319 research outputs found
Stationary and Transient Work-Fluctuation Theorems for a Dragged Brownian Particle
Recently Wang et al. carried out a laboratory experiment, where a Brownian
particle was dragged through a fluid by a harmonic force with constant velocity
of its center. This experiment confirmed a theoretically predicted work related
integrated (I) Transient Fluctuation Theorem (ITFT), which gives an expression
for the ratio for the probability to find positive or negative values for the
fluctuations of the total work done on the system in a given time in a
transient state. The corresponding integrated stationary state fluctuation
theorem (ISSFT) was not observed. Using an overdamped Langevin equation and an
arbitrary motion for the center of the harmonic force, all quantities of
interest for these theorems and the corresponding non-integrated ones (TFT and
SSFT, resp.) are theoretically explicitly obtained in this paper. While the
(I)TFT is satisfied for all times, the (I)SSFT only holds asymptotically in
time. Suggestions for further experiments with arbitrary velocity of the
harmonic force and in which also the ISSFT could be observed, are given. In
addition, a non-trivial long-time relation between the ITFT and the ISSFT was
discovered, which could be observed experimentally, especially in the case of a
resonant circular motion of the center of the harmonic force.Comment: 20 pages, 3 figure
A phenomenological approach to the simulation of metabolism and proliferation dynamics of large tumour cell populations
A major goal of modern computational biology is to simulate the collective
behaviour of large cell populations starting from the intricate web of
molecular interactions occurring at the microscopic level. In this paper we
describe a simplified model of cell metabolism, growth and proliferation,
suitable for inclusion in a multicell simulator, now under development
(Chignola R and Milotti E 2004 Physica A 338 261-6). Nutrients regulate the
proliferation dynamics of tumor cells which adapt their behaviour to respond to
changes in the biochemical composition of the environment. This modeling of
nutrient metabolism and cell cycle at a mesoscopic scale level leads to a
continuous flow of information between the two disparate spatiotemporal scales
of molecular and cellular dynamics that can be simulated with modern computers
and tested experimentally.Comment: 58 pages, 7 figures, 3 tables, pdf onl
Symptoms and feelings valued by patients after a percutaneous coronary intervention: A discrete-choice experiment to inform development of a new patient-reported outcome
Re-use permitted under CC BY-NC. No commercial re-use. See rights and permissions. Published by BMJ. Objective To inform the development of a patient-reported outcome measure, the aim of this study was to identify which symptoms and feelings following percutaneous coronary intervention (PCI) are most important to patients. Design Discrete-choice experiment consisting of two hypothetical scenarios of 10 symptoms and feelings (pain or discomfort; shortness of breath; concern/worry about heart problems; tiredness; confidence to do usual activities; ability to do usual activities; happiness; sleep disturbance; dizziness or light-headedness and bruising) experienced after PCI, described by three levels (never, some of the time, most of the time). Preference weights were estimated using a conditional logit model. Setting Four Australian public hospitals that contribute to the Victorian Cardiac Outcomes Registry (VCOR) and a private insurer's claim database. Participants 138 people aged >18 years who had undergone a PCI in the previous 6 months. Main outcome measures Patient preferences via trade-offs between 10 feelings and symptoms. Results Of the 138 individuals recruited, 129 (93%) completed all 16 choice sets. Conditional logit parameter estimates were mostly monotonic (eg, moving to worse levels for each individual symptom and feeling made the option less attractive). When comparing the magnitude of the coefficients (based on the coefficient of the worst level relative to best level in each item), feeling unhappy was the symptom or feeling that most influenced perception of a least-preferred PCI outcome (OR 0.42, 95% CI 0.34 to 0.51, p<0.0001) and the least influential was bruising (OR 0.81, 95% CI 0.67 to 0.99, p=0.04). Conclusion This study provides new insights into how patients value symptoms and feelings they experience following a PCI
Orientational Ordering in Spatially Disordered Dipolar Systems
This letter addresses basic questions concerning ferroelectric order in
positionally disordered dipolar materials. Three models distinguished by dipole
vectors which have one, two or three components are studied by computer
simulation. Randomly frozen and dynamically disordered media are considered. It
