86 research outputs found
Computing stationary free-surface shapes in microfluidics
A finite-element algorithm for computing free-surface flows driven by
arbitrary body forces is presented. The algorithm is primarily designed for the
microfluidic parameter range where (i) the Reynolds number is small and (ii)
force-driven pressure and flow fields compete with the surface tension for the
shape of a stationary free surface. The free surface shape is represented by
the boundaries of finite elements that move according to the stress applied by
the adjacent fluid. Additionally, the surface tends to minimize its free energy
and by that adapts its curvature to balance the normal stress at the surface.
The numerical approach consists of the iteration of two alternating steps: The
solution of a fluidic problem in a prescribed domain with slip boundary
conditions at the free surface and a consecutive update of the domain driven by
the previously determined pressure and velocity fields. ...Comment: Revised versio
Thick surface flows of granular materials: The effect of the velocity profile on the avalanche amplitude
A few years ago, Bouchaud al. introduced a phenomenological model to describe
surface flows of granular materials [J. Phys. Fr. I, 4, 1383 (1994)]. According
to this model, one can distinguish between a static phase and a rolling phase
that are able to exchange grains through an erosion/accretion mechanism.
Boutreux et al. [Phys. Rev. E, 58, 4692 (1998)] proposed a modification of the
exchange term in order to describe thicker flows where saturation effects are
present. However, these approaches assumed that the downhill convection
velocity of the grains is constant inside the rolling phase, a hypothesis that
is not verified experimentally. In this article, we therefore modify the above
models by introducing a velocity profile in the flow, and study the physical
consequences of this modification in the simple situation of an avalanche in an
open cell. We present a complete analytical description of the avalanche in the
case of a linear velocity profile, and generalize the results for a power-law
dependency. We show, in particular, that the amplitude of the avalanche is
strongly affected by the velocity profile.Comment: 7 figures, accepted for publication in Phys. Rev.
The GRA Beam-Splitter Experiments and Particle-Wave Duality of Light
Grangier, Roger and Aspect (GRA) performed a beam-splitter experiment to
demonstrate the particle behaviour of light and a Mach-Zehnder interferometer
experiment to demonstrate the wave behaviour of light. The distinguishing
feature of these experiments is the use of a gating system to produce near
ideal single photon states. With the demonstration of both wave and particle
behaviour (in two mutually exclusive experiments) they claim to have
demonstrated the dual particle-wave behaviour of light and hence to have
confirmed Bohr's principle of complementarity. The demonstration of the wave
behaviour of light is not in dispute. But we want to demonstrate, contrary to
the claims of GRA, that their beam-splitter experiment does not conclusively
confirm the particle behaviour of light, and hence does not confirm
particle-wave duality, nor, more generally, does it confirm complementarity.
Our demonstration consists of providing a detailed model based on the Causal
Interpretation of Quantum Fields (CIEM), which does not involve the particle
concept, of GRA's which-path experiment. We will also give a brief outline of a
CIEM model for the second, interference, GRA experiment.Comment: 24 pages, 4 figure
Basins of attraction on random topography
We investigate the consequences of fluid flowing on a continuous surface upon
the geometric and statistical distribution of the flow. We find that the
ability of a surface to collect water by its mere geometrical shape is
proportional to the curvature of the contour line divided by the local slope.
Consequently, rivers tend to lie in locations of high curvature and flat
slopes. Gaussian surfaces are introduced as a model of random topography. For
Gaussian surfaces the relation between convergence and slope is obtained
analytically. The convergence of flow lines correlates positively with drainage
area, so that lower slopes are associated with larger basins. As a consequence,
we explain the observed relation between the local slope of a landscape and the
area of the drainage basin geometrically. To some extent, the slope-area
relation comes about not because of fluvial erosion of the landscape, but
because of the way rivers choose their path. Our results are supported by
numerically generated surfaces as well as by real landscapes
Dichotomous Markov noise: Exact results for out-of-equilibrium systems. A review
Nonequilibrium systems driven by additive or multiplicative dichotomous
Markov noise appear in a wide variety of physical and mathematical models. We
review here some prototypical examples, with an emphasis on {\em
analytically-solvable} situations. In particular, it has escaped attention till
recently that the standard results for the long-time properties of such systems
cannot be applied when unstable fixed points are crossed in the asymptotic
regime. We show how calculations have to be modified to deal with these cases
and present a few relevant applications -- the hypersensitive transport, the
rocking ratchet, and the stochastic Stokes' drift. These results reinforce the
impression that dichotomous noise can be put on a par with Gaussian white noise
as far as obtaining analytical results is concerned. They convincingly
illustrate the interplay between noise and nonlinearity in generating
nontrivial behaviors of nonequilibrium systems and point to various practical
applications.Comment: Review article, 85 pages, 24 figures. Some references added.
