1,296 research outputs found
Dynamical heterogeneity in soft particle suspensions under shear
We present experimental measurements of dynamical heterogeneities in a dense
system of microgel spheres, sheared at different rates and at different packing
fractions in a microfluidic channel, and visualized with high speed digital
video microscopy. A four-point dynamic susceptibility is deduced from video
correlations, and is found to exhibit a peak that grows in height and shifts to
longer times as the jamming transition is approached from two different
directions. In particular, the time for particle-size root-mean square relative
displacements is found to scale as where is the strain rate and
is the distance from the random close packing volume
fraction. The typical number of particles in a dynamical heterogeneity is
deduced from the susceptibility peak height and found to scale as . Exponent uncertainties are less than ten
percent. We emphasize that the same power-law behavior is found at packing
fractions above and below . Thus, our results considerably extend a
previous observation of for granular heap flow at
fixed packing below . Furthermore, the implied result compares well with expectation from mode-coupling theory and
with prior observations for driven granular systems
Quantifying stretching and rearrangement in epithelial sheet migration
Although understanding the collective migration of cells, such as that seen
in epithelial sheets, is essential for understanding diseases such as
metastatic cancer, this motion is not yet as well characterized as individual
cell migration. Here we adapt quantitative metrics used to characterize the
flow and deformation of soft matter to contrast different types of motion
within a migrating sheet of cells. Using a Finite-Time Lyapunov Exponent (FTLE)
analysis, we find that - in spite of large fluctuations - the flow field of an
epithelial cell sheet is not chaotic. Stretching of a sheet of cells (i.e.,
positive FTLE) is localized at the leading edge of migration. By decomposing
the motion of the cells into affine and non-affine components using the metric
D, we quantify local plastic rearrangements and describe the motion
of a group of cells in a novel way. We find an increase in plastic
rearrangements with increasing cell densities, whereas inanimate systems tend
to exhibit less non-affine rearrangements with increasing density.Comment: 21 pages, 7 figures This is an author-created, un-copyedited version
of an article accepted for publication in the New Journal of Physics. IOP
Publishing Ltd is not responsible for any errors or omissions in this version
of the manuscript or any version derived from it. The Version of Record is
available online at doi:10.1088/1367-2630/15/2/02503
Role of the hydrological cycle in regulating the planetary climate system of a simple nonlinear dynamical model
International audienceWe present the construction of a dynamic area fraction model (DAFM), representing a new class of models for an earth-like planet. The model presented here has no spatial dimensions, but contains coupled parameterizations for all the major components of the hydrological cycle involving liquid, solid and vapor phases. We investigate the nature of feedback processes with this model in regulating Earth's climate as a highly nonlinear coupled system. The model includes solar radiation, evapotranspiration from dynamically competing trees and grasses, an ocean, an ice cap, precipitation, dynamic clouds, and a static carbon greenhouse effect. This model therefore shares some of the characteristics of an Earth System Model of Intermediate complexity. We perform two experiments with this model to determine the potential effects of positive and negative feedbacks due to a dynamic hydrological cycle, and due to the relative distribution of trees and grasses, in regulating global mean temperature. In the first experiment, we vary the intensity of insolation on the model's surface both with and without an active (fully coupled) water cycle. In the second, we test the strength of feedbacks with biota in a fully coupled model by varying the optimal growing temperature for our two plant species (trees and grasses). We find that the negative feedbacks associated with the water cycle are far more powerful than those associated with the biota, but that the biota still play a significant role in shaping the model climate. third experiment, we vary the heat and moisture transport coefficient in an attempt to represent changing atmospheric circulations
Microfluidic rheology of soft colloids above and below jamming
The rheology near jamming of a suspension of soft colloidal spheres is
studied using a custom microfluidic rheometer that provides stress versus
strain rate over many decades. We find non-Newtonian behavior below the jamming
concentration and yield stress behavior above it. The data may be collapsed
onto two branches with critical scaling exponents that agree with expectations
based on Hertzian contacts and viscous drag. These results support the
conclusion that jamming is similar to a critical phase transition, but with
interaction-dependent exponents.Comment: 4 pages, experimen
Dynamical Compactification and Inflation in Einstein-Yang-Mills Theory with Higher Derivative Coupling
We study cosmology of the Einstein-Yang-Mills theory in ten dimensions with a
quartic term in the Yang-Mills field strength. We obtain analytically a class
of cosmological solutions in which the extra dimensions are static and the
scale factor of the four-dimensional Friedmann-Lemaitre-Robertson-Walker metric
is an exponential function of time. This means that the model can explain
inflation. Then we look for solutions that describe dynamical compactification
of the extra dimensions. The effective cosmological constant in the
four-dimensional universe is determined from the gravitational coupling,
ten-dimensional cosmological constant, gauge coupling and higher derivative
coupling. By numerical integration, the solution with is found to
behave as a matter-dominated universe which asymptotically approaches flat
space-time, while the solution with a non-vanishing approaches de
Sitter space-time in the asymptotic future.Comment: 30 pages, 7 figure
Stiff Stability of the Hydrogen atom in dissipative Fokker electrodynamics
We introduce an ad-hoc electrodynamics with advanced and retarded
Lienard-Wiechert interactions plus the dissipative Lorentz-Dirac
self-interaction force. We study the covariant dynamical system of the
electromagnetic two-body problem, i.e., the hydrogen atom. We perform the
linear stability analysis of circular orbits for oscillations perpendicular to
the orbital plane. In particular we study the normal modes of the linearized
dynamics that have an arbitrarily large imaginary eigenvalue. These large
eigenvalues are fast frequencies that introduce a fast (stiff) timescale into
the dynamics. As an application, we study the phenomenon of resonant
dissipation, i.e., a motion where both particles recoil together in a drifting
circular orbit (a bound state), while the atom dissipates center-of-mass energy
only. This balancing of the stiff dynamics is established by the existence of a
quartic resonant constant that locks the dynamics to the neighborhood of the
recoiling circular orbit. The resonance condition quantizes the angular momenta
in reasonable agreement with the Bohr atom. The principal result is that the
emission lines of quantum electrodynamics (QED) agree with the prediction of
our resonance condition within one percent average deviation.Comment: 1 figure, Notice that Eq. (34) of the Phys. Rev. E paper has a typo;
it is missing the square Brackets of eq. (33), find here the correct e
Graviton confinement inside hypermonopoles of any dimension
We show the generic existence of metastable massive gravitons in the
four-dimensional core of self-gravitating hypermonopoles in any number of
infinite-volume extra-dimensions. Confinement is observed for Higgs and gauge
bosons couplings of the order unity. Provided these resonances are light
enough, they realise the Dvali-Gabadadze-Porrati mechanism by inducing a
four-dimensional gravity law on some intermediate length scales. The effective
four-dimensional Planck mass is shown to be proportional to a negative power of
the graviton mass. As a result, requiring gravity to be four-dimensional on
cosmological length scales may solve the mass hierarchy problem.Comment: 23 pages, 6 figures, uses iopart. Misprints corrected, references
added, matches published versio
- …