11,987 research outputs found
Nanoindentation of virus capsids in a molecular model
A molecular-level model is used to study the mechanical response of empty
cowpea chlorotic mottle virus (CCMV) and cowpea mosaic virus (CPMV) capsids.
The model is based on the native structure of the proteins that consitute the
capsids and is described in terms of the C-alpha atoms. Nanoindentation by a
large tip is modeled as compression between parallel plates. Plots of the
compressive force versus plate separation for CCMV are qualitatively consistent
with continuum models and experiments, showing an elastic region followed by an
irreversible drop in force. The mechanical response of CPMV has not been
studied, but the molecular model predicts an order of magnitude higher
stiffness and a much shorter elastic region than for CCMV. These large changes
result from small structural changes that increase the number of bonds by only
30% and would be difficult to capture in continuum models. Direct comparison of
local deformations in continuum and molecular models of CCMV shows that the
molecular model undergoes a gradual symmetry breaking rotation and accommodates
more strain near the walls than the continuum model. The irreversible drop in
force at small separations is associated with rupturing nearly all of the bonds
between capsid proteins in the molecular model while a buckling transition is
observed in continuum models.Comment: 18 figure
Topology and Bistability in liquid crystal devices
We study nematic liquid crystal configurations in a prototype bistable device
- the Post Aligned Bistable Nematic (PABN) cell. Working within the Oseen-Frank
continuum model, we describe the liquid crystal configuration by a unit-vector
field, in a model version of the PABN cell. Firstly, we identify four distinct
topologies in this geometry. We explicitly construct trial configurations with
these topologies which are used as initial conditions for a numerical solver,
based on the finite-element method. The morphologies and energetics of the
corresponding numerical solutions qualitatively agree with experimental
observations and suggest a topological mechanism for bistability in the PABN
cell geometry
Dynamical density functional theory for the dewetting of evaporating thin films of nanoparticle suspensions exhibiting pattern formation
Recent experiments have shown that the striking structure formation in
dewetting films of evaporating colloidal nanoparticle suspensions occurs in an
ultrathin `postcursor' layer that is left behind by a mesoscopic dewetting
front. Various phase change and transport processes occur in the postcursor
layer, that may lead to nanoparticle deposits in the form of labyrinthine,
network or strongly branched `finger' structures. We develop a versatile
dynamical density functional theory to model this system which captures all
these structures and may be employed to investigate the influence of
evaporation/condensation, nanoparticle transport and solute transport in a
differentiated way. We highlight, in particular, the influence of the subtle
interplay of decomposition in the layer and contact line motion on the observed
particle-induced transverse instability of the dewetting front.Comment: 5 pages, 5 figure
Solidification fronts in supercooled liquids: how rapid fronts can lead to disordered glassy solids
We determine the speed of a crystallisation (or more generally, a
solidification) front as it advances into the uniform liquid phase after the
system has been quenched into the crystalline region of the phase diagram. We
calculate the front speed by assuming a dynamical density functional theory
model for the system and applying a marginal stability criterion. Our results
also apply to phase field crystal (PFC) models of solidification. As the
solidification front advances into the unstable liquid phase, the density
profile behind the advancing front develops density modulations and the
wavelength of these modulations is a dynamically chosen quantity. For shallow
quenches, the selected wavelength is precisely that of the crystalline phase
and so well-ordered crystalline states are formed. However, when the system is
deeply quenched, we find that this wavelength can be quite different from that
of the crystal, so that the solidification front naturally generates disorder
in the system. Significant rearrangement and ageing must subsequently occur for
the system to form the regular well-ordered crystal that corresponds to the
free energy minimum. Additional disorder is introduced whenever a front
develops from random initial conditions. We illustrate these findings with
results obtained from the PFC.Comment: 14 pages, 7 figure
Cognitive performance in multiple system atrophy
The cognitive performance of a group of patients with multiple system atrophy (MSA) of striato-nigral predominance was compared with that of age and IQ matched control subjects, using three tests sensitive to frontal lobe dysfunction and a battery sensitive to memory and learning deficits in Parkinson's disease and dementia of the Alzheimer type. The MSA group showed significant deficits in all three of the tests previously shown to be sensitive to frontal lobe dysfunction. Thus, a significant proportion of patients from the MSA group failed an attentional set-shifting test, specifically at the stage when an extra-dimensional shift was required. They were also impaired in a subject-ordered test of spatial working memory. The MSA group showed deficits mostly confined to measures of speed of thinking, rather than accuracy, on the Tower of London task. These deficits were seen in the absence of consistent impairments in language or visual perception. Moreover, the MSA group showed no significant deficits in tests of spatial and pattern recognition previously shown to be sensitive to patients early in the course of probable Alzheimer's disease and only a few patients exhibited impairment on the Warrington Recognition Memory Test. There were impairments on other tests of visual memory and learning relative to matched controls, but these could not easily be related to fundamental deficits of memory or learning. Thus, on a matching-to-sample task the patients were impaired at simultaneous but not delayed matching to sample, whereas difficulties in a pattern-location learning task were more evident at its initial, easier stages. The MSA group showed no consistent evidence of intellectual deterioration as assessed from their performance on subtests of the Wechsler Adult Intelligence Scale (WAIS) and the National Adult Reading Test (NART). Consideration of individual cases showed that there was some heterogeneity in the pattern of deficits in the MSA group, with one patient showing no impairment, even in the face of considerable physical disability. The results show a distinctive pattern of cognitive deficits, unlike those previously seen using the same tests in patients with Parkinson's and Alzheimer's diseases, and suggesting a prominent frontal-lobe-like component. The implications for concepts of 'subcortical' dementia and 'fronto-striatal' cognitive dysfunction are considered
