393 research outputs found
From Capillary Condensation to Interface Localization Transitions in Colloid Polymer Mixtures Confined in Thin Film Geometry
Monte Carlo simulations of the Asakura-Oosawa (AO) model for colloid-polymer
mixtures confined between two parallel repulsive structureless walls are
presented and analyzed in the light of current theories on capillary
condensation and interface localization transitions. Choosing a polymer to
colloid size ratio of q=0.8 and studying ultrathin films in the range of D=3 to
D=10 colloid diameters thickness, grand canonical Monte Carlo methods are used;
phase transitions are analyzed via finite size scaling, as in previous work on
bulk systems and under confinement between identical types of walls. Unlike the
latter work, inequivalent walls are used here: while the left wall has a
hard-core repulsion for both polymers and colloids, at the right wall an
additional square-well repulsion of variable strength acting only on the
colloids is present. We study how the phase separation into colloid-rich and
colloid-poor phases occurring already in the bulk is modified by such a
confinement. When the asymmetry of the wall-colloid interaction increases, the
character of the transition smoothly changes from capillary condensation-type
to interface localization-type. The critical behavior of these transitions is
discussed, as well as the colloid and polymer density profiles across the film
in the various phases, and the correlation of interfacial fluctuations in the
direction parallel to the confining walls. The experimental observability of
these phenomena also is briefly discussed.Comment: 36 pages, 15 figure
Phase behavior of a system of particles with core collapse
The pressure-temperature phase diagram of a one-component system, with
particles interacting through a spherically symmetric pair potential in two
dimensions is studied. The interaction consists of a hard core plus an
additional repulsion at low energies. It is shown that at zero temperature,
instead of the expected isostructural transition due to core collapse occurring
when increasing pressure, the system passes through a series of ground states
that are not triangular lattices. In particular, and depending on parameters,
structures with squares, chains, hexagons and even quasicrystalline ground
states are found. At finite temperatures the solid-fluid coexistence line
presents a zone with negative slope (which implies melting with decreasing in
volume) and the fluid phase has a temperature of maximum density, similar to
that in water.Comment: 11 pages, 15 figures included. To appear in PRE. Some figures in low
quality format. Better ones available upon request from [email protected]
Partially directed paths in a wedge
The enumeration of lattice paths in wedges poses unique mathematical
challenges. These models are not translationally invariant, and the absence of
this symmetry complicates both the derivation of a functional recurrence for
the generating function, and solving for it. In this paper we consider a model
of partially directed walks from the origin in the square lattice confined to
both a symmetric wedge defined by , and an asymmetric wedge defined
by the lines and Y=0, where is an integer. We prove that the
growth constant for all these models is equal to , independent of
the angle of the wedge. We derive functional recursions for both models, and
obtain explicit expressions for the generating functions when . From these
we find asymptotic formulas for the number of partially directed paths of
length in a wedge when .
The functional recurrences are solved by a variation of the kernel method,
which we call the ``iterated kernel method''. This method appears to be similar
to the obstinate kernel method used by Bousquet-Melou. This method requires us
to consider iterated compositions of the roots of the kernel. These
compositions turn out to be surprisingly tractable, and we are able to find
simple explicit expressions for them. However, in spite of this, the generating
functions turn out to be similar in form to Jacobi -functions, and have
natural boundaries on the unit circle.Comment: 26 pages, 5 figures. Submitted to JCT
Surface Grafting of Poly(L-glutamates). 3. Block Copolymerization
This paper describes for the first time the synthesis of surface-grafted AB-block copolypeptides, consisting of poly(γ-benzyl L-glutamate) (PBLG) as the A-block and poly(γ-methyl L-glutamate) (PMLG) as the B-block. Immobilized primary amine groups of (γ-aminopropyl)triethoxysilane (APS) on silicon wafers initiated the ring-opening polymerization of N-carboxyanhydrides of glutamic acid esters (NCAs). After removal of the BLG-NCA monomer solution after a certain reaction time, the amine end groups of the formed PBLG blocks acted as initiators for the second monomers. This method provides the possibility of making layered structures of surface-grafted block copolymers with tuned properties. Ellipsometry and small-angle X-ray reflection (SAXR) measurements revealed the thickness of the polypeptide layers ranging from 45-100 Å of the first block to 140-270 Å for the total block copolypeptides. The chemical composition of the blocks was determined by X-ray photoelectron spectroscopy (XPS). In addition, Fourier transform infrared transmission spectroscopy (FT-IR) revealed that the polypeptide main chains of both blocks consisted of pure R-helices. The average orientation of the helices ranging from 22-42° with respect to the substrate within the first block to 31-35° in the second block could be derived with FT-IR as well.
