Different Modes Of Growth Research Articles

Modelling surface growth in IBAD with rate equations

AbstractWe develop a rate-equation model for surface growth in ion beam assisted deposition. In terms of the rate coefficients the model describes how island detachment and breakup, adatom diffusion and interlayer transitions of adatoms affect the growth. We identify parameter values corresponding to different modes of growth and discuss our results and ways to improve the model.

Eden growth model for aggregation of charged particles

The stochastic Eden model of charged particles aggregation in two-dimensional systems is presented. This model is governed by two parameters: screening length of electrostatic interaction, $\lambda $, and short range attraction energy, $E$. Different patterns of finite and infinite aggregates are observed. They are of following types of morphologies: linear or linear with bending, warm-like, DBM (dense-branching morphology), DBM with nucleus, and compact Eden-like. The transition between the different modes of growth is studied and phase diagram of the growth structures is obtained in $\lambda, E $ co-ordinates. The detailed aggregate structure analysis, including analysis of their fractal properties, is presented. The scheme of the internal inhomogeneous structure of aggregates is proposed.

Simulated distributions of the density of cell adhesion molecules over branching processes with different geometry and intracellular trafficking

Adhesive cell-cell and cell-substrate interactions mediated by different types of cell adhesion molecules (CAM) are important for growth and migration processes. In simulation study we investigated the impact of geometry of branching cellular processes on the lateral distribution of CAM due to retrograde lateral diffusion from a growing part, where they were delivered by different assumed types of trafficking. The model incorporates trafficking of CAM to and installation in the growing active part of the cell, their lateral diffusive redistribution, formation and dissociation of CAM/ligand complexes, and CAM internalization by endocytosis. Since the rate of growth is two and one order(s) of magnitude slower than the rate of trafficking and lateral diffusion, respectively, steady state distributions of CAM were considered. Three possible types of intracellular CAM partitioning between sister branches were considered: equal, proportional to the branch cross-section area, and proportional to the branch surface area. Asymmetry of branching led to various inhomogeneous distributions of the CAM surface density along the branches, and these distributions depended on the type of intracellular trafficking, which might provide a basis for different modes of growth. One can speculate that, depending on these modes, initially asymmetrical branching can be either reinforced or symmetrized during further development.

Preparation and characterization of hollow fibers of silicalite-1

Hollow fibers consisting solely of silicalite-1 were prepared by a novel method consisting of four basic steps. Carbon fibers were first surface modified to facilitate adsorption of colloidal seed crystals of silicalite-1. Such particles were then adsorbed as a monolayer on the fiber surface and induced to grow into a continuous film of intergrown crystals of silicalite-1. Finally, the carbon fiber used as a template was removed by calcination in air, yielding hollow fibers of silicalite-1. Hollow fibers were prepared in two experimental series employing different synthesis conditions resulting in two different modes of growth. The first experimental series was performed with a clear homogeneous TPAOH/silica solution crystallizing at 100°C, and the second series with a NaOH-TPABr/silica gel crystallizing at 150°C. Materials produced in the first series showed a smooth and homogeneous surface and a relatively uniform fiber wall with a final thickness of 1.1 μm. In these materials the film was formed by crystals growing outward from the surface of the carbon fiber. Materials produced in the second series were less uniform with a final thickness of the fiber wall of about 3.2 μm. The crystal growth in these materials appeared to take place both in an inward and outward direction starting from the seed crystals adsorbed on the carbon fiber. The hollow fibers prepared were characterized by SEM, XRD, FT i.r., and nitrogen adsorption measurements.

