1,040 research outputs found
Distribution and Implications of Sponge Spicules in Surficial Deposits in Ohio
Author Institution: Agronomy Department, The Ohio State University, Columbus, OhioMicroscopic examination of biogenic opal isolated from the 0.05-0.02-mm total mineral fraction of 12 upland soil profiles indicates that fragments of sponge spicules are minor but ubiquitous constituents of Ohio soils, with major concentrations in the upper 10 to 15 inches of the profile. Quantities range from about 30 to 2000 parts per million biogenic opal or 1 to 65 parts per 10 million parts soil. Spicules are absent or extremely rare in calcareous Wisconsin-age till deposits. Their correlation with horizons high in silt content (50-75%), and their size and depth distribution in landscape positions which preclude an authigenic origin, indicate their aeolian transport from aquatic source areas with other loessial materials. Identification of spicules thus provides direct evidence that these horizons have been derived from loess or loess-till admixtures. This microscopic technique may serve useful for the identification of loess when field or laboratory particle-size analysis yields inconclusive evidence
Formation of a cultivated spodosol in east-central Finland
The processes involved in Spodosol (Podzol) formation are still being debated. The pedogenic processes in a Spodosol, 10,700 years-old, at Sotkamo that has been cultivated for about 50 years were studied by characterizing the morphology and analyzing the major chemical properties, texture and mineralogy. Before cultivation, organic acids produced by decomposition of organic matter from pine litter in O and A horizons had weathered primary minerals in A and E horizons releasing Al and Fe. Percolating waters moved the organo-metallic complexes from A and E horizons to Bhsm and Bs horizons where the complexes coated and bridged sand grains eventually forming cemented ortstein. Because of the high biotite content of the parent material, the index of accumulation of Fe and Al in the Bhsm horizon (Al + 0.5 Fe = 4.1%) was the highest reported in Spodosols of Finland. The data support the theory of downward movement of Al and Fe as organo-metallic complexes with formation of some ferrihydrite but little or no formation of imogolite type materials. Little, if any, podzolization has likely occurred since the initiation of cultivation because, after agricultural liming and consequent increase of pH in the Ap horizon, organic compounds are likely to chelate Ca and Mg rather than Al and Fe.;Karkeille hietamaille syntyneet podsolit ovat Suomen kehittyneimpiä maannoksia. Niitä tutkimalla saadaan uutta tietoa tämän koko pohjoisella havumetsävyöhykkeelläyleisen maannostyypin kehittymiseen johtaneista prosesseista, joista edelleenkin vallitsee erilaisia käsityksiä. Tämän tutkimuksen kohteena oli Sotkamossa karkealla hietamaalla oleva noin 50 vuotta viljelty maa, joka on ollut kuivillaan noin 10 700 vuotta. Muokkauskerroksen alapuolella oli huuhtoutumiskerros (valkomaa), joka sisälsi lähes pelkkää kvartsihiekkaa. Sen alapuolella oli noin 10 cm paksu rautapalsi eli iskostunut horisontti, johon ylempää orgaanisina kompleksiyhdisteinä huuhtoutuneet rauta ja alumiini ovat saostuneet. Mikroskoopilla voidaan nähdä, miten nämä saostuneet ainesosat peittävät kvartsihiekan jyväset ja sitovat ne yhteen. Tässä horisontissa oli erittäin runsaasti heikosti kiteytynyttä rautaoksidia, joka on uutettavissa ammoniumoksalaattiliuoksella, kun taas valkomaassa tällaista rautaa oli erittäin vähän. Rikastumiskerroksen alumiinista valtaosa oli pyrofosfaattiin uuttuvassa, oletettavasti orgaanisen aineksen sitomassa muodossa, mikä viittaa aineiden kulkeutuneen tähän horisonttiin nimenomaan kelaatteina eikä epäorgaanisina kolloideina. Syvä kyntö on nostanut valkomaata ja kappaleita rikastumiskerroksen iskostumasta myös muokkauskerrokseen. Rikastumiskerroksen alapuolella kvartsihiekkajyvästen pinnoilla ei ollut paljonkaan rautasaostumia, mutta mikroskoopilla näkyi runsaasti rapautumatonta biotiittia. Rikastumiskerroksen rauta lienee suureksi osaksi peräisin juuri biotiitista, joka on kokonaisuudessaan rapautunut pintamaasta. Vähemmän biotiittia sisältäviin maihin ei todennäköisesti kehity näin vahvaa rikastumiskerrosta maan pienemmän rautapitoisuuden takia. Podsoloituminen on luultavasti pysähtynyt sen jälkeen, kun maa on otettu viljelyyn ja sen pintaosien pH on kalkituksen seurauksena noussut
Self-assembly and crystallisation of indented colloids at a planar wall
We report experimental and simulation studies of the structure of a monolayer
of indented ("lock and key") colloids, on a planar surface. On adding a
non-absorbing polymer with prescribed radius and volume fraction, depletion
interactions are induced between the colloids, with controlled range and
strength. For spherical particles, this leads to crystallisation, but the
indented colloids crystallise less easily than spheres, in both simulation and
experiment. Nevertheless, simulations show that indented colloids do form
plastic (rotator) crystals. We discuss the conditions under which this occurs,
and the possibilities of lower-symmetry crystal states. We also comment on the
kinetic accessibility of these states.Comment: 8 pages, 8 figure