is shown that ferroelectric order is possible in spatially random systems, but
that its existence is very sensitive to the dipole vector dimensionality and
the motion of the medium. A physical analysis of our results provides
significant insight into the nature of ferroelectric transitions.Comment: 4 pages twocolumn LATEX style. 4 POSTSCRIPT figures available from
[email protected]
Ferroelectric and Dipolar Glass Phases of Non-Crystalline Systems
In a recent letter [Phys. Rev. Lett. {\bf 75}, 2360 (1996)] we briefly
discussed the existence and nature of ferroelectric order in positionally
disordered dipolar materials. Here we report further results and give a
complete description of our work. Simulations of randomly frozen and
dynamically disordered dipolar soft spheres are used to study ferroelectric
ordering in non-crystalline systems. We also give a physical interpretation of
the simulation results in terms of short- and long-range interactions. Cases
where the dipole moment has 1, 2, and 3 components (Ising, XY and XYZ models,
respectively) are considered. It is found that the Ising model displays
ferroelectric phases in frozen amorphous systems, while the XY and XYZ models
form dipolar glass phases at low temperatures. In the dynamically disordered
model the equations of motion are decoupled such that particle translation is
completely independent of the dipolar forces. These systems spontaneously
develop long-range ferroelectric order at nonzero temperature despite the
absence of any fined-tuned short-range spatial correlations favoring dipolar
order. Furthermore, since this is a nonequilibrium model we find that the
paraelectric to ferroelectric transition depends on the particle mass. For the
XY and XYZ models, the critical temperatures extrapolate to zero as the mass of
the particle becomes infinite, whereas, for the Ising model the critical
temperature is almost independent of mass and coincides with the ferroelectric
transition found for the randomly frozen system at the same density. Thus in
the infinite mass limit the results of the frozen amorphous systems are
recovered.Comment: 25 pages (LATEX, no macros). 11 POSTSCRIPT figures enclosed.
Submitted to Phisical Review E. Contact: [email protected]
Hot Streaks in Artistic, Cultural, and Scientific Careers
The hot streak, loosely defined as winning begets more winnings, highlights a
specific period during which an individual's performance is substantially
higher than her typical performance. While widely debated in sports, gambling,
and financial markets over the past several decades, little is known if hot
streaks apply to individual careers. Here, building on rich literature on
lifecycle of creativity, we collected large-scale career histories of
individual artists, movie directors and scientists, tracing the artworks,
movies, and scientific publications they produced. We find that, across all
three domains, hit works within a career show a high degree of temporal
regularity, each career being characterized by bursts of high-impact works
occurring in sequence. We demonstrate that these observations can be explained
by a simple hot-streak model we developed, allowing us to probe quantitatively
the hot streak phenomenon governing individual careers, which we find to be
remarkably universal across diverse domains we analyzed: The hot streaks are
ubiquitous yet unique across different careers. While the vast majority of
individuals have at least one hot streak, hot streaks are most likely to occur
only once. The hot streak emerges randomly within an individual's sequence of
works, is temporally localized, and is unassociated with any detectable change
in productivity. We show that, since works produced during hot streaks garner
significantly more impact, the uncovered hot streaks fundamentally drives the
collective impact of an individual, ignoring which leads us to systematically
over- or under-estimate the future impact of a career. These results not only
deepen our quantitative understanding of patterns governing individual
ingenuity and success, they may also have implications for decisions and
policies involving predicting and nurturing individuals with lasting impact
Curvature-coupling dependence of membrane protein diffusion coefficients
We consider the lateral diffusion of a protein interacting with the curvature
of the membrane. The interaction energy is minimized if the particle is at a
membrane position with a certain curvature that agrees with the spontaneous
curvature of the particle. We employ stochastic simulations that take into
account both the thermal fluctuations of the membrane and the diffusive
behavior of the particle. In this study we neglect the influence of the
particle on the membrane dynamics, thus the membrane dynamics agrees with that