Manufacture of zeolite membranes

Hollow fibers consisting solely of silicalite-1 were prepared by a novel method consisting of four basic steps. Carbon fibers were first surface modified to facilitate adsorption of colloidal seed crystals of silicalite-1. Such particles were then adsorbed as a monolayer on the fiber surface and induced to grow into a continuous film of intergrown crystals of silicalite-1. Finally, the carbon fiber used as a template was removed by calcination in air, yielding hollow fibers of silicalite-1. Hollow fibers were prepared in two experimental series employing different synthesis conditions resulting in two different modes of growth. The first experimental series was performed with a clear homogeneous TPAOH/silica solution crystallizing at 100°C, and the second series with a NaOH-TPABr/silica gel crystallizing at 150°C. Materials produced in the first series showed a smooth and homogeneous surface and a relatively uniform fiber wall with a final thickness of 1.1 μm. In these materials the film was formed by crystals growing outward from the surface of the carbon fiber. Materials produced in the second series were less uniform with a final thickness of the fiber wall of about 3.2 μm. The crystal growth in these materials appeared to take place both in an inward and outward direction starting from the seed crystals adsorbed on the carbon fiber. The hollow fibers prepared were characterized by SEM, XRD, FT i.r., and nitrogen adsorption measurements.

Electron microscopy characterization of TiN films on Si, grown by d.c. reactive magnetron sputtering

The structural characteristics of titanium nitride films deposited by d.c. reactive magnetron sputtering on (001) silicon wafers were studied by cross-section transmission electron microscopy and in-situ spectroscopic ellipsometry (SE). The growth habit of the films, the shape and the size of the crystallites were investigated versus deposition temperature, bias voltage and nitrogen flow rate. The surface micro-roughness was also studied by atomic force microscopy. The films exhibit a columnar growth showing preferred orientation along (111) direction. High-resolution transmission electron microscopy (TEM) observations reveal that the Si substrate does not affect the mode of growth. The SE measurements and TEM observations strongly suggest that TiN rich in nitrogen is formed at nitrogen flow rates in the range 1.9–3 sccm. At a nitrogen flow rate F N = 1.7 sccm the stoichiometric compound Ti 2N is formed which exhibits a completely different mode of growth. The ability of the in-situ SE to monitor small structural changes induced by small changes of the growth conditions was tested by the deposition of sequential layers. In this respect SE is a powerful method to control TiN deposition.

Growth, Gravitropism, and Endogenous Ion Currents of Cress Roots (Lepidium sativum L.)

A novel, three-dimensional recording, vibrating probe was used for measuring the density and direction of the endogenous ionic current of cress roots (Lepidium sativum L.) bathed in low salt media (artificial pond water, APW). Roots submerged in regular APW and growing vertically show the following current pattern. Current of 0.7 microampere/square centimeter density enters or leaves the root cap; the current changes direction frequently. Current of 1.6 microamperes/square centimeter enters the meristem zone most of the time. Maximum current with a density of 2.2 microamperes/square centimeter enters the apical elongating zone, i.e. between 0.8 and 1.2 millimeters behind the root tip. The current density decreases to 1.4 microamperes/square centimeter at 2 millimeters, i.e. in the central elongating zone, and to 1.0 microampere/square centimeter at 3 millimeters, i.e. in the basal elongating zone. The current direction changes from inward to predominantly outward between 1.2 and 3 millimeters behind the tip. Measurements on opposite flanks of the roots indicate that the current pattern is fairly symmetrical. After placing the roots horizontally, the density of the endogenous current remains stable, but the current direction changes at the root cap and in the meristem zone. The current leaves the root on the upper side and enters on the lower side, causing a highly asymmetrical current pattern at the very tip. The current pattern at the upper and lower side further away from the tip remains the same as in vertical roots. Roots submerged in low Ca(2+) APW show a very different current pattern, no gravitropism, and no change of the current pattern after horizontal orientation. In these roots current enters the root cap and the basal elongating zone and leaves the apical elongating zone. Three conclusions are drawn from these results: First, plant roots elongate by two different modes of growth that are correlated with different current directions. They grow by cytoplasmic enlargement at sites of inward current and by turgor-driven elongation at sites of outward current. Second, a change in the current pattern at the root cap and in the meristem zone is a clear indicator of later gravitropism. Third, Ca(2+) ions are involved in the gravistimulated change in the current pattern, probably affecting the activity of plasmalemma H(+)-ATPases.

Modeling of crystal growth during rapid solidification

Models for computer simulation of solidification of alloys in two different geometries have been developed. These models allow us to study the roles of crystal nucleation and heat transfer and the effects of different modes of growth on the evolution of the microstructure during solidification. In particular, by applying the powerful and versatile tool comprised of the solute-drag model and the thermodynamic modeling of the alloy system, we are able to predict the formation of a microstructure consisting of alternating layers parallel to the growth front. This type of banded microstructure, experimentally observed in Al-Cu, Al-Fe, and Ag-Cu alloys, has not been satisfactorily explained before.

