3,929 research outputs found
Osmotic force resisting chain insertion in a colloidal suspension
We consider the problem of inserting a stiff chain into a colloidal
suspension of particles that interact with it through excluded volume forces.
The free energy of insertion is associated with the work of creating a cavity
devoid of colloid and sufficiently large to accomodate the chain. The
corresponding work per unit length is the force that resists the entry of the
chain into the colloidal suspension. In the case of a hard sphere fluid, this
work can be calculated straightforwardly within the scaled particle theory; for
solutions of flexible polymers, on the other hand, we employ simple scaling
arguments. The forces computed in these ways are shown, for nanometer chain and
colloid diameters, to be of the order of tens of pN for solution volume
fraction for biophysical processes such as the ejection of DNA from viral
capsids into the cell cytoplasm.Comment: 16 pages,3 figures. Accepted for publication in European Physical
Journal
Requirements for regional short-haul air service and the definition of a flight program to determine neighborhood reactions to small transport aircraft
An evaluation of the current status and future requirements of an intraregional short haul air service is given. A brief definition of the different types of short haul air service is given. This is followed by a historical review of previous attempts to develop short haul air service in high density urban areas and an assessment of the current status. The requirements for intraregional air service, the need for economic and environmental viability and the need for a flight research program are defined. A detailed outline of a research program that would determine urban community reaction to frequent operations of small transport aircraft is also given. Both the operation of such an experiment in a specific region (San Francisco Bay area) and the necessary design modifications of an existing fixed wing aircraft which could be used in the experiment are established. An estimate is made of overall program costs
"Swarm relaxation": Equilibrating a large ensemble of computer simulations
It is common practice in molecular dynamics and Monte Carlo computer
simulations to run multiple, separately-initialized simulations in order to
improve the sampling of independent microstates. Here we examine the utility of
an extreme case of this strategy, in which we run a large ensemble of
independent simulations (a "swarm"), each of which is relaxed to equilibrium.
We show that if is of order , we can monitor the swarm's relaxation
to equilibrium, and confirm its attainment, within , where
is the equilibrium relaxation time. As soon as a swarm of this size
attains equilibrium, the ensemble of final microstates from each run is
sufficient for the evaluation of most equilibrium properties without further
sampling. This approach dramatically reduces the wall-clock time required,
compared to a single long simulation, by a factor of several hundred, at the
cost of an increase in the total computational effort by a small factor. It is
also well-suited to modern computing systems having thousands of processors,
and is a viable strategy for simulation studies that need to produce
high-precision results in a minimum of wall-clock time. We present results
obtained by applying this approach to several test cases.Comment: 12 pages. To appear in Eur. Phy. J. E, 201
Correlation of Positive and Negative Reciprocity Fails to Confer an Evolutionary Advantage: Phase Transitions to Elementary Strategies
Economic experiments reveal that humans value cooperation and fairness. Punishing unfair behavior is therefore common, and according to the theory of strong reciprocity, it is also directly related to rewarding cooperative behavior. However, empirical data fail to confirm that positive and negative reciprocity are correlated. Inspired by this disagreement, we determine whether the combined application of reward and punishment is evolutionarily advantageous. We study a spatial public goods game, where in addition to the three elementary strategies of defection, rewarding, and punishment, a fourth strategy that combines the latter two competes for space. We find rich dynamical behavior that gives rise to intricate phase diagrams where continuous and discontinuous phase transitions occur in succession. Indirect territorial competition, spontaneous emergence of cyclic dominance, as well as divergent fluctuations of oscillations that terminate in an absorbing phase are observed. Yet, despite the high complexity of solutions, the combined strategy can survive only in very narrow and unrealistic parameter regions. Elementary strategies, either in pure or mixed phases, are much more common and likely to prevail. Our results highlight the importance of patterns and structure in human cooperation, which should be considered in future experiments
