317 research outputs found
A Dynamical Theory of Electron Transfer: Crossover from Weak to Strong Electronic Coupling
We present a real-time path integral theory for the rate of electron transfer
reactions. Using graph theoretic techniques, the dynamics is expressed in a
formally exact way as a set of integral equations. With a simple approximation
for the self-energy, the rate can then be computed analytically to all orders
in the electronic coupling matrix element. We present results for the crossover
region between weak (nonadiabatic) and strong (adiabatic) electronic coupling
and show that this theory provides a rigorous justification for the salient
features of the rate expected within conventional electron transfer theory.
Nonetheless, we find distinct characteristics of quantum behavior even in the
strongly adiabatic limit where classical rate theory is conventionally thought
to be applicable. To our knowledge, this theory is the first systematic
dynamical treatment of the full crossover region.Comment: 11 pages, LaTeX, 8 Postscript figures to be published in J. Chem.
Phy
Faraday Instability in a Surface-Frozen Liquid
Faraday surface instability measurements of the critical acceleration, a_c,
and wavenumber, k_c, for standing surface waves on a tetracosanol (C_24H_50)
melt exhibit abrupt changes at T_s=54degC above the bulk freezing temperature.
The measured variations of a_c and k_c vs. temperature and driving frequency
are accounted for quantitatively by a hydrodynamic model, revealing a change
from a free-slip surface flow, generic for a free liquid surface (T>T_s), to a
surface-pinned, no-slip flow, characteristic of a flow near a wetted solid wall
(T < T_s). The change at T_s is traced to the onset of surface freezing, where
the steep velocity gradient in the surface-pinned flow significantly increases
the viscous dissipation near the surface.Comment: 4 pages, 3 figures. Physical Review Letters (in press
Quantum Kinetic Theory of Condensate Growth---Comparison of Experiment and Theory
In a major extension of our previous model (C.W. Gardiner, P. Zoller,
R.J. Ballagh and M.J. Davis, Phys. Rev. Lett. 79, 1793 (1997)) of condensate
growth, we take account of the evolution of the occupations of lower trap
levels, and of the full Bose-Einstein formula for the occupations of higher
trap levels. We find good agreement with experiment, especially at higher
temperatures. We also confirm the picture of the ``kinetic'' region of
evolution, introduced by Kagan et al, for the time up to the initiation of the
condensate. The behavior after initiation essentially follows our original
growth equation, but with a substantially increased rate coefficient W^{+}.Comment: RevTeX, 4 pages and 4 eps figure
Random Sequential Adsorption: From Continuum to Lattice and Pre-Patterned Substrates
The random sequential adsorption (RSA) model has served as a paradigm for
diverse phenomena in physical chemistry, as well as in other areas such as
biology, ecology, and sociology. In the present work, we survey aspects of the
RSA model with emphasis on the approach to and properties of jammed states
obtained for large times in continuum deposition versus that on lattice
substrates, and on pre-patterned surfaces. The latter model has been of recent
interest in the context of efforts to use pre-patterning as a tool to improve
selfassembly in micro- and nanoscale surface structure engineering
Condensate growth in trapped Bose gases
We study the dynamics of condensate formation in an inhomogeneous trapped
Bose gas with a positive interatomic scattering length. We take into account
both the nonequilibrium kinetics of the thermal cloud and the Hartree-Fock
mean-field effects in the condensed and the noncondensed parts of the gas. Our
growth equations are solved numerically by assuming that the thermal component
behaves ergodically and that the condensate, treated within the Thomas-Fermi
approximation, grows adiabatically. Our simulations are in good qualitative
agreement with experiment, however important discrepancies concerning details
of the growth behaviour remain.Comment: 28 pages, 11 figures. Changes made to the introduction, Sec. VI, Sec.