Fronto-striatal cognitive deficits at different stages of Parkinson's disease
Groups of patients with idiopathic Parkinson's disease, either medicated or unmedicated, were compared with matched groups of normal controls on a computerized battery previously shown to be sensitive to frontal lobe dysfunction, including tests of planning, spatial working memory and attentional set-shifting. In a series of problems based on the 'Tower of London' test, medicated patients with Parkinson's disease were shown to be impaired in the amount of time spent thinking about (planning) the solution to each problem. Additionally, an impairment in terms of the accuracy of the solution produced on this test was only evident in those patients with more severe clinical symptoms and was accompanied by deficits in an associated test of spatial short-term memory. Medicated patients with both mild and severe clinical symptoms were also impaired on a related test of spatial working memory. In contrast, a group of patients who were unmedicated and 'early in the course' of the disease were unimpaired in all three of these tests. However, all three Parkinson's disease groups were impaired in the test of attentional set-shifting ability, although unimpaired in a test of pattern recognition which is insensitive to frontal lobe damage. These data are compared with those previously published from a group of young neurosurgical patients with localized excisions of the frontal lobes and are discussed in terms of the specific nature of the cognitive deficit at different stages of Parkinson's disease
Complex Periodic Orbits and Tunnelling in Chaotic Potentials
We derive a trace formula for the splitting-weighted density of states
suitable for chaotic potentials with isolated symmetric wells. This formula is
based on complex orbits which tunnel through classically forbidden barriers.
The theory is applicable whenever the tunnelling is dominated by isolated
orbits, a situation which applies to chaotic systems but also to certain
near-integrable ones. It is used to analyse a specific two-dimensional
potential with chaotic dynamics. Mean behaviour of the splittings is predicted
by an orbit with imaginary action. Oscillations around this mean are obtained
from a collection of related orbits whose actions have nonzero real part
Incentivizing Exploration with Heterogeneous Value of Money
Recently, Frazier et al. proposed a natural model for crowdsourced
exploration of different a priori unknown options: a principal is interested in
the long-term welfare of a population of agents who arrive one by one in a
multi-armed bandit setting. However, each agent is myopic, so in order to
incentivize him to explore options with better long-term prospects, the
principal must offer the agent money. Frazier et al. showed that a simple class
of policies called time-expanded are optimal in the worst case, and
characterized their budget-reward tradeoff.
The previous work assumed that all agents are equally and uniformly
susceptible to financial incentives. In reality, agents may have different
utility for money. We therefore extend the model of Frazier et al. to allow
agents that have heterogeneous and non-linear utilities for money. The
principal is informed of the agent's tradeoff via a signal that could be more
or less informative.
Our main result is to show that a convex program can be used to derive a
signal-dependent time-expanded policy which achieves the best possible
Lagrangian reward in the worst case. The worst-case guarantee is matched by
so-called "Diamonds in the Rough" instances; the proof that the guarantees
match is based on showing that two different convex programs have the same
optimal solution for these specific instances. These results also extend to the
budgeted case as in Frazier et al. We also show that the optimal policy is
monotone with respect to information, i.e., the approximation ratio of the
optimal policy improves as the signals become more informative.Comment: WINE 201
Christianity, paranormal belief and personality: a study among 13- to 16-year-old pupils in England and Wales
Studies concerning the changing landscapes of religiosity and spirituality in the lives of young people in England and Wales draw attention to decline in traditional religiosity and to growth in alternative spiritualities. The present study examined whether such alternative spiritualities occupy the same personality space as traditional religiosity. A sample of 2,950 13- to 16-year-old pupils attending 11 secondary schools in England and Wales completed the Francis Scale of Attitude toward Christianity and an index of paranormal belief, alongside the abbreviated-form Junior Eysenck Personality Questionnaire Revised. The data demonstrated that these two forms of belief were related in different ways to Eysenck's dimensional model of personality space. While attitude toward Christianity occupied the space defined by low psychoticism scores (tendermindedness) and high lie scale scores (social conformity), paranormal belief was related to high psychoticism scores (toughmindedness) and was independent of lie scale scores. These findings support the view that alternative spiritualities may be associated with different personalities
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