TESTING FOR DEVELOPMENTAL CONSTRAINTS: CARPAL FUSIONS IN URODELES.

By definition a developmental constraint is a bias on the production of phenotypic variation caused by the action of developmental processes (Maynard Smith et al., 1985). While there have been many philosophical speculations about the nature of the underlying phenomena and about the impact of developmental constraints on evolution, attempts to measure these biases have been extremely rare. Gould (1989) succeeded in showing an actual constraint at work with his analysis of variation in land snails of the genus Cerion. A sharp transition between juvenile button-like growth and later dome-like growth acts in concert with a-probably ecological -constraint on final size, to produce an allometric transition from very flat shells, where the period of juvenile growth is prolonged relative to the period of dome-like growth, to very high shells, where the conditions are reversed. Gould termed this interplay between ecological restrictions, which set up the dimensions of the frame (size of the adult shell) and variations in the size of pieces (whorls), the jigsawconstraint. Because the growth of a shell is a smooth and continuous process, even if there is a succession of different modes of growth, conventional multivariate statistics can be applied fairly easily. However, heterochrony and differential growth are not the only important evolutionary events: qualitative differences and discontinuous jumps are clearly important (Alberch, 1982; Oster and Alberch, 1983). The analysis of phenomena showing discontinuous jumps usually results in data on the frequencies of different varieties. This would make the chi-square test or other related tests (cf. Sokal and Rohlf, 1981; Chap. 17) natural candidates for statistical evaluation of different developmental hypotheses. Unfortunately developmental hypotheses are just qualitative predictions and not sufficiently accurate to stand quantitative analysis. Most often we wish to determine whether a developmental hypothesis is better than the null hypothesis in which varieties are equally distributed. Hence, a test that provides an index of similarity between developmental hypothesis and empirical data and checks against the null hypothesis discussed above is the most desirable tool for analysis. Mantel's test of quadratic assignment offers these desirable properties (Mantel, 1967): first an index of similarity between two vectors or matrices (say A and B) is calculated (usually the Pearson product-moment correlation coefficient).

Modelling of crystal growth during rapid solidification

A model for computer simulation of solidification of alloys has been developed. This model allows us to study the roles of different nucleation temperatures and heat transfer, and the effects of different modes of growth on the evolution of the microstructure during solidification. In particular, by utilizing a powerful tool comprised of a solute-drag model and the thermodynamic modelling of the alloy system, we are able to predict the formation of a microstructure consisting of alternating layers parallel to the growth front. This type of banded microstructure, experimentally observed in AlCu, AlFe and AgCu alloys, has not been satisfactorily explained before.

Growth in encrusting cheilostome bryozoans: I. Evolutionary trends

Growth of the colony is a basic element of morphological evolution and life history in cheilostome bryozoans. Here we consider the occurrence of different modes of growth in encrusting cheilostomes through geologic time and in well-studied living associations. We assess patterns of zooid formation by direct examination of skeletal characters in species from nearly all diverse fossil assemblages reported from North America and quantify within-assemblage diversities and abundance rankings for fossil encrusting species with different modes of growth. These data document macroevolutionary trends showing a transition from dominance of an apparently primitive mode of budding in the Early Cretaceous to derived modes through the Tertiary. The trends are characterized by their long duration and apparent convergence among systematic subgroups within the Cheilostomata. We then consider the validity of our observations as adaptive trends. Patterns of ecological dominance among living and fossil species indicate that different patterns of zooid formation are important determinants of success of colonies as reflected by their abundance, competitive ability, survivorship, and recovery from injury or predation. The consistency of the long-term trends and evidence for the existence of ecological mechanisms in fossil assemblages suggest a major evolutionary role for biotic interactions.

Complementary Mating Types and Different Modes of Growth in Paramecium aurelia Clone

Summary Mating type differentiation in Paramecium aurelia is seen as controlled by a mechanism which makes it possible to unify the different hereditary systems described until nowadays. The “Odd” or “Even” mating types will be expressed according to the different growths of the experimental lines from sexual reorganization to maturity.

