1,316 research outputs found
Density functional theory for colloidal mixtures of hard platelets, rods, and spheres
A geometry-based density functional theory is presented for mixtures of hard
spheres, hard needles and hard platelets; both the needles and the platelets
are taken to be of vanishing thickness. Geometrical weight functions that are
characteristic for each species are given and it is shown how convolutions of
pairs of weight functions recover each Mayer bond of the ternary mixture and
hence ensure the correct second virial expansion of the excess free energy
functional. The case of sphere-platelet overlap relies on the same
approximation as does Rosenfeld's functional for strictly two-dimensional hard
disks. We explicitly control contributions to the excess free energy that are
of third order in density. Analytic expressions relevant for the application of
the theory to states with planar translational and cylindrical rotational
symmetry, e.g. to describe behavior at planar smooth walls, are given. For
binary sphere-platelet mixtures, in the appropriate limit of small platelet
densities, the theory differs from that used in a recent treatment [L. Harnau
and S. Dietrich, Phys. Rev. E 71, 011504 (2004)]. As a test case of our
approach we consider the isotropic-nematic bulk transition of pure hard
platelets, which we find to be weakly first order, with values for the
coexistence densities and the nematic order parameter that compare well with
simulation results.Comment: 39 pages, 8 figure
The Asakura-Oosawa model in the protein limit: the role of many-body interactions
We study the Asakura-Oosawa model in the "protein limit", where the
penetrable sphere radius is much greater than the hard sphere radius
. The phase behaviour and structure calculated with a full many-body
treatment show important qualitative differences when compared to a description
based on pair potentials alone. The overall effect of the many-body
interactions is repulsive.Comment: 9 pages and 11 figures, submitted to J. Phys.: Condensed Matter,
special issue "Effective many-body interactions and correlations in soft
matter
Coarse-graining polymers as soft colloids
We show how to coarse grain polymers in a good solvent as single particles,
interacting with density-independent or density-dependent interactions. These
interactions can be between the centres of mass, the mid-points or end-points
of the polymers. We also show how to extend these methods to polymers in poor
solvents and mixtures of polymers. Treating polymers as soft colloids can
greatly speed up the simulation of complex many-polymer systems, including
polymer-colloid mixtures.Comment: to appear in Physica A, special STATPHYS 2001 edition. Content of
invited talk by AA
The opposing effects of isotropic and anisotropic attraction on association kinetics of proteins and colloids
The association and dissociation of particles via specific anisotropic interactions is a fundamental
process, both in biology (proteins) and in soft matter (colloidal patchy particles). The presence of
alternative binding sites can lead to multiple productive states and also to non-productive “decoy” or
intermediate states. Besides anisotropic interactions, particles can experience non-specific isotropic
interactions. We employ single replica transition interface sampling to investigate how adding a
non-productive binding site or a nonspecific isotropic interaction alters the dimerization kinetics of
a generic patchy particle model. The addition of a decoy binding site reduces the association rate
constant, independent of the site’s position, while adding an isotropic interaction increases it due
to an increased rebinding probability. Surprisingly, the association kinetics becomes non-monotonic
for a tetramer complex formed by multivalent patchy particles. While seemingly identical to twoparticle
binding with a decoy state, the cooperativity of binding multiple particles leads to a kinetic
optimum. Our results are relevant for the understanding and modeling of biochemical networks and
self-assembly processes. Published by AIP Publishing. https://doi.org/10.1063/1.500648
Molecular Free Energies, Rates, and Mechanisms from Data-Efficient Path Sampling Simulations