Published in the International Journal of Modern Physics B 20, pp. 2825-2888
(2006). Comments and suggestions from the readers are highly welcom
Novel Photosensitizers Trigger Rapid Death of Malignant Human Cells and Rodent Tumor Transplants via Lipid Photodamage and Membrane Permeabilization
BACKGROUND: Apoptotic cascades may frequently be impaired in tumor cells; therefore, the approaches to circumvent these obstacles emerge as important therapeutic modalities. METHODOLOGY/PRINCIPAL FINDINGS: Our novel derivatives of chlorin e(6), that is, its amide (compound 2) and boronated amide (compound 5) evoked no dark toxicity and demonstrated a significantly higher photosensitizing efficacy than chlorin e(6) against transplanted aggressive tumors such as B16 melanoma and M-1 sarcoma. Compound 5 showed superior therapeutic potency. Illumination with red light of mammalian tumor cells loaded with 0.1 µM of 5 caused rapid (within the initial minutes) necrosis as determined by propidium iodide staining. The laser confocal microscopy-assisted analysis of cell death revealed the following order of events: prior to illumination, 5 accumulated in Golgi cysternae, endoplasmic reticulum and in some (but not all) lysosomes. In response to light, the reactive oxygen species burst was concomitant with the drop of mitochondrial transmembrane electric potential, the dramatic changes of mitochondrial shape and the loss of integrity of mitochondria and lysosomes. Within 3-4 min post illumination, the plasma membrane became permeable for propidium iodide. Compounds 2 and 5 were one order of magnitude more potent than chlorin e(6) in photodamage of artificial liposomes monitored in a dye release assay. The latter effect depended on the content of non-saturated lipids; in liposomes consisting of saturated lipids no photodamage was detectable. The increased therapeutic efficacy of 5 compared with 2 was attributed to a striking difference in the ability of these photosensitizers to permeate through hydrophobic membrane interior as evidenced by measurements of voltage jump-induced relaxation of transmembrane current on planar lipid bilayers. CONCLUSIONS/SIGNIFICANCE: The multimembrane photodestruction and cell necrosis induced by photoactivation of 2 and 5 are directly associated with membrane permeabilization caused by lipid photodamage
Experimental and computational investigations of electron dynamics in micro atmospheric pressure radio-frequency plasma jets operated in He/N2 mixtures
The electron power absorption dynamics in radio frequency driven micro atmospheric pressure capacitive plasma jets are studied based on experimental phase resolved optical emission spectroscopy and the computational particle in cell simulations with Monte Carlo treatment of collisions. The jet is operated at 13.56 MHz in He with different admixture concentrations of N2 and at several driving voltage amplitudes. We find the spatio-temporal dynamics of the light emission of the plasma at various wavelengths to be markedly different. This is understood by revealing the population dynamics of the upper levels of selected emission lines/bands based on comparisons between experimental and simulation results. The populations of these excited states are sensitive to different parts of the electron energy distribution function and to contributions from other excited states. Mode transitions of the electron power absorption dynamics from the Ω- to the Penning-mode are found to be induced by changing the N2 admixture concentration and the driving voltage amplitude. Our numerical simulations reveal details of this mode transition and provide novel insights into the operation details of the Penning-mode. The characteristic excitation/emission maximum at the time of maximum sheath voltage at each electrode is found to be based on two mechanisms: (i) a direct channel, i.e. excitation/emission caused by electrons generated by Penning ionization inside the sheaths and (ii) an indirect channel, i.e. secondary electrons emitted from the electrode due to the impact of positive ions generated by Penning ionization at the electrodes
One-node closed multichannel service system: several types of customers and service rates, and random pick-up from the waiting line
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