Nonlinear diffusion model for Rayleigh-Taylor mixing
An heavy fluid placed over a light one is a classical hydrodynamic
instability studied by Lord Rayleigh more than one century ago.
The development of the instability at the interface of the
two fluids generate a mixing layer in which heavy and light fluids
are well mixed by turbulence generated by gravitational forces.
In this Letter we develop a simple closed model for the spatial-temporal
evolution of the mixing layer based on a non-linear diffusion equation.
A variance with usual constant diffusion model, here the diffusivity
is allowed to depend both on time and space, reflecting the complex
structure of the evolving mixing layer.
The nonlinear model reproduces with high accuracy the density
profile of the mixing layer measured in high resolution Direct
Numerical Simulation of the complete Navier-Stokes equations and
predicts the temporal evolution of global quantities such as the
vertical density flux and the width of the mixing layer.
***
LE12465
Understanding the plant cell's self-assembling corset
Growing plant cells show a peculiar structure, the cortical array, made
up of parallel stiff protein tubes called microtubules. It is thought
that the cortical array plays an important role in the growth of cells,
and that it emerges spontaneously from the interactions of the microtubules.
Unlike humans, plants cannot rely on a skeleton for their rigidity;
instead, every cell must contribute by being enclosed in a rigid cell
wall. This presents a challenge, because for the plant to grow in length
these cell walls need to be stretched in a particular direction. Plant
cells have solved this by placing long fibers in the wall, wrapping the
cell like a corset and preventing it from expanding sideways.
These fibers reflect the structure of the microtubules that are inside
the cell, attached to the cell membrane. These microtubules effectively
‘crawl’ across the cell membrane, they grow or shrink, and they can
collide with one another, resulting in shrinkage of a growing
microtubule or its reorientation. It is believed that the collisions
allow the microtubules to ‘negotiate’ a common orientation.
In this paper, we present a model of interacting microtubules, based on
biological observations. The model allows us to make predictions about
the circumstances in which the microtubules can successfully align. We
compare the model predictions with the results from computer
simulations. Our results suggest that the collision-induced shrinkage of
growing microtubules plays a dominant role in their alignment.
***
LK12170BR
SHEDDING LIGHT ON ELECTRONS IN LUMINESCENT ORGANIC SEMICONDUCTOR NANOSTRUCTURES
In organic semiconductors, molecular motifs are the building blocks of periodic architectures, and delocalized electrons within individual molecules interact weakly between molecules. The nature of intermolecular electronic interactions is what defines the semiconductor properties of these advanced materials, including electronic transport and optical properties. These materials are now used in a variety of applications such as organic light-emitting diodes and plastic solar cells. In this letter, an international research team explore the nature of electronic interactions in an organic semiconductor nanostructure by studying the dynamics of electron relaxation to produce light. By exciting the nanostructures with short laser pulses and analysing the time dependence of their subsequent light emission, the research team unravel the spatial extent of electronic intermolecular interactions. Electrons in neighboring molecules interact strongly compared to many organic semiconductor used in applications, such as polymers, allowing electrons to transfer readily between molecules on very fast timescales. However, the spatial extent of excitations resulting from such transfer processes, generated by the laser pulse, is only delocalized over two to three molecules due to strong modulation of the electronic energy by reorganization of the molecular geometry. This work opens the door to the understanding and design of novel organic semiconductor materials for applications in optoelectronics.
***
LK12568
Discovery of the Heaviest Halo Atomic Nucleus
Physicists have measured the radius of Carbon-22, an extremely neutron-rich
carbon isotope with a nucleus comprised of 6 protons and 16 neutrons. In
this paper the radius of Carbon-22 is reported to be twice the
size of the Carbon-12 that predominates in nature. This is a remarkable
finding since the size of a normal nucleus is known to be proportional to
the cube root of its mass number and (22/12)^(1/3) is only 1.2. The radius
of Carbon-22 exceeds that of Lead-208. Nuclei with abnormal sizes are
known as "halo nuclei". The only other halo nuclei observed to date are
Helium-6, Lithium-11, Beryllium-14 and Boron-17. Carbon-22 is the heaviest
halo nucleus found thus far.