VII, and included additional growth curves in Fig. 1
Thermal Properties of Two-Dimensional Advection Dominated Accretion Flow
We study the thermal structure of the widely adopted two-dimensional
advection dominated accretion flow (ADAF) of Narayan & Yi (1995a). The critical
radius for a given mass accretion rate, outside of which the optically thin hot
solutions do not exist in the equatorial plane, agrees with one-dimensional
study. However, we find that, even within the critical radius, there always
exists a conical region of the flow, around the pole, which cannot maintain the
assumed high electron temperature, regardless of the mass accretion rate, in
the absence of radiative heating. This could lead to torus-like advection
inflow shape since, in general, the ions too will cool down. We also find that
Compton preheating is generally important and, if the radiative efficiency,
defined as the luminosity output divided by the mass accretion rate times the
velocity of light squared, is above sim 4x10^-3, the polar region of the flow
is preheated above the virial temperature by Compton heating and it may result
in time-dependent behaviour or outflow while accretion continues in the
equatorial plane. Thus, under most relevant circumstances, ADAF solutions may
be expected to be accompanied by polar outflow winds. While preheating
instabilities exist in ADAF, as for spherical flows, the former are to some
extent protected by their characteristically higher densities and higher
cooling rates, which reduce their susceptibility to Compton driven overheating.Comment: 18 pages including 4 figures. AASTEX. Submitted to Ap
Optimizing Technological Parameters of the Reduction Processes in Treating Steels in a Ladle Furnace
This work reports the possible development of reduction processes when treating the molten metal and slag using a ladle furnace under conditions of intensive stirring with an inert gas. The industrial data have been received, confirming the possibility of decreasing the concentration of ferrous and manganese oxides in the slag and stabilizing the contents of manganese and silicon in the metal
Self-sculpting of a dissolvable body due to gravitational convection
© 2018 American Physical Society. Natural sculpting processes such as erosion or dissolution often yield universal shapes that bear no imprint or memory of the initial conditions. Here we conduct laboratory experiments aimed at assessing the shape dynamics and role of memory for the simple case of a dissolvable boundary immersed in a fluid. Though no external flow is imposed, dissolution and consequent density differences lead to gravitational convective flows that in turn strongly affect local dissolving rates and shape changes, and we identify two distinct behaviors. A flat boundary dissolving from its lower surface tends to retain its overall shape (an example of near perfect memory) while bearing small-scale pits that reflect complex near-body flows. A boundary dissolving from its upper surface tends to erase its initial shape and form an upward spike structure that sharpens indefinitely. We propose an explanation for these different outcomes based on observations of the coupled shape dynamics, concentration fields, and flows
Kinetics of Bose-Condensation
The process of condensation in the system of scalar Bosons with weak interaction is considered. Boltzmann kinetic equation is solved
numerically. Bose condensation proceeds in two stages: At the first stage
condensate is still absent but there is non-zero inflow of particles towards
and the distribution function at grows
from finite values to infinity. At the second stage there are two components,
condensate and particles, reaching their equilibrium values. We show that the
evolution in both stages proceeds in a self - similar way and find the time
needed for condensation, which is finite.Comment: 12 pages, LaTeX RevTeX 3.0, includes 4 eps figure
Diffuse-Charge Dynamics in Electrochemical Systems
The response of a model micro-electrochemical system to a time-dependent
applied voltage is analyzed. The article begins with a fresh historical review
including electrochemistry, colloidal science, and microfluidics. The model
problem consists of a symmetric binary electrolyte between parallel-plate,
blocking electrodes which suddenly apply a voltage. Compact Stern layers on the
electrodes are also taken into account. The Nernst-Planck-Poisson equations are
first linearized and solved by Laplace transforms for small voltages, and
numerical solutions are obtained for large voltages. The ``weakly nonlinear''
limit of thin double layers is then analyzed by matched asymptotic expansions
in the small parameter , where is the
screening length and the electrode separation. At leading order, the system
initially behaves like an RC circuit with a response time of
(not ), where is the ionic diffusivity, but nonlinearity
violates this common picture and introduce multiple time scales. The charging
process slows down, and neutral-salt adsorption by the diffuse part of the
double layer couples to bulk diffusion at the time scale, . In the
``strongly nonlinear'' regime (controlled by a dimensionless parameter
resembling the Dukhin number), this effect produces bulk concentration
gradients, and, at very large voltages, transient space charge. The article
concludes with an overview of more general situations involving surface
conduction, multi-component electrolytes, and Faradaic processes.Comment: 10 figs, 26 pages (double-column), 141 reference
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