The Inherent Structure Landscape Connection Between Liquids, Granular materials and the Jamming Phase Diagram
We provide a comprehensive picture of the jamming phase diagram by connecting
the athermal, granular ensemble of jammed states and the equilibrium fluid
through the inherent structure paradigm for a system hard discs confined to a
narrow channel. The J-line is shown to be divided into packings that are
thermodynamically accessible from the equilibrium fluid and inaccessible
packings. The J-point is found to occur at the transition between these two
sets of packings and is located at the maximum the inherent structure
distribution. A general thermodynamic argument suggests that the density of the
states at the configurational entropy maximum represents a lower bound on the
J-point density in hard sphere systems. Finally, we find that the granular and
fluid systems only occupy the same set of inherent structures, under the same
thermodynamic conditions, at two points, corresponding to zero and infinite
pressures, where they sample the J-point states and the most dense packing
respectively.Comment: 5 pages, 3 Figure
Is Fitspiration Truly an Inspiration? Relationships between Fitspiration, Exercise, and Body Image
Young adults across the United States struggle to meet physical activity recommendations and consume healthy diets, and they often suffer from issues related to body image. Social media influencers dedicated to fitspiration (i.e., fitness inspiration) are purported to have a goal of inspiring others to lead healthier lifestyles. The purpose of this study was to explore the relationships between fitspiration and exercise and body image perceptions amongst college students. Participants (n = 361, mean age = 20.2 years, 78% female) completed surveys that included sociodemographic information, social media usage, fitspiration content engagement, exercise, and body satisfaction. An independent samples t-test assessed differences in exercise by fitspiration viewership, and a chi-square analysis determined relationships between fitspiration and body satisfaction. Participants were routinely active on social media (91% use it for \u3e 1 hour per day), and 61.5% were exposed to fitspiration content. Approximately 41% of respondents have followed exercise advice from fitspiration influencers, though only 11% reported having purchased products. No relationships were reported between following fitspiration and days per week of exercise (M∆ = .02(.20), p = .91). Participants that followed fitspiration were more likely to be dissatisfied with their bodies, X2 (1, n = 316) =7.77, p = .005, compared to participants who did not. Findings demonstrate fitspiration was not related to exercise and was related to poorer body image perceptions among college students. These results are supported by previous findings and indicate a critical misalignment between the purported purpose of fitspiration and the outcome of its viewing
Impact of generalized benefit functions on the evolution of cooperation in spatial public goods games with continuous strategies
Cooperation and defection may be considered as two extreme responses to a
social dilemma. Yet the reality is much less clear-cut. Between the two
extremes lies an interval of ambivalent choices, which may be captured
theoretically by means of continuous strategies defining the extent of the
contributions of each individual player to the common pool. If strategies are
chosen from the unit interval, where 0 corresponds to pure defection and 1
corresponds to the maximal contribution, the question is what is the
characteristic level of individual investments to the common pool that emerges
if the evolution is guided by different benefit functions. Here we consider the
steepness and the threshold as two parameters defining an array of generalized
benefit functions, and we show that in a structured population there exist
intermediate values of both at which the collective contributions are maximal.