of a freely fluctuating membrane. Overall, we find that this curvature-coupling
substantially enhances the diffusion coefficient. We compare the ratio of the
projected or measured diffusion coefficient and the free intramembrane
diffusion coefficient, which is a parameter of the simulations, with analytical
results that rely on several approximations. We find that the simulations
always lead to a somewhat smaller diffusion coefficient than our analytical
approach. A detailed study of the correlations of the forces acting on the
particle indicates that the diffusing inclusion tries to follow favorable
positions on the membrane, such that forces along the trajectory are on average
smaller than they would be for random particle positions.Comment: 16 pages, 8 figure
A multibaker map for shear flow and viscous heating
A consistent description of shear flow and the accompanied viscous heating as
well the associated entropy balance is given in the framework of a
deterministic dynamical system. A laminar shear flow is modeled by a
Hamiltonian multibaker map which drives velocity and temperature fields. In an
appropriate macroscopic limit one recovers the Navier-Stokes and heat
conduction equations along with the associated entropy balance. This indicates
that results of nonequilibrium thermodynamics can be described by means of an
abstract, sufficiently chaotic and mixing dynamics. A thermostating algorithm
can also be incorporated into this framework.Comment: 11 pages; RevTex with multicol+graphicx packages; eps-figure
Characterization of the Rheological, Mucoadhesive, and Drug Release Properties of Highly Structured Gel Platforms for Intravaginal Drug Delivery
This investigation describes the formulation and characterization of rheologically structured vehicles (RSVs) designed for improved drug delivery to the vagina. Interactive, multicomponent, polymeric platforms were manufactured containing hydroxyethylcellulose (HEC, 5 % w/w) polyvinylpyrrolidone (PVP, 4 % w/w), Pluronic (PL, 0 or 10 % w/w), and either polycarbophil (PC, 3 % w/w) or poly(methylvinylether-co-maleic anhydride) (Gantrez S97, 3 % w/w) as a mucoadhesive agent. The rheological (torsional and dynamic), mechanical (com-pressional), and mucoadhesive properties were characterized and shown to be dependent upon the mucoadhesive agent used and the inclusion/exclusion of PL. The dynamic rheological properties of the gel platforms were also assessed following dilution with simulated vaginal fluid (to mimic in vivo dilution). RSVs containing PC were more rheologically structured than comparator formulations containing GAN. This trend was also reflected in formulation hardness, compressibility, consistency, and syringeability. Moreover, formulations containing PL (10% w/w) were more rheologically structured than formulations devoid of PL. Dilution with simulated vaginal fluids significantly decreased rheological structure, although RSVs still retained a highly elastic structure (G ′> G′ ′ and tan δ < 1). Furthermore, RSVs exhibited sustained drug release properties that were shown to be dependent upon their rheological structure. It is considered that these semisolid drug delivery systems may be useful as site-retentive platforms for the sustained delivery of therapeutic agents to the vagina
PIP2-Binding Site in Kir Channels: Definition by Multiscale Biomolecular Simulations†
Phosphatidylinositol bisphosphate (PIP(2)) is an activator of mammalian inwardly rectifying potassium (Kir) channels. Multiscale simulations, via a sequential combination of coarse-grained and atomistic molecular dynamics, enabled exploration of the interactions of PIP(2) molecules within the inner leaflet of a lipid bilayer membrane with possible binding sites on Kir channels. Three Kir channel structures were investigated: X-ray structures of KirBac1.1 and of a Kir3.1-KirBac1.3 chimera and a homology model of Kir6.2. Coarse-grained simulations of the Kir channels in PIP(2)-containing lipid bilayers identified the PIP(2)-binding site on each channel. These models of the PIP(2)-channel complexes were refined by conversion to an atomistic representation followed by molecular dynamics simulation in a lipid bilayer. All three channels were revealed to contain a conserved binding site at the N-terminal end of the slide (M0) helix, at the interface between adjacent subunits of the channel. This binding site agrees with mutagenesis data and is in the proximity of the site occupied by a detergent molecule in the Kir chimera channel crystal. Polar contacts in the coarse-grained simulations corresponded to long-lived electrostatic and H-bonding interactions between the channel and PIP(2) in the atomistic simulations, enabling identification of key side chains
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