33,185 research outputs found
Growth mechanism of nanocrystals in solution: ZnO, a case study
We investigate the mechanism of growth of nanocrystals from solution using
the case of ZnO. Spanning a wide range of values of the parameters, such as the
temperature and the reactant concentration, that control the growth, our
results establish a qualitative departure from the widely accepted diffusion
controlled coarsening (Ostwald ripening) process quantified in terms of the
Lifshitz-Slyozov-Wagner theory. Further, we show that these experimental
observations can be qualitatively and quantitatively understood within a growth
mechanism that is intermediate between the two well-defined limits of diffusion
control and kinetic control.Comment: 10 pages, 4 figure
Growth mechanism of nanostructured superparamagnetic rods obtained by electrostatic co-assembly
We report on the growth of nanostructured rods fabricated by electrostatic
co-assembly between iron oxide nanoparticles and polymers. The nanoparticles
put under scrutiny, {\gamma}-Fe2O3 or maghemite, have diameter of 6.7 nm and
8.3 nm and narrow polydispersity. The co-assembly is driven by i) the
electrostatic interactions between the polymers and the particles, and by ii)
the presence of an externally applied magnetic field. The rods are
characterized by large anisotropy factors, with diameter 200 nm and length
comprised between 1 and 100 {\mu}m. In the present work, we provide for the
first time the morphology diagram for the rods as a function of ionic strength
and concentration. We show the existence of a critical nanoparticle
concentration and of a critical ionic strength beyond which the rods do not
form. In the intermediate regimes, only tortuous and branched aggregates are
detected. At higher concentrations and lower ionic strengths, linear and stiff
rods with superparamagnetic properties are produced. Based on these data, a
mechanism for the rod formation is proposed. The mechanism proceeds in two
steps : the formation and growth of spherical clusters of particles, and the
alignment of the clusters induced by the magnetic dipolar interactions. As far
as the kinetics of these processes is concerned, the clusters growth and their
alignment occur concomitantly, leading to a continuous accretion of particles
or small clusters, and a welding of the rodlike structure.Comment: 15 pages, 10 figures, one tabl
The stability of a rising droplet: an inertialess nonmodal growth mechanism
Prior modal stability analysis (Kojima et al., Phys. Fluids, vol. 27, 1984)
predicted that a rising or sedimenting droplet in a viscous fluid is stable in
the presence of surface tension no matter how small, in contrast to
experimental and numerical results. By performing a non-modal stability
analysis, we demonstrate the potential for transient growth of the interfacial
energy of a rising droplet in the limit of inertialess Stokes equations. The
predicted critical capillary numbers for transient growth agree well with those
for unstable shape evolution of droplets found in the direct numerical
simulations of Koh & Leal (Phys. Fluids, vol. 1, 1989). Boundary integral
simulations are used to delineate the critical amplitude of the most
destabilizing perturbations. The critical amplitude is negatively correlated
with the linear optimal energy growth, implying that the transient growth is
responsible for reducing the necessary perturbation amplitude required to
escape the basin of attraction of the spherical solution.Comment: 11pages, 7 figure
On the linear growth mechanism driving the stationary accretion shock instability
During stellar core collapse, which eventually leads to a supernovae
explosion, the stalled shock is unstable due to the standing accretion shock
instability (SASI). This instability induces large-scale non spherical
oscillations of the shock, which have crucial consequences on the dynamics and
the geometry of the explosion. While the existence of this instability has been
firmly established, its physical origin remains somewhat uncertain. Two
mechanisms have indeed been proposed to explain its linear growth. The first is
an advective-acoustic cycle, where the instability results from the interplay
between advected perturbations (entropy and vorticity) and an acoustic wave.
The second mechanism is purely acoustic and assumes that the shock is able to
amplify trapped acoustic waves. Several arguments favouring the
advective-acoustic cycle have already been proposed, however none was entirely
conclusive for realistic flow parameters. In this article we give two new
arguments which unambiguously show that the instability is not purely acoustic,
and should be attributed to the advective-acoustic cycle. First, we extract a
radial propagation timescale by comparing the frequencies of several unstable
harmonics that differ only by their radial structure. The extracted time
matches the advective-acoustic time but strongly disagrees with a purely
acoustic interpretation. Second, we present a method to compute purely acoustic
modes, by artificially removing advected perturbations below the shock. All
these purely acoustic modes are found to be stable, showing that the advected
wave is essential to the instability mechanism.Comment: 17 pages, 10 figures, accepted for publication in MNRA
Rapid formation of black holes in galaxies: a self-limiting growth mechanism
We present high-quality fluid dynamical simulations of isothermal gas flows
in a rotating barred potential. We show that a large quantity of gas is driven
right into the nucleus of a model galaxy when the potential lacks a central
mass concentration, but the inflow stalls at a nuclear ring in comparison
simulations that include a central massive object. The radius of the nuclear
gas ring increases linearly with the mass of the central object. We argue that
bars drive gas right into the nucleus in the early stages of disk galaxy
formation, where a nuclear star cluster and perhaps a massive black hole could
be created. The process is self-limiting, however, because inflow stalls at a
nuclear ring once the mass of gas and stars in the nucleus exceeds ~1% of the
disk mass, which shuts off rapid growth of the black hole. We briefly discuss
the relevance of these results to the seeding of massive black holes in
galaxies, the merger model for quasar evolution, and the existence of massive
black holes in disk galaxies that lack a significant classical bulge.Comment: 11 pages, 6 figures, accepted to appear in Ap
In situ observation of mono-molecular growth steps on aqueous solution grown crystals and the transport of molecules to the crystals
Direct in situ observation of mono-molecular growth steps on a crystal growing in an aqueous solution became possible. The combination of this method with high resolution Schlieren methods or interferometry, permits the growth mechanism of crystals to be investigated directly. Since the observation of growth steps on crystals is the most direct and sensitive way for investigating a crystal growth mechanism, it would contribute to revealing fundamental differences between the growth in space and on Earth. The method was recently extended to in situ observation of the growth processes at high temperatures (1800K)
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