177 research outputs found
Cracks and fingers: dynamics of ductile fracture in an aqueous foam
Fracture of a quasi-two-dimensional aqueous foam by injection of air can occur via two distinct mechanisms, termed brittle and ductile, which are analogous to crack modes observed for crystalline atomic solids such as metals. In the present work we focus on the dynamics and morphology of the ductile process, in which no films between bubbles are broken. A network modeling approach allows detailed analysis of the foam morphology from individual bubbles to the shape of the propagating crack. This crack develops similarly to fingering instabilities in Hele–Shaw cells filled with homogeneous fluids. We show that the observed width and shape of the crack are compatible this interpretation, and that the discreteness of the bubble structure provides symmetry perturbations and limiting scales characteristic of anomalous fingering. The model thus bridges the gap between fracture of the solid foam lattice and instability growth of interfaces in a fluid system
Response of bubbles to diagnotic ultrasound:a unifying theoretical approach
The scattering of ultrasound from bubbles of m radius, such as used in contrast enhancers for ultrasound diagnostics, is studied. We show that sound scattering and ``active'' emission of sound from oscillating bubbles are not contradictory, but are just two different aspects derived from the same physics. Treating the bubble as a nonlinear oscillator, we arrive at general formulas for scattering and absorption cross-sections. We show that several well-known formulas are recovered in the linear limit of this ansatz. In the case of strongly nonlinear oscillations, however, the cross-sections can be larger than those for linear response by several orders of magnitude. The major part of the incident sound energy is then converted into emitted sound, unlike what happens in the linear case, where the absorption cross-sections exceed the scattering cross-sections
Sonoluminescing air bubbles rectify argon
The dynamics of single bubble sonoluminescence (SBSL) strongly depends on the
percentage of inert gas within the bubble. We propose a theory for this
dependence, based on a combination of principles from sonochemistry and
hydrodynamic stability. The nitrogen and oxygen dissociation and subsequent
reaction to water soluble gases implies that strongly forced air bubbles
eventually consist of pure argon. Thus it is the partial argon (or any other
inert gas) pressure which is relevant for stability. The theory provides
quantitative explanations for many aspects of SBSL.Comment: 4 page
Sonoluminescence: Nature's Smallest BlackBody
The Spectrum of the light emitted by a sonoluminescing bubble is extremely
well fit by the spectrum of a blackbody. Furthermore the radius of emission can
be smaller than the wavelength of the light. Consequences, for theories of
sonoluminescence are discussed.Comment: 8 pages, 3 Figure
Comment on Mie Scattering from a Sonoluminescing Bubble with High Spatial and Temporal Resolution [Physical Review E 61, 5253 (2000)]
A key parameter underlying the existence of sonoluminescence (SL)is the time
relative to SL at which acoustic energy is radiated from the collapsed bubble.
Light scattering is one route to this quantity. We disagree with the statement
of Gompf and Pecha that -highly compressed water causes the minimum in
scattered light to occur 700ps before SL- and that this effect leads to an
overestimate of the bubble wall velocity. We discuss potential artifacts in
their experimental arrangement and correct their description of previous
experiments on Mie scattering.Comment: 10 pages, 2 figure
Statistical Mechanics of Two-dimensional Foams
The methods of statistical mechanics are applied to two-dimensional foams
under macroscopic agitation. A new variable -- the total cell curvature -- is
introduced, which plays the role of energy in conventional statistical
thermodynamics. The probability distribution of the number of sides for a cell
of given area is derived. This expression allows to correlate the distribution
of sides ("topological disorder") to the distribution of sizes ("geometrical
disorder") in a foam. The model predictions agree well with available
experimental data
Cavitation induced by explosion in a model of ideal fluid
We discuss the problem of an explosion in the cubic-quintic superfluid model,
in relation to some experimental observations. We show numerically that an
explosion in such a model might induce a cavitation bubble for large enough
energy. This gives a consistent view for rebound bubbles in superfluid and we
indentify the loss of energy between the successive rebounds as radiated waves.
We compute self-similar solution of the explosion for the early stage, when no
bubbles have been nucleated. The solution also gives the wave number of the
excitations emitted through the shock wave.Comment: 21 pages,13 figures, other comment
Sonoluminescence as a QED vacuum effect. I: The Physical Scenario
Several years ago Schwinger proposed a physical mechanism for
sonoluminescence in terms of changes in the properties of the
quantum-electrodynamic (QED) vacuum state. This mechanism is most often phrased
in terms of changes in the Casimir Energy: changes in the distribution of
zero-point energies and has recently been the subject of considerable
controversy. The present paper further develops this quantum-vacuum approach to
sonoluminescence: We calculate Bogolubov coefficients relating the QED vacuum
states in the presence of a homogeneous medium of changing dielectric constant.
In this way we derive an estimate for the spectrum, number of photons, and
total energy emitted. We emphasize the importance of rapid spatio-temporal
changes in refractive indices, and the delicate sensitivity of the emitted
radiation to the precise dependence of the refractive index as a function of
wavenumber, pressure, temperature, and noble gas admixture. Although the
physics of the dynamical Casimir effect is a universal phenomenon of QED,
specific experimental features are encoded in the condensed matter physics
controlling the details of the refractive index. This calculation places rather
tight constraints on the possibility of using the dynamical Casimir effect as
an explanation for sonoluminescence, and we are hopeful that this scenario will
soon be amenable to direct experimental probes. In a companion paper we discuss
the technical complications due to finite-size effects, but for reasons of
clarity in this paper we confine attention to bulk effects.Comment: 25 pages, LaTeX 209, ReV-TeX 3.2, eight figures. Minor revisions:
Typos fixed, references updated, minor changes in numerical estimates, minor
changes in some figure
An Alternative Method to Deduce Bubble Dynamics in Single Bubble Sonoluminescence Experiments
In this paper we present an experimental approach that allows to deduce the
important dynamical parameters of single sonoluminescing bubbles (pressure
amplitude, ambient radius, radius-time curve) The technique is based on a few
previously confirmed theoretical assumptions and requires the knowledge of
quantities such as the amplitude of the electric excitation and the phase of
the flashes in the acoustic period. These quantities are easily measurable by a
digital oscilloscope, avoiding the cost of expensive lasers, or ultrafast
cameras of previous methods. We show the technique on a particular example and
compare the results with conventional Mie scattering. We find that within the
experimental uncertainties these two techniques provide similar results.Comment: 8 pages, 5 figures, submitted to Phys. Rev.
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