Vertex analysis of neural tree structures containing trichotomous nodes

Vertex analysis defines dichotomous networks in terms of the relative frequencies of pendant ( V P) and nodal ( V d) vertices. V a, V b and V c comprise the 3 classes of V d which connect 2, 1 and 0 V p respectively. The vertex ratio ( V R) of V a/ V b may be used to define topology since V a and V c approach equality in large networks. The introduction of trichotomous nodes ( V t, comprising V a′, V b′, V c′ and V d′, connecting 3, 2, 1 and 0 V p confounds vertex analysis in as much as the values of V R must be recalculated to maintain the definition of topology. Each V t is equivalent to 2 V d. V a′, V c′ and V d′ may be unequivocally transformed into V a + V b; V b + V c; and 2 V c respectively. However, V b, potentially contains V a, V b and V c in proportions which must be arbitrarily set, since the mode of growth only determines the frequencies of V t and not their transformation into V d pairs. We have assumed that all possible pairs of transformed V b, are equally likely and, accordingly, V b, is equivalent to V a/3 + 4 V b/3 + V c/3. The values of V R for different modes of growth, featuring different frequencies of V t, may thus be obtained by computer growth simulation. The mode of growth of observed networks may thereby be ascertained from V R, providing all V t are detectable in the tree.

Characterization of surface defects and determination of overlayer nucleation and growth by surface-sensitive diffraction

The physical properties of crystalline materials are strongly influenced by disturbances of the long-range order. In thin films, the extent and nature of the defects responsible for this loss of order are governed by the interatomic forces of the constituent material and the conditions under which the film is grown. Surface-sensitive diffraction techniques (such as LEED, RHEED and small-angle X-ray diffraction) are sensitive to the degree of order over areas large enough to be statistically representative of the surface conditions. We discuss the usefulness of surface-sensitive diffraction techniques not only in establishing the existence of ordered regions on the surface but in characterizing the nature of the disorder present at the surface. Qualitative features of diffraction patterns are discussed and the applicability of quantitative diffraction techniques are illustrated in determining size distributions of stepped surfaces and in investigations of the mechanism of nucleation and growth of overlayer films. For overlayer islands growing on an ordered substrate, we relate the separation of islands to the nucleation behavior and the distribution of island sizes to the mode of growth. The intensity distribution of diffracted beams is calculated under specific assumptions for the nucleation and growth of two-dimensional overlayer islands and it is shown that such diffraction measurements can differentiate between different modes of growth.

Hydrometeor Evolution in Rainbands over the California Valley

Abstract Hydrometeor distributions were measured in two rainbands that passed over the California Valley. The ground radar was used to vector the University of Wyoming's instrumented King Air aircraft to the top of the rainband at which time an onboard computer algorithm was used to make multiple penetrations of an ensemble of ice particles that were assumed to descend at 1 or 2 m s−1 while drifting downwind. Distribution parameters were calculated for each penetration and the changes in these parameters were used to infer the different modes of growth that the particles undergo. It was found that there were five distinct regions of particle growth. Nucleation and depositional growth were the dominant processes near the top of the rainband. The hydrometeors then fell through a region of aggregation from the −20°C level to the − 10°C level. Ice crystal multiplication was the dominant process from the −10°C level to the −4° level. From −4 to O°C, aggregation was again the dominant processes. Below the 0°C l...

Postnatal modifications of the dendritic tree of cells in the inferior colliculus of the cat. A quantitative Golgi analysis.

