, as expected theoretically. The parametrized form of ⟨kblob × Nblob⟩ was used as a calibration curve against which the internal dynamics of several polypeptides and poly(methyl acrylate) could be compared in DMSO. This study illustrates the ability of PEF measurements to determine the persistence length and quantify the internal dynamics of polymers in solution, two important parameters in the characterization of macromolecules.

Structure of polyvinylpyrrolidone in water/ethanol mixture in dilute and semi-dilute regimes: Roles of solvents mixture composition and polymer concentration

This study aims to describe the structure of polyvinylpyrrolidone (PVP, Mw = 40.000 g/mol) in water/ethanol solution in dilute and semi dilute regimes at 25 °C by viscosity measurements. Intrinsic viscosity of the polymer is determined for different ethanol molar fractions, XA. The cononsolvency of PVP in water/ethanolis evident and manifested by a contraction in the 0.05–0.4ethanol molar fraction range. The overlapping concentration,CPVP∗(XA), of PVP in water/ethanol increases with increasing XA. The Flory exponent, ν(XA), and the Mark-Houwink-Sakurada(MHS) coefficients,a(XA), are deduced by plotting the double logarithm of the relative viscosity versus polymer concentration in semi-dilute regime. The Flory exponent and the MHS values are shown to be dependent on XA. The blob model theory is extended and used to study the structure of a polymer in its mixed solvents. So, the structure of PVP in water/ethanol mixture is described by determining the structural parameters: density of monomers, monomer size, correlation length and the number of monomer in each blob for the PVP concentration ranged from CPVP∗(XA) to 2CPVP∗(XA). A spectacular reduction of the correlation length is observed in the 0.1–0.3 ethanol molar fraction.

Synergetic Effects of Alanine and Glycine in Blob-Based Methods for Predicting Protein Folding Times.

The polypeptide PGlyAlaGlu was prepared with 20 mol % glycine (Gly), 36 mol % d,l-alanine (Ala), and 44 mol % d,l-glutamic acid (Glu) and labeled with the dye 1-pyrenemethylamine to yield a series of Py-PGlyAlaGlu samples. The fluorescence decays of the Py-PGlyAlaGlu samples were analyzed according to the fluorescence blob model (FBM) to obtain the number Nblobexp of amino acids (aa's) encompassed inside the subvolume Vblob of the polypeptide probed by an excited pyrene. An Nblobexp value of 29 (±2) was retrieved for Py-PGlyAlaGlu, which was much larger than for any of the copolypeptide PGlyGlu or PAlaGlu prepared with either Gly and Glu or Ala and Glu, respectively. The continuous increase in Nblobexp with decreasing side chain size (SCS) from 10 aa's for PGlu to 16 aa's for PAlaGlu and 22 aa's for PGlyGlu was used earlier to define the reach of an aa and determine the groups of aa's that could interact with each other along a polypeptide backbone according to their SCS. These groups of aa's, referred to as blobs, led to the implementation of blob-based models (BBM) to predict the folding time τFtheo,BBM of 145 proteins, which was found to match their experimental folding time τFexp with a relatively high 0.71 correlation coefficient. Nevertheless, the much higher Nblobexp value found for Py-PGlyAlaGlu compared to all other pyrene-labeled polypeptides studied to date indicates that the reach of aa's along a polypeptide sequence is affected not only by SCS but also by synergetic effects between different aa's. Following this new insight, a revised BBM was implemented to predict τFtheo,BBM for 195 proteins assuming the existence or absence of synergies to control the interactions between aa's along a polypeptide sequence. Similarly good correlation coefficients of 0.71 and 0.74 were obtained for a direct 1:1 comparison of τFexp and τFtheo,BBM for the 195 proteins without and with synergies, respectively. This result suggests that synergetic effects between different aa's have little effect on τFtheo,BBM predicted from BBM underlying the robustness of this methodology.

Topology-driven spatial organization of ring polymers under confinement.

Entropic repulsion between DNA ring polymers under confinement is a key mechanism governing the spatial segregation of bacterial DNA before cell division. Here we establish that "internal" loops within a modified-ring polymer architecture enhance entropic repulsion between two overlapping polymers confined in a cylinder. Interestingly, they also induce entropy-driven spatial organization of polymer segments as seen in vivo. Here we design polymers of different architectures in our simulations by introducing a minimal number of cross-links between specific monomers along the ring-polymer contour. The cross-links are likely induced by various bridging proteins inside living cells. We investigate the segregation of two polymers with modified topologies confined in a cylinder, which initially had spatially overlapping configurations. This helps us to identify the architectures that lead to higher success rates of segregation. We also establish the mechanism that leads to localization of specific polymer segments. We use the blob model to provide a theoretical understanding of why certain architectures lead to enhanced entropic repulsive forces between the polymers. Lastly, we establish a correspondence between the organizational patterns of the chromosome of the C.crescentus bacterium and our results for a specifically designed polymer architecture. However, the principles outlined here pertaining to the organization of polymeric segments are applicable to both synthetic and biological polymers.

