818 research outputs found
Effective Interactions and Volume Energies in Charge-Stabilized Colloidal Suspensions
Charge-stabilized colloidal suspensions can be conveniently described by
formally reducing the macroion-microion mixture to an equivalent one-component
system of pseudo-particles. Within this scheme, the utility of a linear
response approximation for deriving effective interparticle interactions has
been demonstrated [M. J. Grimson and M. Silbert, Mol. Phys. 74, 397 (1991)].
Here the response approach is extended to suspensions of finite-sized macroions
and used to derive explicit expressions for (1) an effective electrostatic pair
interaction between pseudo-macroions and (2) an associated volume energy that
contributes to the total free energy. The derivation recovers precisely the
form of the DLVO screened-Coulomb effective pair interaction for spherical
macroions and makes manifest the important influence of the volume energy on
thermodynamic properties of deionized suspensions. Excluded volume corrections
are implicitly incorporated through a natural modification of the inverse
screening length. By including nonlinear response of counterions to macroions,
the theory may be generalized to systematically investigate effective many-body
interactions.Comment: 13 pages (J. Phys.: Condensed Matter, in press
Hysteresis and the dynamic phase transition in thin ferromagnetic films
Hysteresis and the non-equilibrium dynamic phase transition in thin magnetic
films subject to an oscillatory external field have been studied by Monte Carlo
simulation. The model under investigation is a classical Heisenberg spin system
with a bilinear exchange anisotropy in a planar thin film geometry with
competing surface fields. The film exhibits a non-equilibrium phase transition
between dynamically ordered and dynamically disordered phases characterized by
a critical temperature Tcd, whose location of is determined by the amplitude H0
and frequency w of the applied oscillatory field. In the presence of competing
surface fields the critical temperature of the ferromagnetic-paramagnetic
transition for the film is suppressed from the bulk system value, Tc, to the
interface localization-delocalization temperature Tci. The simulations show
that in general Tcd < Tci for the model film. The profile of the time-dependent
layer magnetization across the film shows that the dynamically ordered and
dynamically disordered phases coexist within the film for T < Tcd. In the
presence of competing surface fields, the dynamically ordered phase is
localized at one surface of the film.Comment: PDF file, 21 pages including 8 figure pages; added references,typos
added; to be published in PR
Structure and thermodynamics of multi-component/multi-Yukawa mixtures
New small angle scattering experiments reveal new peaks in colloidal systems
(S.H. Chen et al) in the structure function S(k), in a region that was
inaccessible with older instruments. We propose here general closure of the
Ornstein Zernike equation, that is the sum of an arbitrary number of yukawas,
and that that will go well beyond the MSA . For this closure we get for the
Laplace transform of the pair correlation function . This function is easily
transformed into S(k) by replacing the Laplace variable by the Fourier
wariable. Although the method is general and valid for polydisperse systems, an
explicit continued fraction solution is found for the monodisperse case.Comment: 16 page
Stability of Colloidal Quasicrystals
Freezing of charge-stabilized colloidal suspensions and relative stabilities
of crystals and quasicrystals are studied using thermodynamic perturbation
theory. Macroion interactions are modelled by effective pair potentials
combining electrostatic repulsion with polymer-depletion or van der Waals
attraction. Comparing free energies -- counterion terms included -- for
elementary crystals and rational approximants to icosahedral quasicrystals,
parameters are identified for which one-component quasicrystals are stabilized
by a compromise between packing entropy and cohesive energy.Comment: 6 pages, 4 figure
Effective Interactions and Volume Energies in Charged Colloids: Linear Response Theory
Interparticle interactions in charge-stabilized colloidal suspensions, of
arbitrary salt concentration, are described at the level of effective
interactions in an equivalent one-component system. Integrating out from the
partition function the degrees of freedom of all microions, and assuming linear
response to the macroion charges, general expressions are obtained for both an
effective electrostatic pair interaction and an associated microion volume
energy. For macroions with hard-sphere cores, the effective interaction is of
the DLVO screened-Coulomb form, but with a modified screening constant that
incorporates excluded volume effects. The volume energy -- a natural
consequence of the one-component reduction -- contributes to the total free
energy and can significantly influence thermodynamic properties in the limit of
low-salt concentration. As illustrations, the osmotic pressure and bulk modulus
are computed and compared with recent experimental measurements for deionized
