1,005 research outputs found
Kinetics of Gravity-Driven Water Channels Under Steady Rainfall
We investigate the formation of fingered flow in dry granular media under
simulated rainfall using a quasi-2D experimental set-up composed of a random
close packing of mono-disperse glass beads. Using controlled experiments, we
analyze the finger instabilities that develop from the wetting front as a
function of fundamental granular (particle size) and fluid properties
(rainfall, viscosity).These finger instabilities act as precursors for water
channels, which serve as outlets for water drainage. We look into the
characteristics of the homogeneous wetting front and channel size as well as
estimate relevant time scales involved in the instability formation and the
velocity of the channel finger tip. We compare our experimental results with
that of the well-known prediction developed by Parlange and Hill [1976]. This
model is based on linear stability analysis of the growth of perturbations
arising at the interface between two immiscible fluids. Results show that in
terms of morphology, experiments agree with the proposed model. However, in
terms of kinetics we nevertheless account for another term that describes the
homogenization of the wetting front. This result shows that the manner we
introduce the fluid to a porous medium can also influence the formation of
finger instabilities.Comment: 13 pages, 7 figure
Influence of a local change of depth on the behavior of bouncing oil drops
The work of Couder \textit{et al} (see also Bush \textit{et al}) inspired
consideration of the impact of a submerged obstacle, providing a local change
of depth, on the behavior of oil drops in the bouncing regime. In the linked
videos, we recreate some of their results for a drop bouncing on a uniform
depth bath of the same liquid undergoing vertical oscillations just below the
conditions for Faraday instability, and show a range of new behaviors
associated with change of depth.
This article accompanies a fluid dynamics video entered into the Gallery of
Fluid Motion of the 66th Annual Meeting of the APS Division of Fluid Dynamics.Comment: High and low resolutions videos included as ancillary file
Both Chromosome Decondensation and Condensation Are Dependent on DNA Replication in C.elegans Embryos
SummaryDuring cell division, chromatin alternates between a condensed state to facilitate chromosome segregation and a decondensed form when DNA replicates. In most tissues, S phase and mitosis are separated by defined G1 and G2 gap phases, but early embryogenesis involves rapid oscillations between replication and mitosis. Using Caenorhabditis elegans embryos as a model system, we show that chromosome condensation and condensin II concentration on chromosomal axes require replicated DNA. In addition, we found that, during late telophase, replication initiates on condensed chromosomes and promotes the rapid decondensation of the chromatin. Upon replication initiation, the CDC-45-MCM-GINS (CMG) DNA helicase drives the release of condensin I complexes from chromatin and the activation or displacement of inactive MCM-2–7 complexes, which together with the nucleoporin MEL-28/ELYS tethers condensed chromatin to the nuclear envelope, thereby promoting chromatin decondensation. Our results show how, in an early embryo, the chromosome-condensation cycle is functionally linked with DNA replication
Optomechanical sideband cooling of a micromechanical oscillator close to the quantum ground state
Cooling a mesoscopic mechanical oscillator to its quantum ground state is
elementary for the preparation and control of low entropy quantum states of
large scale objects. Here, we pre-cool a 70-MHz micromechanical silica
oscillator to an occupancy below 200 quanta by thermalizing it with a 600-mK
cold 3He gas. Two-level system induced damping via structural defect states is
shown to be strongly reduced, and simultaneously serves as novel thermometry
method to independently quantify excess heating due to the cooling laser. We
demonstrate that dynamical backaction sideband cooling can reduce the average
occupancy to 9+-1 quanta, implying that the mechanical oscillator can be found
(10+- 1)% of the time in its quantum ground state.Comment: 11 pages, 5 figure
Cascade metathesis reactions for the synthesis of taxane and isotaxane derivatives
Tricyclic isotaxane and taxane derivatives and have been synthesized by a very efficient cascade ring-closing dienyne metathesis (RCDEYM) reaction, which formed the A and B rings in one operation. When the alkyne is present at C13 (with no neighboring gem-dimethyl group), the RCEDYM reaction leads to 14,15-isotaxanes 16a,b and 18b with the gem-dimethyl group on the A ring. If the alkyne is at the C11 position (and thus flanked by a gem-dimethyl group), RCEDYM reaction only proceeds in the presence of a trisubstituted olefin at C13, which disfavors the competing diene ring-closing metathesis reaction, to give the tricyclic core of Taxol 44
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