102 research outputs found
Non-linear effects and shock formation in the focusing of a spherical acoustic wave : Numerical simulations and experiments in liquid helium
The focusing of acoustic waves is used to study nucleation phenomena in
liquids. At large amplitude, non-linear effects are important so that the
magnitude of pressure or density oscillations is difficult to predict. We
present a calculation of these oscillations in a spherical geometry.
We show that the main source of non-linearities is the shape of the equation
of state of the liquid, enhanced by the spherical geometry. We also show that
the formation of shocks cannot be ignored beyond a certain oscillation
amplitude. The shock length is estimated by an analytic calculation based on
the characteristics method. In our numerical simulations, we have treated the
shocks with a WENO scheme. We obtain a very good agreement with experimental
measurements which were recently performed in liquid helium. The comparison
between numerical and experimental results allows in particular to calibrate
the vibration of the ceramics used to produce the wave, as a function of the
applied voltage.Comment: 20 pages, 26 figures. Submitted to The European Physical Journal
Freezing of He-4 and its liquid-solid interface from Density Functional Theory
We show that, at high densities, fully variational solutions of solid-like
type can be obtained from a density functional formalism originally designed
for liquid 4He. Motivated by this finding, we propose an extension of the
method that accurately describes the solid phase and the freezing transition of
liquid 4He at zero temperature. The density profile of the interface between
liquid and the (0001) surface of the 4He crystal is also investigated, and its
surface energy evaluated. The interfacial tension is found to be in
semiquantitative agreement with experiments and with other microscopic
calculations. This opens the possibility to use unbiased DF methods to study
highly non-homogeneous systems, like 4He interacting with strongly attractive
impurities/substrates, or the nucleation of the solid phase in the metastable
liquid.Comment: 5 pages, 4 figures, submitted to Phys. Rev.
Detecting vapour bubbles in simulations of metastable water
International audienceThe investigation of cavitation in metastable liquids with molecular simulations requires an appropriate definition of the volume of the vapour bubble forming within the metastable liquid phase. Commonly used approaches for bubble detection exhibit two significant flaws: first, when applied to water they often identify the voids within the hydrogen bond network as bubbles thus masking the signature of emerging bubbles and, second, they lack thermodynamic consistency. Here, we present two grid-based methods, the M-method and the V-method, to detect bubbles in metastable water specifically designed to address these shortcomings. The M-method incorporates information about neighbouring grid cells to distinguish between liquid- and vapour-like cells, which allows for a very sensitive detection of small bubbles and high spatial resolution of the detected bubbles. The V-method is calibrated such that its estimates for the bubble volume correspond to the average change in system volume and are thus thermodynamically consistent. Both methods are computationally inexpensive such that they can be used in molecular dynamics and Monte Carlo simulations of cavitation. We illustrate them by computing the free energy barrier and the size of the critical bubble for cavitation in water at negative pressure
Cavitation pressure in liquid helium
Recent experiments have suggested that, at low enough temperature, the
homogeneous nucleation of bubbles occurs in liquid helium near the calculated
spinodal limit. This was done in pure superfluid helium 4 and in pure normal
liquid helium 3. However, in such experiments, where the negative pressure is
produced by focusing an acoustic wave in the bulk liquid, the local amplitude
of the instantaneous pressure or density is not directly measurable. In this
article, we present a series of measurements as a function of the static
pressure in the experimental cell. They allowed us to obtain an upper bound for
the cavitation pressure P_cav (at low temperature, P_cav < -2.4 bar in helium
3, P_cav < -8.0 bar in helium 4). From a more precise study of the acoustic
transducer characteristics, we also obtained a lower bound (at low temperature,
P_cav > -3.0 bar in helium 3, P_cav > - 10.4 bar in helium 4). In this article
we thus present quantitative evidence that cavitation occurs at low temperature
near the calculated spinodal limit (-3.1 bar in helium 3 and -9.5 bar in helium
4). Further information is also obtained on the comparison between the two
helium isotopes. We finally discuss the magnitude of nonlinear effects in the
focusing of a sound wave in liquid helium, where the pressure dependence of the
compressibility is large.Comment: 11 pages, 9 figure
Anomaly in the stability limit of liquid helium 3
We propose that the liquid-gas spinodal line of helium 3 reaches a minimum at
0.4 K. This feature is supported by our cavitation measurements. We also show
that it is consistent with extrapolations of sound velocity measurements.
Speedy [J. Phys. Chem. 86, 3002 (1982)] previously proposed this peculiar
behavior for the spinodal of water and related it to a change in sign of the
expansion coefficient alpha, i. e. a line of density maxima. Helium 3 exhibits
such a line at positive pressure. We consider its extrapolation to negative
pressure. Our discussion raises fundamental questions about the sign of alpha
in a Fermi liquid along its spinodal.Comment: 5 pages, 3 figure
Quantum cavitation in liquid He: dissipation effects
We have investigated the effect that dissipation may have on the cavitation
process in normal liquid He. Our results indicate that a rather small
dissipation decreases sizeably the quantum-to-thermal crossover temperature
for cavitation in normal liquid He. This is a possible explanation
why recent experiments have not yet found clear evidence of quantum cavitation
at temperatures below the predicted by calculations which neglect
dissipation.Comment: To be published in Physical Review B6
Effect of dissolved salt on the anomalies of water at negative pressure
Adding salt to water at ambient pressure affects its thermodynamic
properties. At low salt concentration, anomalies such as the density maximum
are shifted to lower temperature, while at large enough salt concentration they
cannot be observed any more. Here we investigate the effect of salt on an
anomaly recently observed in pure water at negative pressure: the existence of
a sound velocity minimum along isochores. We compare experiments and
simulations for an aqueous solution of sodium chloride with molality around
, reaching pressures beyond .
We also discuss the origin of the minima in the sound velocity and emphasize
the importance of the relative position of the temperatures of sound velocity
and density anomalies.Comment: 10 pages, 13 figures, 1 appendix. Added minor simulation detail
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