Confinement free energy for a polymer chain: Corrections to scaling.

Spatial confinement of a polymer chain results in a reduction of conformational entropy. For confinement of a flexible N-mer chain in a planar slit or cylindrical pore (confining dimension D), a blob model analysis predicts the asymptotic scaling behavior ΔF/N ∼ D-γ with γ ≈ 1.70, where ΔF is the free energy increase due to confinement. Here, we extend this scaling analysis to include the variation of local monomer density upon confinement giving ΔF/N ∼ D-γ(1 - h(N, D)), where the correction-to-scaling term has the form h ∼ Dy/NΔ with exponents y = 3 - γ ≈ 1.30 and Δ = 3/γ - 1 ≈ 0.76. To test these scaling predictions, we carry out Wang-Landau simulations of confined and unconfined tangent-hard-sphere chains (bead diameter σ) in the presence of a square-well trapping potential. The fully trapped chain provides a common reference state, allowing for an absolute determination of the confinement free energy. Our simulation results for 32 ≤ N ≤ 1024 and 3 ≤ D/σ ≤ 14 are well-described by the extended scaling relation giving exponents of γ = 1.69(1), y = 1.25(2), and Δ = 0.75(6).

Conservative integrators for vortex blob methods on the plane

Conservative integrators have recently been derived for many-body systems using the Discrete Multiplier Method, specifically for the point vortex problems in [1]. In the present paper, we extend this work to derive conservative symmetric second-order one-step integrators for a family of vortex blob models. A rational function approximation was used to approximate the exponential integral which appears in the Hamiltonian. Conservative properties and second order convergence are proved. Numerical experiments are shown to verify the conservative property of these integrators, their second-order accuracy, and as well as the resulting spatial and temporal accuracy of the vortex blob method. Moreover, the derived implicit conservative integrators are shown to better preserve conserved quantities than standard higher-order explicit integrators on comparable computation times.

Relationship between Gel Mesh and Particle Size in Determining Nanoparticle Diffusion in Hydrogel Nanocomposites.

The diffusion of poly(ethylene glycol) methyl ether thiol (PEGSH)-functionalized gold nanoparticles (NPs) was measured in polyacrylamide gels with various cross-linking densities. The molecular weight of the PEGSH ligand and particle core size were both varied to yield particles with hydrodynamic diameters ranging from 7 to 21 nm. The gel mesh size was varied from approximately 36 to 60 nm by controlling the cross-linking density of the gel. Because high-molecular-weight ligands are expected to yield more compressible particles, we expected the diffusion constants of the NPs to depend on their hard/soft ratios (where the hard component of the particle consists of the particle core and the soft component of the particle consists of the ligand shell). However, our measurements revealed that NP diffusion coefficients resulted primarily from changes in the overall hydrodynamic diameter and not the ratio of particle core size to ligand size. Across all particles and gels, we found that the diffusion coefficient was well predicted by the confinement ratio calculated from the diameter of the particle and an estimate of the gel mesh size obtained from the elastic blob model and was well described using a hopping model for nanoparticle diffusion. These results suggest that the elastic blob model provides a reasonable estimate of the mesh size that particles "see" as they diffuse through the gel. This work brings new insights into the factors that dictate how NPs move through polymer gels and will inform the development of hydrogel nanocomposites for applications such as drug delivery in heterogeneous, viscoelastic biological materials.

Pyrene Excimer Formation (PEF) and Its Application to the Study of Polypeptide Dynamics.

