15 research outputs found
Reconstruction and thermal stability of the cubic SiC(001) surfaces
The (001) surfaces of cubic SiC were investigated with ab-initio molecular
dynamics simulations. We show that C-terminated surfaces can have different
c(2x2) and p(2x1) reconstructions, depending on preparation conditions and
thermal treatment, and we suggest experimental probes to identify the various
reconstructed geometries. Furthermore we show that Si-terminated surfaces
exhibit a p(2x1) reconstruction at T=0, whereas above room temperature they
oscillate between a dimer row and an ideal geometry below 500 K, and sample
several patterns including a c(4x2) above 500 K.Comment: 12 pages, RevTeX, figures 1 and 2 available in gif form at
http://irrmawww.epfl.ch/fg/sic/fig1.gif and
http://irrmawww.epfl.ch/fg/sic/fig2.gi
Aggregation and settling in aqueous polydisperse alumina nanoparticle suspensions
Nanoparticle suspensions (also called nanofluids) are often polydisperse and
tend to settle with time. Settling kinetics in these systems are known to be
complex and hence challenging to understand. In this work, polydisperse
spherical alumina (Al2O3) nanoparticles in the size range of ~10-100nm were
dispersed in water and examined for aggregation and settling behaviour near its
isoelectric point (IEP). A series of settling experiments were conducted and
the results were analysed by photography and by Small Angle X-ray Scattering
(SAXS). The settling curve obtained from standard bed height measurement
experiments indicated two different types of behaviour, both of which were also
seen in the SAXS data. But the SAXS data were remarkably able to pick out the
rapid settling regime as a result of the high temporal resolution (10s) used.
By monitoring the SAXS intensity, it was further possible to record the
particle aggregation process for the first time. Optical microscopy images were
produced on drying and dried droplets extracted from the suspension at various
times. Dried deposits showed the rapid decrease in the number of very large
particles with time which qualitatively validates the SAXS prediction, and
therefore its suitability as a tool to study unstable polydisperse colloids.
Keywords: Nanoparticles, nanofluids, polydisperse, aggregation, settling,
alumina, microscopy, SAX
Acoustic measurements of a liquefied cohesive sediment bed under waves
In this article the response of a cohesive sediment deposit
under the action of water waves is studied with the help of
laboratory experiments and an analytical model. Under the same regular wave condition
three different bed responses were observed depending on the degree of
consolidation of the deposit: no bed motion, bed motion of the upper layer
after the action of the first waves, and massive bed motion after several
waves. The kinematic of the upper 3 cm of the deposit were measured
with an ultrasound acoustic profiler, while the pore-water pressure inside
the bed was simultaneously measured using several pore pressure sensors. A
poro-elastic model was developed to interpret the experimental observations.
The model showed that the amplitude of the shear stress increased down into
the bed. Then it is possible that the lower layers of the deposit experience
plastic deformations, while the upper layers present just elastic
deformations. Since plastic deformations in the lower layers are necessary
for pore pressure build-up, the analytical model was used to interpret the
experimental results and to state that liquefaction of a self consolidated
cohesive sediment bed would only occur if the bed yield stress falls within
the range defined by the amplitude of the shear stress inside the bed