9,313 research outputs found
Breaking the Rayleigh-Plateau instability limit using thermocavitation within a droplet
We report on the generation of liquid columns that extend far beyond the traditional Rayleigh-Plateau instability onset. The columns are driven by the acoustic pressure wave emitted after bubble collapse. A high-speed video imaging device, which records images at a rate of up to 105 fps, was employed to follow their dynamics. These bubbles, commonly termed thermocavitation bubbles, are generated by focusing a midpower (275 mW) continuous wavelength laser into a highly absorbing liquid droplet. A simple model of the propagation of the pressure wavefront emitted after the bubble collapse shows that focusing the pressure wave at the liquid-air interface drives the evolution of the liquid columns. Control over the aspect ratio of the liquid column is realized by adjusting the cavitation bubble's size, beam focus position, and droplet volume. © 2013 by Begell House, Inc
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Controllable direction of liquid jets generated by thermocavitation within a droplet.
A high-velocity fluid stream ejected from an orifice or nozzle is a common mechanism to produce liquid jets in inkjet printers or to produce sprays among other applications. In the present research, we show the generation of liquid jets of controllable direction produced within a sessile water droplet by thermocavitation. The jets are driven by an acoustic shock wave emitted by the collapse of a hemispherical vapor bubble at the liquid-solid/substrate interface. The generated shock wave is reflected at the liquid-air interface due to acoustic impedance mismatch generating multiple reflections inside the droplet. During each reflection, a force is exerted on the interface driving the jets. Depending on the position of the generation of the bubble within the droplet, the mechanical energy of the shock wave is focused on different regions at the liquid-air interface, ejecting cylindrical liquid jets at different angles. The ejected jet angle dependence is explained by a simple ray tracing model of the propagation of the acoustic shock wave inside the droplet
Open cluster candidates in the VVVX area: VVVX CL 076 and CL 077
We are reporting some basic parameters of two newly discovered clusters, VVVX CL 076 and CL 077, recently discovered in the galactic disk area covered by the VISTA Variables in the Via Lactea eXtended (VVVX) ESO Public Survey. The preliminary analysis shows that both clusters are young and relatively close to the Sun.Peer reviewedFinal Published versio
Compton Echoes from Gamma-Ray Bursts: Unveiling Misaligned Jets in Nearby Type Ib/c Supernovae
There is now compelling evidence of a link between long-duration gamma-ray
bursts (GRBs) and Type Ib/c supernovae (SNe). These core-collapse explosions
are conjectured to radiate an anisotropic, beamed component associated with a
decelerating, relativistic outflow and an unbeamed, isotropic component
associated with the slowly expanding stellar debris. The anisotropic emission
remains at a very low level until the Doppler cone of the beam intersects the
observer's line of sight, making off-axis GRB jets directly detectable only at
long wavelengths and late times. Circumstellar material, however, will Compton
scatter the prompt gamma-ray and afterglow radiation flux and give rise to a
reflection echo. We show that the Compton echo of a misaligned GRB carries an
X-ray luminosity that may exceed by many orders of magnitude that produced by
the underlying subrelativistic SN during the first few weeks. Bright scattering
echoes may therefore provide a means for detecting a population of misaligned
GRBs associated with nearby Type Ib/c SNe and yield crucial information on the
environment surrounding a massive star at the time of its death. The question
of whether the interpretation of GRB980425 as an ordinary GRB observed off-axis
is consistent with the lack of an X-ray echo is addressed, along with the
constraints derived on the possible existence of misaligned GRB jets in
SN1993J, SN1994I, SN1999em, and SN2002ap.Comment: 9 pages, 2 figures, to appear in the ApJ Letter
Effects of site dilution on the magnetic properties of geometrically frustrated antiferromagnets
The effect of site dilution by non magnetic impurities on the susceptibility
of geometrically frustrated antiferromagnets (kagome and pyrochlore lattices)
is discussed in the framework of the Generalized Constant Coupling model, for
both classical and quantum Heisenberg spins. For the classical diluted
pyrochlore lattice, excellent agreement is found when compared with Monte Carlo
data. Results for the quantum case are also presented and discussed.Comment: 5 pages, 3 figure
First Detection of 3He+ in the Planetary Nebula IC 418
The He isotope is important to many fields of astrophysics, including
stellar evolution, chemical evolution, and cosmology. The isotope is produced
in low-mass stars which evolve through the planetary nebula (PN) phase. He
abundances in PNe can help test models of the chemical evolution of the Galaxy.