This Perspective describes how the fluorescence blob model (FBM) has been developed and applied over the past 30 years to characterize the long-range backbone dynamics (LRBD) of polymers in solution. In these experiments, the polymers are randomly labeled with the dye pyrene, which forms an excimer upon the encounter between an excited and a ground-state pyrenyl label inside a finite subvolume of the polymer coil referred to as a blob representing the volume probed by the excited pyrene. By compartmentalizing the polymer coil into a cluster of identical blobs, FBM analysis of the fluorescence decays acquired with the polymers yields the number Nblob of structural units inside a blob. Since a flexible or rigid backbone will result in an Nblob that is either large or small, Nblob can be used as a measure of the flexibility of a given polymer. After having established that these experiments based on pyrene excimer formation (PEF) yielded quantitative information about the LRBD of a variety of polymers in solution, control experiments were carried out to characterize the effects that different molecular variables, such as the side-chain size (SCS) of a structural unit or the length of the linker connecting pyrene to the polymeric backbone, had on the parameters retrieved with the FBM. At this point, the FBM was applied to study the LRBD of polypeptides prepared from racemic mixtures of amino acids (aa's). These studies led to the establishment of simple rules that could be developed into mathematical equations to describe the LRBD of polypeptides. The Nblob values retrieved from the FBM analysis of the fluorescence decays acquired with the pyrene-labeled polypeptides could then be employed to predict the total conformational search time (τtcs) of any polypeptide based on their sequence. Strong correlations were found between the predicted τtcs and the experimental folding times of 145 proteins. The good quality of these correlations suggests that the blob-based approach described in this report might represent an interesting mathematical means for studying protein folding.

A bioinspired synthetic soft hydrogel for the treatment of dry eye.

Natural soft hydrogels are unique elastic soft materials utilized by living organisms for protecting delicate tissues. Under a theoretical framework derived from the Blob model, we chemically crosslinked high molecular weight hyaluronic acid at a concentration close to its overlap concentration (c*), and created synthetic soft hydrogels that exhibited unique rheological properties similar to a natural soft hydrogel: being dominantly elastic under low shear stress while being viscous when the stress is above a small threshold. We explored a potential application of the hyaluronic acid‐based soft hydrogel as a long‐acting ocular surface lubricant and evaluated its therapeutic effects for dry eye. The soft hydrogel was found to be biocompatible after topical instillation on experimental animals' and companion dogs' eyes. In a canine clinical study, twice‐a‐day ocular instillation of the soft hydrogel in combination with cyclosporine for 1 month improved the clinical signs in more than 65% of dog patients previously unresponsive to cyclosporine treatment.

Unfolding of Helical Poly(L-Glutamic Acid) in N,N-Dimethylformamide Probed by Pyrene Excimer Fluorescence (PEF).

The denaturation undergone by α–helical poly(L-glutamic acid) (PLGA) in N,N-dimethylformamide upon addition of guanidine hydrochloride (GdHCl) was characterized by comparing the fluorescence of a series of PLGA constructs randomly labeled with the dye pyrene (Py-PLGA) to that of a series of Py-PDLGA samples prepared from a racemic mixture of D,L-glutamic acid. The process of pyrene excimer formation (PEF) was taken advantage of to probe changes in the conformation of α–helical Py-PLGA. Fluorescence Blob Model (FBM) analysis of the fluorescence decays of the Py-PLGA and Py-PDLGA constructs yielded the average number (<Nblob>) of glutamic acids located inside a blob, which represented the volume probed by an excited pyrenyl label. <Nblob> remained constant for randomly coiled Py-PDLGA but decreased from ~20 to ~10 glutamic acids for the Py-PLGA samples as GdHCl was added to the solution. The decrease in <Nblob> reflected the decrease in the local density of PLGA as the α–helix unraveled in solution. The changes in <Nblob> with GdHCl concentration was used to determine the change in Gibbs energy required to denature the PLGA α–helix in DMF. The relationship between <Nblob> and the local density of macromolecules can now be applied to characterize the conformation of macromolecules in solution.

Electrostatics, conformation, and rheology of unentangled semidilute polyelectrolyte solutions

Polyelectrolyte (PE) solutions, which are charged polymers in polar solvents, are ubiquitous and essential to life. Due to the electrostatic interactions among the charged monomers and mobile ions, the dependence of the rheological properties on the polymer concentration of PE solutions differs significantly from that of solutions of uncharged macromolecules. In addition, salt in PE solutions, whether added intentionally or intrinsically present, can affect the properties of the solutions. Here, we analyze the ion distribution near a monomer using the nonlinear Poisson–Boltzmann equation for scenarios with nonoverlapping and overlapping electric double layers. Consequently, by incorporating the electrostatic interactions into the blob model and Zimm–Rouse dynamic model, we obtain different scaling laws for the electrostatic energy per monomer, correlation length, end-to-end distance, relaxation time and viscosity η of semidilute, unentangled PE solutions in consecutive regimes of polymer concentration np, and salt concentration ns, which are summarized in tables. With our theory, we anticipate that the empirical Fuoss law η∝np1/2 is expected for solutions prepared with salt-contaminated PE samples, while η∝np0.68 might be found for those with purer PE samples. A new critical charge fraction φc is defined, where we predict that the peak, which is present in the measurements of the reduced viscosity η/np as a function of np, is only expected for weakly charged PEs φ&amp;lt;φc prepared with pure PE samples. Comparisons with the experimental data as well as the classical scaling theories are provided, and the range of applicability of the theory is discussed.