Postnatal modifications of dendrites have been quantitatively studied by network analysis of the dendritic tree in the two central nuclei of the inferior colliculus in the cat. This analysis revealed cells with two types of branching patterns suggesting two different modes of growth. The predominant pattern is characterized by dichotomous branching on random segments (DR cells). However, a purely collateral branching pattern is particular to certain cells (CB cells). These two branching patterns were found in both nuclei of the IC in adult and young cats, but the exact significance of these two cell types remains unclear. The dendritic trees of cells in kittens differed from those of the adult cat. Also, the types of modification were different in the two functionally distinct nuclei of the inferior colliculus that we studied. The most dramatic modifications were observed in the dendritic tree of DR cells in the central nucleus, which receives fibers from the auditory nuclei in the brainstem. Two parameters were modified: the mean number of terminal segments and the mean total length of segments. Both parameters increased in the sagittal plane and decreased in the frontal plane. These modifications indicate a reorientation of the dendritic tree in the sagittal plane, along the incoming axons from the auditory nuclei. As these afferents become functionally mature only after birth, this spatial remodeling of the dendrites seems closely related to functional maturation of secondary auditory axons. In the dorsomedial nucleus that receives fibers from the auditory cortices, the dendritic tree of DR cells also undergoes spatial reorientation. This is more evident in the horizontal plane and with respect to the incoming axons. Our results suggest that the characteristic orientation of the dendritic tree of cells observed in the inferior colliculus of the adult cat is established only after the first postnatal weeks. This orientation seems to result from an active process of remodeling concomitant with the functional maturation of afferents, a fact already established for various cell types in the nervous system.

A quantitative cytochemical study of the growth of individual mast cells.

For individual mast cells, relationships between their dry mass and their content of heparin and 5-hydroxytryptamine (5-HT) were studied. This was achieved by measuring these parameters successively on identical cells, by means of quantitative cytochemical techniques. The peritoneal mast cells have a very long life span and a slow turnover of granule components. Increase of the dry weight of the cells may therefore be taken as an expression of cellular growth. Mast cell populations from younger and older animals were analysed in an attempt to evaluate the influence of cell-aging and animal-aging on the growth of the mast cells. The analysis was based on allometric (log-log) plots and linear regressions. Within the cell populations there were strong mutual correlations between the cell parameters studied, without any obvious deviations from linearity. However, the slopes of the allometric lines indicated a somewhat different mode of growth for mast cell from younger and older animals. The capacity of the mast cells to accumulate 5-HT after a single injection of its precursor, 5-hydroxytryptophan, was used as a functional test. In relation to the cell weight, the induced increase of 5-HT was greater for lighter than for heavier mast cells. This difference between light and heavy mast cells was greater for cells from younger than from older animals. These differences in growth and functional properties between mast cells from younger and older animals were interpreted as an effect of the animals rather than of aging of the cells.

Cisternal abnormalities produced by clinical tumours in the posterior cranial fossa. III. Tumours of the brain stem and pontine angle.

The pneumographic appearances of the subarachnoid cisterns have been analysed in detail in a clinical series of 17 patients with brain stem tumours and in 40 with pontine angle tumours. The cisternal distortions may be used to distinguish tumours in the pons from tumours in the medulla oblongata and also to distinguish tumours of the pontine angle with different modes of growth, e.g. tumours mainly indenting the cerebellar hemisphere from those mainly indenting the brain stem. The results should be applicable also for an analysis of cisternal pathology by other methods such as computer tomography and cisternography with positive contrast media.

Optical Absorption Studies of the Growth of Microcrystals in Nascent Suspensions III. Absorption Spectra of Nascent Silver Iodide Hydrosols

AbstractThe optical absorption of nascent silver iodide hydrosols prepared by rapid mixing of dilute solutions of silver and iodide salts has been studied in the 350-500 nm region. The heights and wavelengths of characteristic absorption peaks and shoulders of the sols varied with time in a manner depending on excess lattice ion and the protective colloid or the surfactant .spe~ies added. and their concentrations. The different modes of variation of the spectra have been Interpreted as orlgmatmg from different modes of growth of silver iodide sub-microcrystals in the sols. Electron microscopy of the cry tals confirmed thi interpretation.

Crystal habit modification by water soluble polymers

The crystal habits of SrSO 4 crystals grown from 0.015 M solution with and without the addition of polyvinyl sulfonate are compared. Pure strontium sulfate precipitate consists of single crystals, about some tens of microns long, with distinctly visible growth steps. Crystals grown with polyvinyl sulfonate in stirred solutions show a striking change of habit: they are sponge-like structures, composed of minute crystallites, of size in the nm range. The different modes of growth and the time-lag for precipitation in the presence of the polymer are explained by the stabilization of the critical nuclei of SrSO 4, possibly nucleating on the polymer molecules.