Adsorption transition of a self-avoiding polymer chain onto a rigid rod
The subject of this work is the adsorption transition of a long flexible
self-avoiding polymer chain onto a rigid thin rod. The rod is represented by a
cylinder of radius R with a short-ranged attractive surface potential for the
chain monomers. General scaling results are obtained by using renormalization
group arguments in conjunction with available results for quantum field
theories with curved boundaries [McAvity and Osborn 1993 Nucl. Phys. B 394,
728]. Relevant critical exponents are identified and estimated using geometric
arguments.Comment: 19 pages, 4 figures. To appear in: J. Phys.: Condens. Matter, special
issue dedicated to Lothar Schaefer on the occasion of his 60th birthda
The fundamental links between climate change and marine plastic pollution
Plastic pollution and climate change have commonly been treated as two separate issues and sometimes are even seen as competing. Here we present an alternative view that these two issues are fundamentally linked. Primarily, we explore how plastic contributes to greenhouse gas (GHG) emissions from the beginning to the end of its life cycle. Secondly, we show that more extreme weather and floods associated with climate change, will exacerbate the spread of plastic in the natural environment. Finally, both issues occur throughout the marine environment, and we show that ecosystems and species can be particularly vulnerable to both, such as coral reefs that face disease spread through plastic pollution and climate-driven increased global bleaching events. A Web of Science search showed climate change and plastic pollution studies in the ocean are often siloed, with only 0.4% of the articles examining both stressors simultaneously. We also identified a lack of regional and industry-specific life cycle analysis data for comparisons in relative GHG contributions by materials and products. Overall, we suggest that rather than debate over the relative importance of climate change or marine plastic pollution, a more productive course would be to determine the linking factors between the two and identify solutions to combat both crises
Microplastics: an introduction to environmental transport processes
Microplastic pollution is widespread across the globe, pervading land, water, and air. These environments are commonly considered independently, however, in reality these are closely linked. This review gives an overview of the background knowledge surrounding sources, fate and transport of microplastics within the environment. We introduce a new “Plastic Cycle” concept in order to better understand the processes influencing flux and retention of microplastics between and across the wide range of environmental matrices. As microplastics are a pervasive, persistent and potentially harmful pollutant, an understanding of these processes will allow for assessment of exposure to better determine the likely long‐term ecological and human health implications of microplastic pollution
RCT of web-based personalized normative feedback for college drinking prevention: Are typical student norms good enough?
Objectives: Personalized normative feedback (PNF) interventions are generally effective at correcting normative misperceptions and reducing risky alcohol consumption among college students. However, research has yet to establish what level of reference group specificity is most efficacious in delivering PNF. This study compared the efficacy of a web-based PNF intervention employing eight increasingly-specific reference groups against a Web-BASICS intervention and a repeated-assessment control in reducing risky drinking and associated consequences. Method: Participants were 1663 heavy drinking Caucasian and Asian undergraduates at two universities. The referent for web-based PNF was either the typical same-campus student, or a same-campus student at one (either gender, race, or Greek-affiliation), or a combination of two (e.g., gender and race), or all three levels of specificity (i.e., gender, race, and Greek-affiliation). Hypotheses were tested using quasi-Poisson generalized linear models fit by generalized estimating equations. Results: The PNF intervention participants showed modest reductions in all four outcomes (average total drinks, peak drinking, drinking days, and drinking consequences) compared to control participants. No significant differences in drinking outcomes were found between the PNF group as a whole and the Web-BASICS group. Among the eight PNF conditions, participants receiving typical student PNF demonstrated greater reductions in all four outcomes compared to those receiving PNF for more specific reference groups. Perceived drinking norms and discrepancies between individual behavior and actual norms mediated the efficacy of the intervention. Conclusions: Findings suggest a web-based PNF intervention using the typical student referent offers a parsimonious approach to reducing problematic alcohol use outcomes among college students
RCT of Web-based Personalized Normative Feedback for College Drinking Prevention: Are Typical Student Norms Good Enough?
Objectives Personalized normative feedback (PNF) interventions are generally effective at correcting normative misperceptions and reducing risky alcohol consumption among college students. However, research has yet to establish what level of reference group specificity is most efficacious in delivering PNF. This study compared the efficacy of a web-based PNF intervention employing eight increasingly-specific reference groups against a Web-BASICS intervention and a repeated-assessment control in reducing risky drinking and associated consequences. Method Participants were 1663 heavy drinking Caucasian and Asian undergraduates at two universities. The referent for web-based PNF was either the typical same-campus student, or a same-campus student at one (either gender, race, or Greek-affiliation), or a combination of two (e.g., gender and race), or all three levels of specificity (i.e., gender, race, and Greek-affiliation). Hypotheses were tested using quasi-Poisson generalized linear models fit by generalized estimating equations. Results The PNF intervention participants showed modest reductions in all four outcomes (average total drinks, peak drinking, drinking days, and drinking consequences) compared to control participants. No significant differences in drinking outcomes were found between the PNF group as a whole and the Web-BASICS group. Among the eight PNF conditions, participants receiving typical student PNF demonstrated greater reductions in all four outcomes compared to those receiving PNF for more specific reference groups. Perceived drinking norms and discrepancies between individual behavior and actual norms mediated the efficacy of the intervention. Conclusions Findings suggest a web-based PNF intervention using the typical student referent offers a parsimonious approach to reducing problematic alcohol use outcomes among college students
Self-Consistent Field study of Polyelectrolyte Brushes
We formulate a self-consistent field theory for polyelectrolyte brushes in
the presence of counterions. We numerically solve the self-consistent field
equations and study the monomer density profile, the distribution of
counterions, and the total charge distribution. We study the scaling relations
for the brush height and compare them to the prediction of other theories. We
find a weak dependence of the brush height on the grafting density.We fit the
counterion distribution outside the brush by the Gouy-Chapman solution for a
virtual charged wall. We calculate the amount of counterions outside the brush
and find that it saturates as the charge of the polyelectrolytes increases
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