How do lotus leaves achieve anti-dew water repellency?
Many plants exhibit remarkable water repellency owing to their rough surface. The textured surface traps air underneath water drops and the air cushioning gives rise to the water repellency. A long-standing puzzle is that, unlike their biological counterparts, engineered rough surfaces do not retain water repellency when subjected to naturally occurring condensations. Researchers at Duke University uncovered an ingenious mechanism used by lotus leaves to stay dry after repeated condensations. Physicists have now shown that even the lotus leaf does not retain water repellency under condensation when fixed; however, the same leaf becomes water-repellent again when vibrated. There is a reason for the big leaf sitting on a slim stem – natural vibrations supply energy for dewetting the condensate-penetrated textures and restoring the air cushioning. The research on anti-dew water repellency is pointing toward a new direction in engineering robust self-cleaning systems.
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LE12614
Mushrooms' spore ejection process reproduced on water-repellent surfaces
Mushrooms are known to use surface energy to discharge a spore from the tip of its sterigma. The discharge process is triggered by the coalescence of the wetted spore with a condensate drop at its base. Researchers at Duke University discovered a similar process on man-made water-repellent surfaces. Researchers have now observed that when condensate drops coalesced on water-repellent surfaces, the merged drop spontaneously jumped out of the surface. Like the spore ejection, the jumping drops are powered by surface energy released upon drop coalescence. The research is the first known engineering reproduction of the ballistospore ejection process. Their work also has immediate applications in energy harvesting and thermal management. For example, the spontaneous jumping motion offers an internal mechanism, independent of gravity, to remove liquid condensate from the condensers in power plants and spacecrafts.
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LF12743AR
Radio-controlled atoms
Scientists have succeeded in using
radio-frequency radiation to control the way atoms collide with each
other. Cold gases of atoms have been a hot topic for many years,
particularly since 1995, when a new state of matter known as a
Bose-Einstein condensate was formed for the first time. Many experiments
since then have depended on being able to precisely control the
interactions between the atoms – tuning them to be strong or weak,
attractive or repulsive. One common way of doing this is by tuning a
magnetic field to certain values where the interaction properties change
quickly. In this new work, the authors demonstrate that radiofrequency
radiation provides a second degree of control. This can be used together
with a magnetic field to provide greater levels of control. One can, for
instance independently control the interactions between different
components of a gas with three or more types of atom. This is of
interest to other programs using cold gases, such as many-body physics
and quantum information
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LF11943
Exploring the limits of antiferromagnetism in nanostructured materials
Researchers at Argonne National Laboratory and Politecnico di Milano in Italy have recently explored the limits of antiferromagnetism in a nanostructured material for the first time, measuring the temperature required to support antiferromagnetic order in atomic monolayers of manganese on tungsten as the dimensions of the structures are reduced. While these boundaries are well understood in ferromagnetic materials, antiferromagnetic materials Ð where neighboring magnetic moments cancel rather than add together Ð have proven much more challenging to unravel. In this work, the authors exploited the unique properties of manganese spin spirals on tungsten to correlate spin-sensitive scanning tunneling microscopy techniques on the atomic scale with electronic signatures, showing that the ordering temperature for the antiferromagnetic structure depends both on its size and its orientation with respect to the crystal lattice. Atomic-scale spin-sensitive investigations such as this will help guide the way to next generation platforms for ultra-high-density data storage and novel sensing capabilities.
Figure: (Top Panel) Schematic of the spin structure of the Mn monolayer on W(110). Nearest neighbor spins are slightly canted resulting in a cycloidal spin spiral with alternating out-of-plane and in-plane regions that repeat with a period of about 6 nm. (a) Topography and (b) differential conductance of the Mn on W(110) at 40 K. In the inset, high resolution topographic data taken with a spin-sensitive tip shows the atomically resolved spin structure, the electronic signature of which can be seen as light and dark stripes in the differential conductance map. The contrast of these stripes is used to determine the degree of antiferromagnetic order in the nanostructure.
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LE12264BR
"Mirror, Mirror on the Wall": The Many Faces of Silver Nanoparticles
Imagine peering into a mirror and seeing a different image look back at
you! And if that is not enough, the image changes with time, twinkling
like stars in the night-time sky.
150 years ago, Bernhard Tollens invented a simple chemical route to
growing large silver mirrors. We demonstrate that such a mirror -
composed of myriads of tiny "nanoparticles" - may itself shine: rather
than merely reflecting light, the mirror actually generates light.
This effect arises because light waves behave a bit like lightning: give
them a conducting surface of a particular shape, and the metal will
absorb and transmit radiation. The consequence is that light energy
becomes highly focused, leading to optical amplification phenomena we do
not usually experience. One such phenomenon is hyperscattering of light:
much like blowing too hard on a whistle generates an overtone,
scattering intense light fields generates radiation of twice the
frequency – the mirror changes the color of light at discrete “hot spots”.
Remarkably, the radiation from these “hot spots” is not static: the
mirror constantly but reversibly changes the way in which it responds to
the incident light field on nanometer length scales.
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EE10730
Understanding of complex behaviour of vehicular trajectories measured on multi-lane American freeways
The physics of spatiotemporal phase transitions in traffic flow on multi-lane freeways is revealed based on data analyses of vehicular traffic in the framework of three-phase traffic theory. The complex dynamics of moving jams observed in single vehicle data measured by video-cameras on American highways (http://ngsim.camsys.com) is explained by the nucleation-interruption effect in synchronized flow, i.e., the spontaneous nucleation of a narrow moving jam with the subsequent jam dissolution.
A dual role of lane changing in vehicular traffic is revealed: (1) lane changing can lead to the emergence of a nucleus for a phase transition, in particular for moving jam emergence; and in contrast, (2) lane changing can lead to moving jam dissolution.
This dual role of lane changing is responsible for a very complex spatiotemporal behaviour of vehicular traffic on multi-lane roads.