suspensions. For macroions of sufficient charge and concentration, it is shown
that the counterions can act to soften or destabilize colloidal crystals.Comment: 14 pages, including 3 figure
Reviewing the integration of patient data: how systems are evolving in practice to meet patient needs
<p>Abstract</p> <p>Background</p> <p>The integration of Information Systems (IS) is essential to support shared care and to provide consistent care to individuals – patient-centred care. This paper identifies, appraises and summarises studies examining different approaches to integrate patient data from heterogeneous IS.</p> <p>Methods</p> <p>The literature was systematically reviewed between 1995–2005 to identify articles mentioning patient records, computers and data integration or sharing.</p> <p>Results</p> <p>Of 3124 articles, 84 were included describing 56 distinct projects. Most of the projects were on a regional scale. Integration was most commonly accomplished by messaging with pre-defined templates and middleware solutions. HL7 was the most widely used messaging standard. Direct database access and web services were the most common communication methods. The user interface for most systems was a Web browser. Regarding the type of medical data shared, 77% of projects integrated diagnosis and problems, 67% medical images and 65% lab results. More recently significantly more IS are extending to primary care and integrating referral letters.</p> <p>Conclusion</p> <p>It is clear that Information Systems are evolving to meet people's needs by implementing regional networks, allowing patient access and integration of ever more items of patient data. Many distinct technological solutions coexist to integrate patient data, using differing standards and data architectures which may difficult further interoperability.</p
Introduction revisiting the Argentine crisis a decade on: changes and continuities
This introductory chapter to the book "Argentina Since the 2001 Crisis Recovering the Past, Reclaiming the Future" analyses crisis and its associated responses and subsequent recovery in the context of Argentina’s multiple implosion of 2001-02 whilst also assessing its legacies for the country’s social, cultural, economic and political realms during the last decade. It recognises that "crisis" is a term that is much used in the post-Lehman Brothers world and that the subsequent responses and associated recoveries (or lack of) have been the subject of a cascade of academic, government, media, and think-tank investigation ever since. The chapter instead seeks to understand the nature of how crisis and its impacts should be investigated and interrogated, by rejecting false dichotomies of ‘old’ and ‘new’ and synthesising understanding to form an analysis that draws both elements of continuity and elements of change. Secondly, it argues that crisis manifests itself in a number of realms, and that heuristic devices employed to investigate them must subsequently also be drawn from across a range of disciplinary perspectives. Thirdly, it examines how the 2001-02 crisis in Argentina led to a series of responses that both rejected the neoliberal model yet also recovered elements of it. Finally it outlines the structure of the rest of the book, briefly summarising the chapters in turn
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Overview of mathematical approaches used to model bacterial chemotaxis II: bacterial populations
We review the application of mathematical modeling to understanding the behavior of populations of chemotactic bacteria. The application of continuum mathematical models, in particular generalized Keller–Segel models, is discussed along with attempts to incorporate the microscale (individual) behavior on the macroscale, modeling the interaction between different species of bacteria, the interaction of bacteria with their environment, and methods used to obtain experimentally verified parameter values. We allude briefly to the role of modeling pattern formation in understanding collective behavior within bacterial populations. Various aspects of each model are discussed and areas for possible future research are postulated
Exchange anisotropy and the dynamic phase transition in thin ferromagnetic Heisenberg films
Monte Carlo simulations have been performed to investigate the dependence of
the dynamic phase behavior on the bilinear exchange anisotropy of a classical
Heisenberg spin system. The system under consideration is a planar thin
ferromagnetic film with competing surface fields subject to a pulsed
oscillatory external field. The results show that the films exhibit a single
discontinuous dynamic phase transition (DPT) as a function of the anisotropy of
the bilinear exchange interaction in the Hamiltonian. Furthermore there is no
evidence of stochastic resonance (SR) associated with the DPT. These results
are in marked contrast to the continuous DPT observed in the same system as a
function of temperature and applied field strength for a fixed bilinear
exchange anisotropy.Comment: 11 pages including 3 figure pages; submitted to PR
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