We present the detection of the He emission line using the single dish
Deep Space Station 63, towards the PN IC418. We derived a He/H
abundance in the range 1.740.810 to
5.81.710, depending on whether part of the line arises in
an outer ionized halo. The lower value for He/H ratio approaches values
predicted by stellar models which include thermohaline mixing, but requires
that large amounts of He are produced inside low-mass stars which enrich
the interstellar medium (ISM). However, this over-predicts the He abundance
in HII regions, the ISM, and proto-solar grains, which is known to be of the
order of 10. This discrepancy questions our understanding of the
evolution of the He, from circumstellar environments to the ISM.Comment: 5 pages, 4 figures, accepted for publication in the Monthly Notices
of the Royal Astronomical Society Letter
Critical behavior of 2 and 3 dimensional ferro- and antiferromagnetic spin ice systems in the framework of the Effective Field Renormalization Group technique
In this work we generalize and subsequently apply the Effective Field
Renormalization Group technique to the problem of ferro- and
antiferromagnetically coupled Ising spins with local anisotropy axes in
geometrically frustrated geometries (kagome and pyrochlore lattices). In this
framework, we calculate the various ground states of these systems and the
corresponding critical points. Excellent agreement is found with exact and
Monte Carlo results. The effects of frustration are discussed. As pointed out
by other authors, it turns out that the spin ice model can be exactly mapped to
the standard Ising model but with effective interactions of the opposite sign
to those in the original Hamiltonian. Therefore, the ferromagnetic spin ice is
frustrated, and does not order. Antiferromagnetic spin ice (in both 2 and 3
dimensions), is found to undergo a transition to a long range ordered state.
The thermal and magnetic critical exponents for this transition are calculated.
It is found that the thermal exponent is that of the Ising universality class,
whereas the magnetic critical exponent is different, as expected from the fact
that the Zeeman term has a different symmetry in these systems. In addition,
the recently introduced Generalized Constant Coupling method is also applied to
the calculation of the critical points and ground state configurations. Again,
a very good agreement is found with both exact, Monte Carlo, and
renormalization group calculations for the critical points. Incidentally, we
show that the generalized constant coupling approach can be regarded as the
lowest order limit of the EFRG technique, in which correlations outside a
frustrated unit are neglected, and scaling is substituted by strict equality of
the thermodynamic quantities.Comment: 28 pages, 9 figures, RevTeX 4 Some minor changes in the conclussions.
One reference adde
Emergent excitations in a geometrically frustrated magnet
Frustrated systems are ubiquitous and interesting because their behavior is
difficult to predict. Magnetism offers extreme examples in the form of spin
lattices where all interactions between spins cannot be simultaneously
satisfied. Such geometrical frustration leads to macroscopic degeneracies, and
offers the possibility of qualitatively new states of matter whose nature has
yet to be fully understood. Here we have discovered how novel composite spin
degrees of freedom can emerge from frustrated interactions in the cubic spinel
ZnCr2O4. Upon cooling, groups of six spins self-organize into weakly
interacting antiferromagnetic loops whose directors, defined as the unique
direction along which the spins are aligned parallel or antiparallel, govern
all low temperature dynamics. The experimental evidence comes from a
measurement of the magnetic form factor by inelastic neutron scattering. While
the data bears no resemblance to the atomic form factor for chromium, they are
perfectly consistent with the form factor for hexagonal spin loop directors.
The hexagon directors are to a first approximation decoupled from each other
and hence their reorientations embody the long-sought local zero energy modes
for the pyrochlore lattice.Comment: 10 pages, 4 figures upon reques
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