2,789 research outputs found
Probing the causes of thermal hysteresis using tunable N-agg micelles with linear and brush-like thermoresponsive coronas
Self-assembled thermoresponsive polymers in aqueous solution have great potential as smart, switchable materials for use in biomedical applications. In recent years, attention has turned to the reversibility of these polymers’ thermal transitions, which has led to debate over what factors influence discrepancies in the transition temperature when heating the system compared to the temperature obtained when cooling the system, known as the thermal hysteresis. Herein, we synthesize micelles with tunable aggregation numbers (Nagg) whose cores contain poly(n-butyl acrylate-co-N,N-dimethylacrylamide) (p(nBA-co-DMA)) and four different thermoresponsive corona blocks, namely poly(N-isopropylacrylamide) (pNIPAM), poly(N,N-diethylacrylamide) (pDEAm), poly(diethylene glycol monomethyl ether methacrylate) (pDEGMA) and poly(oligo(ethylene glycol) monomethyl ether methacrylate) (pOEGMA). By studying their thermoresponsive behavior, we elucidate the effects of changing numerous important characteristics both in the thermoresponsive chain chemistry and architecture, and in the structure of their self-assemblies. Our findings demonstrate large deviations in the reversibility between the self-assemblies and the corresponding thermoresponsive homopolymers; specifically we find that micelles whose corona consist of polymers with a brush-like architecture (pDEGMA and pOEGMA) exhibit irreversible phase transitions at a critical chain density. These results lead to a deeper understanding of stimuli-responsive self-assemblies and demonstrate the potential of tunable Nagg micelles for uncovering structure–property relationships in responsive polymer systems
Axial morphology along the Southern Chile Rise
Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Marine Geology 315-318 (2012): 58-63, doi:10.1016/j.margeo.2012.06.001.Morphology of four spreading segments on the southern Chile Rise is described based on
multi-beam bathymetric data collected along the axial zones. The distribution of axial volcanoes,
the character of rift valley scarps, and the average depths vary between Segment 1 in the south,
terminating at the Chile Triple Junction, and Segment 4 in the north, which are separated by
three intervening transform faults. Despite this general variability, there is a consistent pattern of
clockwise rotation of the southern-most axial volcanic ridge within each of Segments 2, 3, and 4,
relative to the overall trend of the rift valley. A combination of local ridge-transform intersection
stresses and regional tectonics may influence spreading axis evolution in this sense.This work was
supported by NOAA/OE grant NA08OAR4600757 and University of California Ship Funds
On particle acceleration and trapping by Poynting flux dominated flows
Using particle-in-cell (PIC) simulations, we study the evolution of a
strongly magnetized plasma slab propagating into a finite density ambient
medium. Like previous work, we find that the slab breaks into discrete magnetic
pulses. The subsequent evolution is consistent with diamagnetic relativistic
pulse acceleration of \cite{liangetal2003}. Unlike previous work, we use the
actual electron to proton mass ratio and focus on understanding trapping vs.
transmission of the ambient plasma by the pulses and on the particle
acceleration spectra. We find that the accelerated electron distribution
internal to the slab develops a double-power law. We predict that emission from
reflected/trapped external electrons will peak after that of the internal
electrons. We also find that the thin discrete pulses trap ambient electrons
but allow protons to pass through, resulting in less drag on the pulse than in
the case of trapping of both species. Poynting flux dominated scenarios have
been proposed as the driver of relativistic outflows and particle acceleration
in the most powerful astrophysical jets.Comment: 25 pages, Accepted by Plasma Physics and Controlled Fusio
Assessing the Feasibility of Alternative Livelihood Options for Communities Surrounding the Molinière-Beauséjour Marine Protected Area, Grenada
Scale Free Cluster Distributions from Conserving Merging-Fragmentation Processes
We propose a dynamical scheme for the combined processes of fragmentation and
merging as a model system for cluster dynamics in nature and society displaying
scale invariant properties. The clusters merge and fragment with rates
proportional to their sizes, conserving the total mass. The total number of
clusters grows continuously but the full time-dependent distribution can be
rescaled over at least 15 decades onto a universal curve which we derive
analytically. This curve includes a scale free solution with a scaling exponent
of -3/2 for the cluster sizes.Comment: 4 pages, 3 figure
MHD Stellar and Disk Winds: Application to Planetary Nebulae
