13,337 research outputs found
Mapping of dissipative particle dynamics in fluctuating hydrodynamics simulations
Dissipative particle dynamics (DPD) is a novel particle method for mesoscale
modeling of complex fluids. DPD particles are often thought to represent
packets of real atoms, and the physical scale probed in DPD models are
determined by the mapping of DPD variables to the corresponding physical
quantities. However, the non-uniqueness of such mapping has led to difficulties
in setting up simulations to mimic real systems and in interpreting results.
For modeling transport phenomena where thermal fluctuations are important
(e.g., fluctuating hydrodynamics), an area particularly suited for DPD method,
we propose that DPD fluid particles should be viewed as only 1) to provide a
medium in which the momentum and energy are transferred according to the
hydrodynamic laws and 2) to provide objects immersed in the DPD fluids the
proper random "kicks" such that these objects exhibit correct fluctuation
behaviors at the macroscopic scale. We show that, in such a case, the choice of
system temperature and mapping of DPD scales to physical scales are uniquely
determined by the level of coarse-graining and properties of DPD fluids. We
also verified that DPD simulation can reproduce the macroscopic effects of
thermal fluctuation in particulate suspension by showing that the Brownian
diffusion of solid particles can be computed in DPD simulations with good
accuracy
Accelerating charging dynamics in sub-nanometer pores
Having smaller energy density than batteries, supercapacitors have
exceptional power density and cyclability. Their energy density can be
increased using ionic liquids and electrodes with sub-nanometer pores, but this
tends to reduce their power density and compromise the key advantage of
supercapacitors. To help address this issue through material optimization, here
we unravel the mechanisms of charging sub-nanometer pores with ionic liquids
using molecular simulations, navigated by a phenomenological model. We show
that charging of ionophilic pores is a diffusive process, often accompanied by
overfilling followed by de-filling. In sharp contrast to conventional
expectations, charging is fast because ion diffusion during charging can be an
order of magnitude faster than in bulk, and charging itself is accelerated by
the onset of collective modes. Further acceleration can be achieved using
ionophobic pores by eliminating overfilling/de-filling and thus leading to
charging behavior qualitatively different from that in conventional, ionophilic
pores
What if pulsars are born as strange stars?
The possibility and the implications of the idea, that pulsars are born as
strange stars, are explored. Strange stars are very likely to have atmospheres
with typical mass of but bare polar caps almost
throughout their lifetimes, if they are produced during supernova explosions. A
direct consequence of the bare polar cap is that the binding energies of both
positively and negatively charged particles at the bare quark surface are
nearly infinity, so that the vacuum polar gap sparking scenario as proposed by
Ruderman & Sutherland should operate above the cap, regardless of the sense of
the magnetic pole with respect to the rotational pole. Heat can not accumulate
on the polar cap region due to the large thermal conductivity on the bare quark
surface. We test this ``bare polar cap strange star'' (BPCSS) idea with the
present broad band emission data of pulsars, and propose several possible
criteria to distinguish BPCSSs from neutron stars.Comment: 31 pages in Latex. Accepted by AstroParticle Physic
Correlated two-photon scattering in cavity optomechanics
We present an exact analytical solution of the two-photon scattering in a
cavity optomechanical system. This is achieved by solving the quantum dynamics
of the total system, including the optomechanical cavity and the cavity-field
environment, with the Laplace transform method. The long-time solution reveals
detailed physical processes involved as well as the corresponding resonant
photon frequencies. We characterize the photon correlation induced in the
scattering process by calculating the two-photon joint spectrum of the
long-time state. Clear evidence for photon frequency anti-correlation can be
observed in the joint spectrum. In addition, we calculate the equal-time
second-order correlation function of the cavity photons. The results show that
the radiation pressure coupling can induce photon blockade effect, which is
strongly modulated by the phonon sideband resonance. In particular, we obtain
an explicit expression of optomechanical coupling strength determining these
sideband modulation peaks based on the two-photon resonance condition.Comment: 10 pages, 6 figure
Electrical fixing of photorefractive holograms in Sr0.75Ba0.25Nb2O6
Photorefractive holograms stored in Sr0.75Ba0.25Nb2O6 crystals are electrically fixed at room temperature. The fixed holograms can be read out directly or after a positive-voltage pulse is applied that can dramatically enhance the diffraction efficiency. Single gratings as well as images are recorded and fixed
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