Blob-Based Predictions of Protein Folding Times from the Amino Acid-Dependent Conformation of Polypeptides in Solution

A combination of fluorescence blob model and molecular mechanics optimizations was applied to determine the number (Nblob) of amino acids (aa’s) located in the volume probed by an excited pyrene for poly(l-lysine) labeled with 1-pyrenebutyric acid (PyBu-PLL), as it adopted an extended conformation in DMSO. The Nblob value of PyBu-PLL was then compared to those of other pyrene-labeled polypeptides adopting a similarly extended conformation over the length scale of a blob to quantify the contribution of the pyrenyl label to Nblob. After subtracting the pyrene contribution, the plot of the corrected Nblob as a function of aa side chain reach (SCR) led to the conclusion that, for a polypeptide in an extended conformation, increasing the SCR of an aa by one bond resulted in an ∼1.8 aa increase in Nblob. This information could then be applied to predict the intrinsic Nblob value (N(SCR)) of any aa based on its SCR as long as this aa was part of an aa sequence in an extended conformation. The increase in Nblob resulting from the conformational freedom imparted by smaller aa’s to a polypeptide backbone was accounted for by multiplying N(SCR) with the bending function fb(SCS), which was determined experimentally for the different side chain sizes (SCS) of aa’s. By scanning the sequence of a protein, an Nblob value was calculated for each aa from its N(SCR) and fb(SCS) values. The Nblob values were then sorted from largest to lowest and averaged to yield ⟨Nblob⟩. Renormalization group theory was applied to determine the number (Ω) of conformations that would be available to a protein based on its ⟨Nblob⟩ value and the number of blobs found in the protein sequence. Multiplying Ω by the time required by an aa to adopt its possible conformations yielded the protein folding time (τF). A correlation coefficient of 0.73 was obtained from the comparison of the calculated and experimentally determined τF values, demonstrating the ability of this blob-based approach to predict the protein folding times within ±1.4 orders of magnitude from the experimentally determined value.

Automatic Detection of Individual Trees from VHR Satellite Images Using Scale-Space Methods.

This research investigates the use of scale-space theory to detect individual trees in orchards from very-high resolution (VHR) satellite images. Trees are characterized by blobs, for example, bell-shaped surfaces. Their modeling requires the identification of local maxima in Gaussian scale space, whereas location of the maxima in the scale direction provides information about the tree size. A two-step procedure relates the detected blobs to tree objects in the field. First, a Gaussian blob model identifies tree crowns in Gaussian scale space. Second, an improved tree crown model modifies this model in the scale direction. The procedures are tested on the following three representative cases: an area with vitellaria trees in Mali, an orchard with walnut trees in Iran, and one case with oil palm trees in Indonesia. The results show that the refined Gaussian blob model improves upon the traditional Gaussian blob model by effectively discriminating between false and correct detections and accurately identifying size and position of trees. A comparison with existing methods shows an improvement of 10–20% in true positive detections. We conclude that the presented two-step modeling procedure of tree crowns using Gaussian scale space is useful to automatically detect individual trees from VHR satellite images for at least three representative cases.

Isospin Effect on fragment productions and reaction mechanisms for Ni+Ca systems at 25 AMeV

Main features of fragments produced in 58Ni+40Ca and 58Ni+48Ca systems at 25 AMeV, collected by Chimera multidetector at INFN Laboratori Nazionali del Sud (Italy), are analysed in order to study properties of moderately excited systems (Ex=2-5 AMeV) formed in central collisions. By means of correlations between specific global variables, sensitive to the centrality of the collision, a stringent selection of Fusion-Evaporation residues, Multifragmentation sources and their decay products has been performed. The influence of the isospin contents of the systems on the reaction mechanism is carefully investigated and relevant observables such as isospin asymmetry N/Z or charge and mass of fragments, are discussed in order to probe the fragmentation path, characterised by short living ( ∼ 100fm/c) low density states of nuclear matter out of equilibrium. Preliminary comparisons with predictions of the BLOB model, in a semi-classical mean field framework, are presented, investigating cluster production emerging from the occurrence of low-density instabilities in central collisions.