118 research outputs found
Entropy-induced smectic phases in rod-coil copolymers
We present a self-consistent field theory (SCFT) of semiflexible (wormlike)
diblock copolymers, each consisting of a rigid and a flexible part. The
segments of the polymers are otherwise identical, in particular with regard to
their interactions, which are taken to be of an Onsager excluded-volume type.
The theory is developed in a general three-dimensional form, as well as in a
simpler one-dimensional version. Using the latter, we demonstrate that the
theory predicts the formation of a partial-bilayer smectic-A phase in this
system, as shown by profiles of the local density and orientational
distribution functions. The phase diagram of the system, which includes the
isotropic and nematic phases, is obtained in terms of the mean density and
rigid-rod fraction of each molecule. The nematic-smectic transition is found to
be second order. Since the smectic phase is induced solely by the difference in
the rigidities, the onset of smectic ordering is shown to be an entropic effect
and therefore does not have to rely on additional Flory-Huggins-type repulsive
interactions between unlike chain segments. These findings are compared with
other recent SCFT studies of similar copolymer models and with computer
simulations of several molecular models.Comment: 13 pages, 8 figure
Tests of Dynamical Scaling in 3-D Spinodal Decomposition
We simulate late-stage coarsening of a 3-D symmetric binary fluid. With
reduced units l,t (with scales set by viscosity, density and surface tension)
our data extends two decades in t beyond earlier work. Across at least four
decades, our own and others' individual datasets (< 1 decade each) show viscous
hydrodynamic scaling (l ~ a + b t), but b is not constant between runs as this
scaling demands. This betrays either the unexpected intrusion of a
discretization (or molecular) lengthscale, or an exceptionally slow crossover
between viscous and inertial regimes.Comment: Submitted to Phys. Rev.
3D Spinodal Decomposition in the Inertial Regime
We simulate late-stage coarsening of a 3D symmetric binary fluid using a
lattice Boltzmann method. With reduced lengths and times l and t respectively
(scales set by viscosity, density and surface tension) our data sets cover 1 <
l
100 we find clear evidence of Furukawa's inertial scaling (l ~ t^{2/3}),
although the crossover from the viscous regime (l ~ t) is very broad. Though it
cannot be ruled out, we find no indication that Re is self-limiting (l ~
t^{1/2}) as proposed by M. Grant and K. R. Elder [Phys. Rev. Lett. 82, 14
(1999)].Comment: 4 pages, 3 eps figures, RevTex, minor changes to bring in line with
published version. Mobility values added to Table
Scientists' warning on extreme wildfire risks to water supply
2020 is the year of wildfire records. California experienced its three largest fires early in its fire season. The Pantanal, the largest wetland on the planet, burned over 20% of its surface. More than 18 million hectares of forest and bushland burned during the 2019â2020 fire season in Australia, killing 33 people, destroying nearly 2500 homes, and endangering many endemic species. The direct cost of damages is being counted in dozens of billion dollars, but the indirect costs on waterârelated ecosystem services and benefits could be equally expensive, with impacts lasting for decades. In Australia, the extreme precipitation (â200âmmâday â1 in several locationâ) that interrupted the catastrophic wildfire season triggered a series of watershed effects from headwaters to areas downstream. The increased runoff and erosion from burned areas disrupted water supplies in several locations. These postâfire watershed hazards via source water contamination, flash floods, and mudslides can represent substantial, systemic longâterm risks to drinking water production, aquatic life, and socioâeconomic activity. Scenarios similar to the recent event in Australia are now predicted to unfold in the Western USA. This is a new reality that societies will have to live with as uncharted fire activity, water crises, and widespread human footprint collide allâaround of the world. Therefore, we advocate for a more proactive approach to wildfireâwatershed risk governance in an effort to advance and protect water security. We also argue that there is no easy solution to reducing this risk and that investments in both green (i.e., natural) and grey (i.e., built) infrastructure will be necessary. Further, we propose strategies to combine modern data analytics with existing tools for use by water and land managers worldwide to leverage several decades worth of data and knowledge on postâfire hydrology
A solution scan of societal options to reduce transmission and spread of respiratory viruses: SARS-CoV-2 as a case study
Societal biosecurity â measures built into everyday society to minimize risks from pests and diseases â is an important aspect of managing epidemics and pandemics. We aimed to identify societal options for reducing the transmission and spread of respiratory viruses. We used SARS-CoV-2 (severe acute respiratory syndrome coronavirus 2) as a case study to meet the immediate need to manage the COVID-19 pandemic and eventually transition to more normal societal conditions, and to catalog options for managing similar pandemics in the future. We used a âsolution scanningâ approach. We read the literature; consulted psychology, public health, medical, and solution scanning experts; crowd-sourced options using social media; and collated comments on a preprint. Here, we present a list of 519 possible measures to reduce SARS-CoV-2 transmission and spread. We provide a long list of options for policymakers and businesses to consider when designing biosecurity plans to combat SARS-CoV-2 and similar pathogens in the future. We also developed an online application to help with this process. We encourage testing of actions, documentation of outcomes, revisions to the current list, and the addition of further options.</p
Potential bioavailability of representative pyrogenic organic matter compounds in comparison to natural dissolved organic matter pools
Pyrogenic organic matter (PyOM) from wildfires impacts river
corridors globally and is widely regarded as resistant to biological
degradation. Though recent work suggests PyOM may be more bioavailable than
historically perceived, estimating bioavailability across its chemical
spectrum remains elusive. To address this knowledge gap, we assessed
potential bioavailability of representative PyOM compounds relative to
ubiquitous dissolved organic matter (DOM) with a substrate-explicit model.