MHD winds can emanate from both stars and surrounding accretion disks. It is
of interest to know how much wind power is available and which (if either) of
the two rotators dominates that power. We investigate this in the context of
multi-polar planetary nebulae (PNe) and proto-planetary nebulae (PPNe), for
which recent observations have revealed the need for a wind power source in
excess of that available from radiation driving, and a possible need for
magnetic shaping. We calculate the MHD wind power from a coupled disk and star,
where the former results from binary disruption. The resulting wind powers
depend only on the accretion rate and stellar properties. We find that if the
stellar envelope were initially slowly rotating, the disk wind would dominate
throughout the evolution. If the envelope of the star were rapidly rotating,
the stellar wind could initially be of comparable power to the disk wind until
the stellar wind carries away the star's angular momentum. Since an initially
rapidly rotating star can have its spin and magnetic axes misaligned to the
disk, multi-polar outflows can result from this disk wind system. For times
greater than a spin-down time, the post-AGB stellar wind is slaved to the disk
for both slow and rapid initial spin cases and the disk wind luminosity
dominates. We find a reasonably large parameter space where a hybrid star+disk
MHD driven wind is plausible and where both or either can account for PPNe and
PNe powers. We also speculate on the morphologies which may emerge from the
coupled system. The coupled winds might help explain the shapes of a number of
remarkable multi-shell or multi-polar nebulae. Magnetic activity such as X-ray
flares may be associated with the both central star and the disk and would be a
valuable diagnostic for the dynamical role of MHD processes in PNe.Comment: ApJ accepted version, incorporating some important revisions. 25
Pages, LaTex, + 5 fig
GP-SUM. Gaussian Processes Filtering of non-Gaussian Beliefs
This work studies the problem of stochastic dynamic filtering and state
propagation with complex beliefs. The main contribution is GP-SUM, a filtering
algorithm tailored to dynamic systems and observation models expressed as
Gaussian Processes (GP), and to states represented as a weighted sum of
Gaussians. The key attribute of GP-SUM is that it does not rely on
linearizations of the dynamic or observation models, or on unimodal Gaussian
approximations of the belief, hence enables tracking complex state
distributions. The algorithm can be seen as a combination of a sampling-based
filter with a probabilistic Bayes filter. On the one hand, GP-SUM operates by
sampling the state distribution and propagating each sample through the dynamic
system and observation models. On the other hand, it achieves effective
sampling and accurate probabilistic propagation by relying on the GP form of
the system, and the sum-of-Gaussian form of the belief. We show that GP-SUM
outperforms several GP-Bayes and Particle Filters on a standard benchmark. We
also demonstrate its use in a pushing task, predicting with experimental
accuracy the naturally occurring non-Gaussian distributions.Comment: WAFR 2018, 16 pages, 7 figure
Self-Lensing Models of the LMC
All of the proposed explanations for the microlensing events observed towards
the LMC have difficulties. One of these proposed explanations, LMC
self-lensing, which invokes ordinary LMC stars as the long sought-after lenses,
has recently gained considerable popularity as a possible solution to the
microlensing conundrum. In this paper, we carefully examine the set of LMC
self-lensing models. In particular, we review the pertinent observations made
of the LMC, and show how these observations place limits on such self-lensing
models. We find that, given current observational constraints, no purely LMC
disk models are capable of producing optical depths as large as that reported
in the MACHO collaboration 2-year analysis. Besides pure disk, we also consider
alternate geometries, and present a framework which encompasses the previous
studies of LMC self-lensing. We discuss which model parameters need to be
pushed in order for such models to succeed. For example, like previous workers,
we find that an LMC halo geometry may be able to explain the observed events.
However, since all known LMC tracer stellar populations exhibit disk-like
kinematics, such models will have difficulty being reconciled with
observations. For SMC self-lensing, we find predicted optical depths differing
from previous results, but more than sufficient to explain all observed SMC
microlensing. In contrast, for the LMC we find a self-lensing optical depth
contribution between 0.47e-8 and 7.84e-8, with 2.44e-8 being the value for the
set of LMC parameters most consistent with current observations.Comment: 20 pages, Latex, 14 figures, submitted to Ap
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