Molecular dynamics is a powerful tool for studying the thermodynamics and kinetics of complex molecular events. However, these simulations can rarely sample the required time scales in practice. Transition path sampling overcomes this limitation by collecting unbiased trajectories and capturing the relevant events. Moreover, the integration of machine learning can boost the sampling while simultaneously learning a quantitative representation of the mechanism. Still, the resulting trajectories are by construction non-Boltzmann-distributed, preventing the calculation of free energies and rates. We developed an algorithm to approximate the equilibrium path ensemble from machine-learning-guided path sampling data. At the same time, our algorithm provides efficient sampling, mechanism, free energy, and rates of rare molecular events at a very moderate computational cost. We tested the method on the folding of the mini-protein chignolin. Our algorithm is straightforward and data-efficient, opening the door to applications in many challenging molecular systems
Density functional theory and demixing of binary hard rod-polymer mixtures
A density functional theory for a mixture of hard rods and polymers modeled
as chains built of hard tangent spheres is proposed by combining the functional
due to Yu and Wu for the polymer mixtures [J. Chem. Phys. {\bf 117}, 2368
(2002)] with the Schmidt's functional [Phys. Rev. E {\bf 63}, 50201 (2001)] for
rod-sphere mixtures. As a simple application of the functional, the demixing
transition into polymer-rich and rod-rich phases is examined. When the chain
length increases, the phase boundary broadens and the critical packing fraction
decreases. The shift of the critical point of a demixing transition is most
noticeable for short chains.Comment: 4 pages,2 figures, in press, PR
Statics and dynamics of free and hydrogen-bonded OH groups at the air/water interface
We use classical atomistic molecular dynamics simulations of two water models (SPC/E and TIP4P/2005) to investigate the orientation and reorientation dynamics of two subpopulations of OH groups belonging to water molecules at the air/water interface at 300 K: those OH groups that donate a hydrogen bond (called “bonded”) and those that do not (called “free”). Free interfacial OH groups reorient in two distinct regimes: a fast regime from 0 to 1 ps and a slow regime thereafter. Qualitatively similar behavior was reported by others for free OH groups near extended hydrophobic surfaces. In contrast, the net reorientation of bonded OH groups occurs at a rate similar to that of bulk water. This similarity in reorientation rate results from compensation of two effects: decreasing frequency of hydrogen-bond breaking/formation (i.e., hydrogen-bond exchange) and faster rotation of intact hydrogen bonds. Both changes result from the decrease in density at the air/water interface relative to the bulk. Interestingly, because of the presence of capillary waves, the slowdown of hydrogen-bond exchange is signiffcantly smaller than that reported for water near extended hydrophobic surfaces, but it is almost identical to that reported for water near small hydrophobic solutes. In this sense water at the air/water interface has characteristics of water of hydration of both small and extended hydrophobic solutes.SARA Computing and Networking Services (www.sara.nl)Nederlandse Organisatie voor Wetenschappelijk Onderzoe
Relating Teaching Qualifications and Basic Need Satisfaction in Medical Teaching
Contains fulltext :
182164.pdf (publisher's version ) (Open Access)INTRODUCTION: Teaching Qualifications (TQs) have been implemented in University Medical Centers, but their relation to teachers’ motivation for medical teaching is unknown. Because teacher motivation influences important outcomes, it is crucial to study how TQs are related to promoting teacher motivation, by fulfilling the basic needs of feeling autonomous, competent, and related towards medical teaching.
AIMS: To explore relations between TQs and feelings of autonomy, competence, and relatedness towards medical teaching.
METHODS: An online questionnaire was used to collect data about teaching at a university hospital. We measured feelings of autonomy, competence, and relatedness towards medical teaching using the Teaching-related Basic Need Satisfaction scale (T-BNS). We applied multivariate regression analysis to examine relations between TQs and basic need satisfaction in teaching.
RESULTS: A total of 767 medical teachers participated. TQs appear to be related to feeling competent in teaching. Higher TQ levels are not related to higher feelings of autonomy, competence, and relatedness towards medical teaching.
CONCLUSIONS: The results imply that appealing to non-qualified teachers’ feelings of competence towards medical teaching may stir up their enthusiasm for TQ policy. They also call for robust teaching positions to build teaching experience, preferably as early as possible, and for assessing the importance applicants attach to education in job interviews
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