20,180 research outputs found
Considerations on bubble fragmentation models
n this paper we describe the restrictions that the probability density function (p.d.f.) of the size of particles resulting from the rupture of a drop or bubble must satisfy. Using conservation of volume, we show that when a particle of diameter, D0, breaks into exactly two fragments of sizes D and D2 = (D30−D3)1/3 respectively, the resulting p.d.f., f(D; D0), must satisfy a symmetry relation given by D22 f(D; D0) = D2 f(D2; D0), which does not depend on the nature of the underlying fragmentation process. In general, for an arbitrary number of resulting particles, m(D0), we determine that the daughter p.d.f. should satisfy the conservation of volume condition given by m(D0) ∫0D0 (D/D0)3 f(D; D0) dD = 1. A detailed analysis of some contemporary fragmentation models shows that they may not exhibit the required conservation of volume condition if they are not adequately formulated. Furthermore, we also analyse several models proposed in the literature for the breakup frequency of drops or bubbles based on different principles, g(ϵ, D0). Although, most of the models are formulated in terms of the particle size D0 and the dissipation rate of turbulent kinetic energy, ϵ, and apparently provide different results, we show here that they are nearly identical when expressed in dimensionless form in terms of the Weber number, g*(Wet) = g(ϵ, D0) D2/30 ϵ−1/3, with Wet ~ ρ ϵ2/3 D05/3/σ, where ρ is the density of the continuous phase and σ the surface tension
Heat flow in the postquasistatic approximation
We apply the postquasistatic approximation to study the evolution of
spherically symmetric fluid distributions undergoing dissipation in the form of
radial heat flow. For a model which corresponds to an incompressible fluid
departing from the static equilibrium, it is not possible to go far from the
initial state after the emission of a small amount of energy. Initially
collapsing distributions of matter are not permitted. Emission of energy can be
considered as a mechanism to avoid the collapse. If the distribution collapses
initially and emits one hundredth of the initial mass only the outermost layers
evolve. For a model which corresponds to a highly compressed Fermi gas, only
the outermost shell can evolve with a shorter hydrodynamic time scale.Comment: 5 pages, 5 figure
Disorder-Induced First Order Transition and Curie Temperature Lowering in Ferromagnatic Manganites
We study the effect that size disorder in the cations surrounding manganese
ions has on the magnetic properties of manganites. This disorder is mimic with
a proper distribution of spatially disordered Manganese energies. Both, the
Curie temperature and the order of the transition are strongly affected by
disorder. For moderate disorder the Curie temperature decreases linearly with
the the variance of the distribution of the manganese site energies, and for a
disorder comparable to that present in real materials the transition becomes
first order. Our results provide a theoretical framework to understand disorder
effects on the magnetic behavior of manganites.Comment: 4 pages, three figures include
Glassy dynamics, aging and thermally activated avalanches in interface pinning at finite temperatures
We study numerically the out-of-equilibrium dynamics of interfaces at finite
temperatures when driven well below the zero-temperature depinning threshold.
We go further than previous analysis by including the most relevant
non-equilibrium correction to the elastic Hamiltonian. We find that the
relaxation dynamics towards the steady-state shows glassy behavior, aging and
violation of the fluctuation-dissipation theorem. The interface roughness
exponent alpha approx 0.7 is found to be robust to temperature changes. We also
study the instantaneous velocity signal in the low temperature regime and find
long-range temporal correlations. We argue 1/f-noise arises from the merging of
local thermally-activated avalanches of depinning events.Comment: 4 pages, 4 figure
El transporte público en la ciudad. Subsidio a las mayorías o "realismo económico"
La problemática del transporte urbano de pasajeros está determinada en gran parte por la propia dimensión y organización territorial de la ciudad. Es el caso de la zona metropolitana del Distrito Federal, donde se concentran 16 millones de personasen una superficie de aproximadamente 1200 kms. cuadrados. En el momento actual de crisis, uno de los aspectos más relevantes de la política estatal de los transportes es el del subsidio que funciona como salario indirecto para los sectores mayoritarios de la población urbana. El retiro intempestivo sería una medida errónea de consecuencias económicas y sociales imprevisibles
Lesiones traumáticas en cráneos aislados de los poblados ibéricos de Ullastret (Girona)
X Congreso Nacional de Paleopatología. Univesidad Autónoma de Madrid, septiembre de 200
El presupuesto de la ciudad de México o el subsidio de la nación.
El presupuesto programado para 1984, aún en su austeridad, expresa también el creciente subsidio que el conjunto de la nación da a la ciudad de México a través de la participación del Distrito Federal en las aportaciones del Gobierno Federal. Las posibles alternativas a la profunda problemática de las finanzas del DDF enfrentan marcos muy estrechos que limitan seriamente las supuestas alternativas; la ausencia de una reforma fiscal nacional integral, la crisis económica y el particular papel de la ciudad en el conjunto del país, son, entre otros, algunos de los límites más importantes
Ultrafast non-linear optical signal from a single quantum dot: exciton and biexciton effects
We present results on both the intensity and phase-dynamics of the transient
non-linear optical response of a single quantum dot (SQD).
The time evolution of the Four Wave Mixing (FWM) signal on a subpicosecond
time scale is dominated by biexciton effects. In particular, for the
cross-polarized excitation case a biexciton bound state is found. In this
latter case, mean-field results are shown to give a poor description of the
non-linear optical signal at small times. By properly treating exciton-exciton
effects in a SQD, coherent oscillations in the FWM signal are clearly
demonstrated. These oscillations, with a period corresponding to the inverse of
the biexciton binding energy, are correlated with the phase dynamics of the
system's polarization giving clear signatures of non-Markovian effects in the
ultrafast regime.Comment: 10 pages, 3 figure
- …