However, as the cost-to-benefit ratio of cooperation increases the
characteristic threshold decreases, while the corresponding steepness
increases. Our observations remain valid if more complex sigmoid functions are
used, thus reenforcing the importance of carefully adjusted benefits for high
levels of public cooperation.Comment: 8 two-column pages, 8 figures; accepted for publication in Physical
Review
Test of classical nucleation theory on deeply supercooled high-pressure simulated silica
We test classical nucleation theory (CNT) in the case of simulations of
deeply supercooled, high density liquid silica, as modelled by the BKS
potential. We find that at density ~g/cm, spontaneous nucleation
of crystalline stishovite occurs in conventional molecular dynamics simulations
at temperature T=3000 K, and we evaluate the nucleation rate J directly at this
T via "brute force" sampling of nucleation events. We then use parallel,
constrained Monte Carlo simulations to evaluate , the free energy
to form a crystalline embryo containing n silicon atoms, at T=3000, 3100, 3200
and 3300 K. We find that the prediction of CNT for the n-dependence of fits reasonably well to the data at all T studied, and at 3300 K yields a
chemical potential difference between liquid and stishovite that matches
independent calculation. We find that , the size of the critical nucleus,
is approximately 10 silicon atoms at T=3300 K. At 3000 K, decreases to
approximately 3, and at such small sizes methodological challenges arise in the
evaluation of when using standard techniques; indeed even the
thermodynamic stability of the supercooled liquid comes into question under
these conditions. We therefore present a modified approach that permits an
estimation of at 3000 K. Finally, we directly evaluate at T=3000
K the kinetic prefactors in the CNT expression for J, and find physically
reasonable values; e.g. the diffusion length that Si atoms must travel in order
to move from the liquid to the crystal embryo is approximately 0.2 nm. We are
thereby able to compare the results for J at 3000 K obtained both directly and
based on CNT, and find that they agree within an order of magnitude.Comment: corrected calculation, new figure, accepted in JC
Conditional strategies and the evolution of cooperation in spatial public goods games
The fact that individuals will most likely behave differently in different
situations begets the introduction of conditional strategies. Inspired by this,
we study the evolution of cooperation in the spatial public goods game, where
besides unconditional cooperators and defectors, also different types of
conditional cooperators compete for space. Conditional cooperators will
contribute to the public good only if other players within the group are likely
to cooperate as well, but will withhold their contribution otherwise. Depending
on the number of other cooperators that are required to elicit cooperation of a
conditional cooperator, the latter can be classified in as many types as there
are players within each group. We find that the most cautious cooperators, such
that require all other players within a group to be conditional cooperators,
are the undisputed victors of the evolutionary process, even at very low
synergy factors. We show that the remarkable promotion of cooperation is due
primarily to the spontaneous emergence of quarantining of defectors, which
become surrounded by conditional cooperators and are forced into isolated
convex "bubbles" from where they are unable to exploit the public good. This
phenomenon can be observed only in structured populations, thus adding to the
relevance of pattern formation for the successful evolution of cooperation.Comment: 7 two-column pages, 7 figures; accepted for publication in Physical
Review
Evolutionary Roots of Property Rights; The Natural and Cultural Nature of Human Cooperation
Debates about the role of natural and cultural selection in the development of prosocial, antisocial and socially neutral mechanisms and behavior raise questions that touch property rights, cooperation, and conflict. For example, some researchers suggest that cooperation and prosociality evolved by natural selection (Hamilton 1964, Trivers 1971, Axelrod and Hamilton 1981, De Waal 2013, 2014), while others claim that natural selection is insufficient for the evolution of cooperation, which required in addition cultural selection (Sterelny 2013, Bowles and Gintis 2003, Seabright 2013, Norenzayan 2013). Some scholars focus on the complexity and hierarchical nature of the evolution of cooperation as involving different tools associated with lower and the higher levels of competition (Nowak 2006, Okasha 2006); others suggest that humans genetically inherited heuristics that favor prosocial behavior such as generosity, forgiveness or altruistic punishment (Ridley 1996, Bowles and Gintis 2004, Rolls 2005). We argue these mechanisms are not genetically inherited; rather, they are features inherited through cultural selection. To support this view we invoke inclusive fitness theory, which states that individuals tend to maximize their inclusive fitness, rather than maximizing group fitness. We further reject the older notion of natural group selection - as well as more recent versions (West, Mouden, Gardner 2011) – which hold that natural selection favors cooperators within a group (Wynne-Edwards 1962). For Wynne-Edwards, group selection leads to group adaptations; the survival of individuals therefore depends on the survival of the group and a sharing of resources. Individuals who do not cooperate, who are selfish, face extinction due to rapid and over-exploitation of resources
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