Interfacial tension of the isotropic--nematic interface in suspensions of soft spherocylinders
The isotropic to nematic transition in a system of soft spherocylinders is
studied by means of grand canonical Monte Carlo simulations. The probability
distribution of the particle density is used to determine the coexistence
density of the isotropic and the nematic phases. The distributions are also
used to compute the interfacial tension of the isotropic--nematic interface,
including an analysis of finite size effects. Our results confirm that the
Onsager limit is not recovered until for very large elongation, exceeding at
least L/D=40, with L the spherocylinder length and D the diameter. For smaller
elongation, we find that the interfacial tension increases with increasing L/D,
in agreement with theoretical predictions.Comment: 8 pages, 7 figures, and also 1 tabl
Critical phenomena in colloid-polymer mixtures: interfacial tension, order parameter, susceptibility and coexistence diameter
The critical behavior of a model colloid-polymer mixture, the so-called AO
model, is studied using computer simulations and finite size scaling
techniques. Investigated are the interfacial tension, the order parameter, the
susceptibility and the coexistence diameter. Our results clearly show that the
interfacial tension vanishes at the critical point with exponent 2\nu ~ 1.26.
This is in good agreement with the 3D Ising exponent. Also calculated are
critical amplitude ratios, which are shown to be compatible with the
corresponding 3D Ising values. We additionally identify a number of subtleties
that are encountered when finite size scaling is applied to the AO model. In
particular, we find that the finite size extrapolation of the interfacial
tension is most consistent when logarithmic size dependences are ignored. This
finding is in agreement with the work of Berg et al.[Phys. Rev. B, V47 P497
(1993)]Comment: 13 pages, 16 figure
Phase behavior of a fluid with competing attractive and repulsive interactions
Fluids in which the interparticle potential has a hard core, is attractive at
moderate separations, and repulsive at greater separations are known to exhibit
novel phase behavior, including stable inhomogeneous phases. Here we report a
joint simulation and theoretical study of such a fluid, focusing on the
relationship between the liquid-vapor transition line and any new phases. The
phase diagram is studied as a function of the amplitude of the attraction for a
certain fixed amplitude of the long ranged repulsion. We find that the effect
of the repulsion is to substitute the liquid-vapor critical point and a portion
of the associated liquid-vapor transition line, by two first order transitions.
One of these transitions separates the vapor from a fluid of spherical
liquidlike clusters; the other separates the liquid from a fluid of spherical
voids. At low temperature, the two transition lines intersect one another and a
vapor-liquid transition line at a triple point. While most integral equation
theories are unable to describe the new phase transitions, the Percus Yevick
approximation does succeed in capturing the vapor-cluster transition, as well
as aspects of the structure of the cluster fluid, in reasonable agreement with
the simulation results.Comment: 15 pages, 20 figure
Wetting of a symmetrical binary fluid mixture on a wall
We study the wetting behaviour of a symmetrical binary fluid below the
demixing temperature at a non-selective attractive wall. Although it demixes in
the bulk, a sufficiently thin liquid film remains mixed. On approaching
liquid/vapour coexistence, however, the thickness of the liquid film increases
and it may demix and then wet the substrate. We show that the wetting
properties are determined by an interplay of the two length scales related to
the density and the composition fluctuations. The problem is analysed within
the framework of a generic two component Ginzburg-Landau functional
(appropriate for systems with short-ranged interactions). This functional is
minimized both numerically and analytically within a piecewise parabolic
potential approximation. A number of novel surface transitions are found,
including first order demixing and prewetting, continuous demixing, a
tricritical point connecting the two regimes, or a critical end point beyond
which the prewetting line separates a strongly and a weakly demixed film. Our
results are supported by detailed Monte Carlo simulations of a symmetrical
binary Lennard-Jones fluid at an attractive wall.Comment: submitted to Phys. Rev.
Modelling colloids with Baxter's adhesive hard sphere model
The structure of the Baxter adhesive hard sphere fluid is examined using
computer simulation. The radial distribution function (which exhibits unusual
discontinuities due to the particle adhesion) and static structure factor are
calculated with high accuracy over a range of conditions and compared with the
predictions of Percus--Yevick theory. We comment on rigidity in percolating
clusters and discuss the role of the model in the context of experiments on
colloidal systems with short-range attractive forces.Comment: 14 pages, 7 figures. (For proceedings of "Structural arrest in
colloidal systems with short-range attractive forces", Messina, December
2003
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