The range of potential bioavailability of PyOM was greater than natural DOM;
however, the predicted thermodynamics, metabolic rates, and carbon use
efficiencies (CUEs) overlapped significantly between all OM pools. Compound type
(e.g., natural versus PyOM) had approximately 6-fold less impact on predicted
respiration rates than simulated carbon and oxygen limitations. Within PyOM,
the metabolism of specific chemistries differed strongly between unlimited
and oxygen-limited conditions â degradations of anhydrosugars, phenols, and polycyclic aromatic hydrocarbons (PAHs) were more favorable under oxygen
limitation than other molecules. Notably, amino sugar-like, protein-like, and lignin-like PyOM had lower carbon use efficiencies relative to natural DOM
of the same classes, indicating potential impacts in process-based model
representations. Overall, our work illustrates how similar PyOM
bioavailability may be to that of natural DOM in the river corridor,
furthering our understanding of how PyOM may influence riverine
biogeochemical cycling.</p
Cheaters allow cooperators to prosper
Cooperation based on the production of costly common goods is observed throughout nature. This is puzzling, as cooperation is vulnerable to exploitation by defectors which enjoy a fitness advantage by consuming the common good without contributing fairly. Depletion of the common good can lead to population collapse and the destruction of cooperation. However, population collapse implies small population size, which, in a structured population, is known to favor cooperation. This happens because small population size increases variability in cooperator frequency across different locations. Since individuals in cooperator-dominated locations (which are most likely cooperators) will grow more than those in defector-dominated locations (which are most likely defectors), cooperators can outgrow defectors globally despite defectors outgrowing cooperators in each location. This raises the possibility that defectors can lead to conditions that sometimes rescue cooperation from defector-induced destruction. We demonstrate multiple mechanisms through which this can occur, using an individual-based approach to model stochastic birth, death, migration, and mutation events. First, during defector-induced population collapse, defectors occasionally go extinct before cooperators by chance, which allows cooperators to grow. Second, empty locations, either preexisting or created by defector-induced population extinction, can favor cooperation because they allow cooperator but not defector migrants to grow. These factors lead to the counterintuitive result that the initial presence of defectors sometimes allows better survival of cooperation compared to when defectors are initially absent. Finally, we find that resource limitation, inducible by defectors, can select for mutations adaptive to resource limitation. When these mutations are initially present at low levels or continuously generated at a moderate rate, they can favor cooperation by further reducing local population size. We predict that in a structured population, small population sizes precipitated by defectors provide a "built-in" mechanism for the persistence of cooperation
Liquid crystals and their defects
These lecture notes discuss classical models of liquid crystals, and the
different ways in which defects are described according to the different
models.Comment: CIME lecture course, Cetraro, 201
Tapping into non-English-language science for the conservation of global biodiversity.
The widely held assumption that any important scientific information would be available in English underlies the underuse of non-English-language science across disciplines. However, non-English-language science is expected to bring unique and valuable scientific information, especially in disciplines where the evidence is patchy, and for emergent issues where synthesising available evidence is an urgent challenge. Yet such contribution of non-English-language science to scientific communities and the application of science is rarely quantified. Here, we show that non-English-language studies provide crucial evidence for informing global biodiversity conservation. By screening 419,679 peer-reviewed papers in 16 languages, we identified 1,234 non-English-language studies providing evidence on the effectiveness of biodiversity conservation interventions, compared to 4,412 English-language studies identified with the same criteria. Relevant non-English-language studies are being published at an increasing rate in 6 out of the 12 languages where there were a sufficient number of relevant studies. Incorporating non-English-language studies can expand the geographical coverage (i.e., the number of 2° à 2° grid cells with relevant studies) of English-language evidence by 12% to 25%, especially in biodiverse regions, and taxonomic coverage (i.e., the number of species covered by the relevant studies) by 5% to 32%, although they do tend to be based on less robust study designs. Our results show that synthesising non-English-language studies is key to overcoming the widespread lack of local, context-dependent evidence and facilitating evidence-based conservation globally. We urge wider disciplines to rigorously reassess the untapped potential of non-English-language science in informing decisions to address other global challenges. Please see the Supporting information files for Alternative Language Abstracts
Coincident molecular auxeticity and negative order parameter in a liquid crystal elastomer
Auxetic materials have negative Poisson's ratios and so expand rather than contract in one or several direction(s) perpendicular to applied extensions. The auxetics community has long sought synthetic molecular auxetics - non-porous, inherently auxetic materials which are simple to fabricate and avoid porosity-related weakening. Here, we report, synthetic molecular auxeticity for a non-porous liquid crystal elastomer. For strains above ~0.8 applied perpendicular to the liquid crystal director, the liquid crystal elastomer becomes auxetic with the maximum negative Poisson's ratio measured to date being -0.74â±â0.03 - larger than most values seen in naturally occurring molecular auxetics. The emergence of auxeticity coincides with the liquid crystal elastomer backbone adopting a negative order parameter, QBâ=â-0.41â±â0.01 - further implying negative liquid crystal ordering. The reported behaviours consistently agree with theoretical predictions from Warner and Terentjev liquid crystal elastomer theory. Our results open the door for the design of synthetic molecular auxetics
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