12,451 research outputs found
Soft modes near the buckling transition of icosahedral shells
Icosahedral shells undergo a buckling transition as the ratio of Young's
modulus to bending stiffness increases. Strong bending stiffness favors smooth,
nearly spherical shapes, while weak bending stiffness leads to a sharply
faceted icosahedral shape. Based on the phonon spectrum of a simplified
mass-and-spring model of the shell, we interpret the transition from smooth to
faceted as a soft-mode transition. In contrast to the case of a disclinated
planar network where the transition is sharply defined, the mean curvature of
the sphere smooths the transitition. We define elastic susceptibilities as the
response to forces applied at vertices, edges and faces of an icosahedron. At
the soft-mode transition the vertex susceptibility is the largest, but as the
shell becomes more faceted the edge and face susceptibilities greatly exceed
the vertex susceptibility. Limiting behaviors of the susceptibilities are
analyzed and related to the ridge-scaling behavior of elastic sheets. Our
results apply to virus capsids, liposomes with crystalline order and other
shell-like structures with icosahedral symmetry.Comment: 28 pages, 6 figure
Sudden collapse of a colloidal gel
Metastable gels formed by weakly attractive colloidal particles display a
distinctive two-stage time-dependent settling behavior under their own weight.
Initially a space-spanning network is formed that for a characteristic time,
which we define as the lag time \taud, resists compaction. This solid-like
behavior persists only for a limited time. Gels whose age \tw is greater than
\taud yield and suddenly collapse. We use a combination of confocal
microscopy, rheology and time-lapse video imaging to investigate both the
process of sudden collapse and its microscopic origin in an refractive-index
matched emulsion-polymer system. We show that the height of the gel in the
early stages of collapse is well described by the surprisingly simple
expression, h(\ts) = \h0 - A \ts^{3/2}, with \h0 the initial height and
\ts = \tw-\taud the time counted from the instant where the gel first yields.
We propose that this unexpected result arises because the colloidal network
progressively builds up internal stress as a consequence of localized
rearrangement events which leads ultimately to collapse as thermal equilibrium
is re-established.Comment: 14 pages, 11 figures, final versio
Choosing Life Stories: Body As Teacher
Biological science and the larger society interact with each other. Biologists tell stories--stories such as fertilization, body development, and evolution--using the narrative structures given to them by the larger society. These stories have to be consistent with the scientific data; but what data are collected is also a social judgement. The stories that biologists have told have often emphasized competition and have often marginalized cooperative efforts. New research has shown that these competitive stories offer a very incomplete version of what is happening in our bodies, and that mutual cooperation is a major part of how the body develops and evolves. Understanding the body is key to understanding how parts integrate into wholes and provide new narratives of who we are and what is considered normative in our society
SmartHeLP: Smartphone-based Hemoglobin Level Prediction Using an Artificial Neural Network
Blood hemoglobin level (Hgb) measurement has a vital role in the diagnosis, evaluation, and management of numerous diseases. We describe the use of smartphone video imaging and an artificial neural network (ANN) system to estimate Hgb levels non-invasively. We recorded 10 second-300 frame fingertip videos using a smartphone in 75 adults. Red, green, and blue pixel intensities were estimated for each of 100 area blocks in each frame and the patterns across the 300 frames were described. ANN was then used to develop a model using the extracted video features to predict hemoglobin levels. In our study sample, with patients 20-56 years of age, and gold standard hemoglobin levels of 7.6 to 13.5 g/dL., we observed a 0.93 rank order of correlation between model and gold standard hemoglobin levels. Moreover, we identified specific regions of interest in the video images which reduced the required feature space
Multistability of free spontaneously-curved anisotropic strips
Multistable structures are objects with more than one stable conformation,
exemplified by the simple switch. Continuum versions are often elastic
composite plates or shells, such as the common measuring tape or the slap
bracelet, both of which exhibit two stable configurations: rolled and unrolled.
Here we consider the energy landscape of a general class of multistable
anisotropic strips with spontaneous Gaussian curvature. We show that while
strips with non-zero Gaussian curvature can be bistable, strips with positive
spontaneous curvature are always bistable, independent of the elastic moduli,
strips of spontaneous negative curvature are bistable only in the presence of
spontaneous twist and when certain conditions on the relative stiffness of the
strip in tension and shear are satisfied. Furthermore, anisotropic strips can
become tristable when their bending rigidity is small. Our study complements
and extends the theory of multistability in anisotropic shells and suggests new
design criteria for these structures.Comment: 20 pages, 10 figure
Behavioural and morphological changes in fish exposed to ecologically relevant boat noises
There is increasing concern about the effect of underwater noise on fish due to rising levels of anthropogenic noise. We performed experiments on the black bullhead (Ameiurus melas), a species with known hearing specializations and located within the Laurentian Great Lakes where there is considerable commercial and recreational boat traffic. We tested and compared physiology (baseline cortisol), behaviour (activity, sheltering), and morphology (ciliary bundles of hair cells) of bullhead to boat noise. At 140 dB re 1 μPa (−54.84 dB re 1 m·s−2), we saw clear behavioural effects in terms of both activity and sheltering levels despite no obvious morphological or physiological stress. Following both short-and long-period acute exposure to higher — but environmentally relevant — noise levels, bullhead were less active and sheltered more and also exhibited a decrease in ciliary bundles. These results suggest that there are sublethal effects of anthropogenic noise on fish behaviour and ciliary bundles, which may have direct implications on population health. Moreover, commonly used metrics such as stress hormones may not always offer the most relevant biomarker of the response to anthropogenic boat noise
Discrete elastic model for stretching-induced flagellar polymorphs
Force-induced reversible transformations between coiled and normal polymorphs
of bacterial flagella have been observed in recent optical-tweezer experiment.
We introduce a discrete elastic rod model with two competing helical states
governed by a fluctuating spin-like variable that represents the underlying
conformational states of flagellin monomers. Using hybrid Brownian dynamics
Monte-Carlo simulations, we show that a helix undergoes shape transitions
dominated by domain wall nucleation and motion in response to externally
applied uniaxial tension. A scaling argument for the critical force is
presented in good agreement with experimental and simulation results.
Stretching rate-dependent elasticity including a buckling instability are
found, also consistent with the experiment
Semi-classical buckling of stiff polymers
A quantitative theory of the buckling of a worm like chain based on a
semi-classical approximation of the partition function is presented. The
contribution of thermal fluctuations to the force-extension relation that
allows to go beyond the classical Euler buckling is derived in the linear and
non-linear regime as well. It is shown that the thermal fluctuations in the
nonlinear buckling regime increase the end-to-end distance of the semiflexible
rod if it is confined to 2 dimensions as opposed to the 3 dimensional case. Our
approach allows a complete physical understanding of buckling in D=2 and in D=3
below and above the Euler transition.Comment: Revtex, 17 pages, 4 figure
Nonequilibrium Microscopic Distribution of Thermal Current in Particle Systems
A nonequilibrium distribution function of microscopic thermal current is
studied by a direct numerical simulation in a thermal conducting steady state
of particle systems. Two characteristic temperatures of the thermal current are
investigated on the basis of the distribution. It is confirmed that the
temperature depends on the current direction; Parallel temperature to the
heat-flux is higher than antiparallel one. The difference between the parallel
temperature and the antiparallel one is proportional to a macroscopic
temperature gradient.Comment: 4 page
Scalar GW detection with a hollow spherical antenna
We study the response and cross sections for the absorption of GW energy in a
Jordan-Brans-Dicke theory by a resonant mass detector shaped as a hollow
sphere.Comment: latex file, 9 page
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