12,007 research outputs found
Comment on ``Solidification of a Supercooled Liquid in a Narrow Channel''
Comment on PRL v. 86, p. 5084 (2001) [cond-mat/0101016]. We point out that
the authors' simulations are consistent with the known theory of steady-state
solutions in this system
Microscopic Selection of Fluid Fingering Pattern
We study the issue of the selection of viscous fingering patterns in the
limit of small surface tension. Through detailed simulations of anisotropic
fingering, we demonstrate conclusively that no selection independent of the
small-scale cutoff (macroscopic selection) occurs in this system. Rather, the
small-scale cutoff completely controls the pattern, even on short time scales,
in accord with the theory of microscopic solvability. We demonstrate that
ordered patterns are dynamically selected only for not too small surface
tensions. For extremely small surface tensions, the system exhibits chaotic
behavior and no regular pattern is realized.Comment: 6 pages, 5 figure
Dynamics of Conformal Maps for a Class of Non-Laplacian Growth Phenomena
Time-dependent conformal maps are used to model a class of growth phenomena
limited by coupled non-Laplacian transport processes, such as nonlinear
diffusion, advection, and electro-migration. Both continuous and stochastic
dynamics are described by generalizing conformal-mapping techniques for viscous
fingering and diffusion-limited aggregation, respectively. A general notion of
time in stochastic growth is also introduced. The theory is applied to
simulations of advection-diffusion-limited aggregation in a background
potential flow. A universal crossover in morphology is observed from
diffusion-limited to advection-limited fractal patterns with an associated
crossover in the growth rate, controlled by a time-dependent effective Peclet
number. Remarkably, the fractal dimension is not affected by advection, in
spite of dramatic increases in anisotropy and growth rate, due to the
persistence of diffusion limitation at small scales.Comment: 4 pages, 2 figures (six color plates
Solution of an infection model near threshold
We study the Susceptible-Infected-Recovered model of epidemics in the
vicinity of the threshold infectivity. We derive the distribution of total
outbreak size in the limit of large population size . This is accomplished
by mapping the problem to the first passage time of a random walker subject to
a drift that increases linearly with time. We recover the scaling results of
Ben-Naim and Krapivsky that the effective maximal size of the outbreak scales
as , with the average scaling as , with an explicit form for
the scaling function
Taylor dispersion of gyrotactic swimming micro-organisms in a linear flow
The theory of generalized Taylor dispersion for suspensions of Brownian particles is developed to study the dispersion of gyrotactic swimming micro-organisms in a linear shear flow. Such creatures are bottom-heavy and experience a gravitational torque which acts to right them when they are tipped away from the vertical. They also suffer a net viscous torque in the presence of a local vorticity field. The orientation of the cells is intrinsically random but the balance of the two torques results in a bias toward a preferred swimming direction. The micro-organisms are sufficiently large that Brownian motion is negligible but their random swimming across streamlines results in a mean velocity together with diffusion. As an example, we consider the case of vertical shear flow and calculate the diffusion coefficients for a suspension of the alga <i>Chlamydomonas nivalis</i>. This rational derivation is compared with earlier approximations for the diffusivity
Quantum network of neutral atom clocks
We propose a protocol for creating a fully entangled GHZ-type state of
neutral atoms in spatially separated optical atomic clocks. In our scheme,
local operations make use of the strong dipole-dipole interaction between
Rydberg excitations, which give rise to fast and reliable quantum operations
involving all atoms in the ensemble. The necessary entanglement between distant
ensembles is mediated by single-photon quantum channels and collectively
enhanced light-matter couplings. These techniques can be used to create the
recently proposed quantum clock network based on neutral atom optical clocks.
We specifically analyze a possible realization of this scheme using neutral Yb
ensembles.Comment: 13 pages, 11 figure
Evidence for J and H-band excess in classical T Tauri stars and the implications for disk structure and estimated ages
We argue that classical T Tauri stars (cTTs) possess significant non-
photospheric excess in the J and H bands. We first show that normalizing the
spectral energy distributions (SEDs) of cTTs to the J-band leads to a poor fit
of the optical fluxes, while normalizing the SEDs to the Ic-band produces a
better fit to the optical bands and in many cases reveals the presence of a
considerable excess at J and H. NIR spectroscopic veiling measurements from the
literature support this result. We find that J and H-band excesses correlate
well with the K-band excess, and that the J-K and H-K colors of the excess
emission are consistent with that of a black body at the dust sublimation
temperature (~ 1500-2000 K). We propose that this near-IR excess originates at
a hot inner rim, analogous to those suggested to explain the near-IR bump in
the SEDs of Herbig Ae/Be stars. To test our hypothesis, we use the model
presented by Dullemond et al. (2001) to fit the photometry data between 0.5 um
and 24 um of 10 cTTs associated with the Chamaeleon II molecular cloud. The
models that best fit the data are those where the inner radius of the disk is
larger than expected for a rim in thermal equilibrium with the photospheric
radiation field alone. In particular, we find that large inner rims are
necessary to account for the mid infrared fluxes (3.6-8.0 um) obtained by the
Spitzer Space Telescope. Finally, we argue that deriving the stellar
luminosities of cTTs by making bolometric corrections to the J-band fluxes
systematically overestimates these luminosities. The overestimated luminosities
translate into underestimated ages when the stars are placed in the H-R
diagram. Thus, the results presented herein have important implications for the
dissipation timescale of inner accretion disks.Comment: 45 pages, 13 figure
Simulation and analysis of in vitro DNA evolution
We study theoretically the in vitro evolution of a DNA sequence by binding to
a transcription factor. Using a simple model of protein-DNA binding and
available binding constants for the Mnt protein, we perform large-scale,
realistic simulations of evolution starting from a single DNA sequence. We
identify different parameter regimes characterized by distinct evolutionary
behaviors. For each regime we find analytical estimates which agree well with
simulation results. For small population sizes, the DNA evolutional path is a
random walk on a smooth landscape. While for large population sizes, the
evolution dynamics can be well described by a mean-field theory. We also study
how the details of the DNA-protein interaction affect the evolution.Comment: 11 pages, 11 figures. Submitted to PNA
Influence of the temperature on the depinning transition of driven interfaces
We study the dynamics of a driven interface in a two-dimensional random-field
Ising model close to the depinning transition at small but finite temperatures
T using Glauber dynamics. A square lattice is considered with an interface
initially in (11)-direction. The drift velocity v is analyzed for the first
time using finite size scaling at T = 0 and additionally finite temperature
scaling close to the depinning transition. In both cases a perfect data
collapse is obtained from which we deduce beta = 1/3 for the exponent which
determines the dependence of v on the driving field, nu = 1 for the exponent of
the correlation length and delta = 5 for the exponent which determines the
dependence of v on T.Comment: 5 pages, Latex, Figures included, to appear in Europhys. Let
Phase-Field Model of Mode III Dynamic Fracture
We introduce a phenomenological continuum model for mode III dynamic fracture
that is based on the phase-field methodology used extensively to model
interfacial pattern formation. We couple a scalar field, which distinguishes
between ``broken'' and ``unbroken'' states of the system, to the displacement
field in a way that consistently includes both macroscopic elasticity and a
simple rotationally invariant short scale description of breaking. We report
two-dimensional simulations that yield steady-state crack motion in a strip
geometry above the Griffith threshold.Comment: submitted to PR
- …