Binary Droplet Collision at High Weber Number
Droplet collisions are relevant to miscellaneous nature and practical problems, of which splashing of multiple droplets may be one of the most astonishing outcomes impressing the public. While such phenomena have been studied extensively in droplet impact upon a surface, they have rarely been investigated in collisions between two droplets (as an elemental unit of related structure such as spray processes), partly due to the difficulty to achieve the desired experimental conditions. In this work, by using techniques for creating high-speed droplets, we found new regimes of binary droplet collision with relatively large impact inertia (head-on between two stable droplets at 23 m/s) and identified the transition boundaries for various break-up mechanisms that shed light on the fundamental structure of droplet collision dynamics and further application to industrial purposes. Various liquids were also used to explore the effects of viscosity and surface tension.
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LE12526
Has PAMELA really detected dark matter?
"Scientists have detected particles that may come from invisible dark
matter" said the BBC on 1 April 2009. This was not an April Fool joke
but a reference to the observation by the PAMELA satellite of high
energy positrons in excess of what is expected from cosmic ray
collisions with interstellar matter. The annihilation of dark matter in
the Galaxy naturally generates such positrons so this result attracted
wide interest. It was noted however that nearby pulsars can also emit
such positrons. A third possibility is that the positrons are indeed
cosmic ray secondaries but accelerated by the same shock wave that
created their parent particles.
In this paper, we outline a test to discriminate between these models.
Apart from positrons, rare nuclei like boron are also created by cosmic
ray collisions. If PAMELA finds their flux to follow the relative
increase in the positron flux with energy then the third possibility
above is the correct one. Even if the dark matter explanation is ruled
out, there is the equally exciting prospect of establishing an
accelerator of cosmic rays nearby. This may soon be directly seen in TeV
gamma-rays or neutrinos by experiments such as HESS, MILAGRO and IceCube.
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LE12232
SAILING BEFORE THE LIGHT
Just as the wind boosts a sail, the pressure of light may accelerate a
reflecting object. A few years after the invention of the laser, the
idea of a
rocket boosted by an Earth-based laser system was proposed for interstellar
travel. With the recent advent of ultrapowerful laser systems the "Light
Sail"
concept, scaled down to lengths of a few microns and times of one
millionth of
millionth of a second, may yield acceleration values similar to those
near the
surface of a Black Hole and provide compact sources of high-energy ions for
applications. In this paper it is shown that the relativity textbook
model of a
light-accelerated mirror is surprisingly accurate in predicting the final
velocity of a laser-driven ultrathin metallic foil although the underlying
dynamics is complex, involving self-organization of the system and
leading to
apparent paradoxes in its physical description due to the different way
ultraintense light acts on electrons and ions.
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LE11948
Is it possible to rectify the acoustic energy flux?
By coupling a superlattice and a strongly-nonlinear medium, we present a simple but efficient "acoustic diode" model that first makes it possible to control the acoustic energy flux. Remarkable similarity is revealed between the behaviors of an acoustic diode and an electric diode. The acoustic energy flux is allowed to transport in one direction but cannot propagate as its incident direction is reversed. This is different from our traditional knowledge and may be identified as a significant rectifying effect on acoustic energy flux that could not be fulfilled by any existing acoustical system. By employing such an acoustic diode, it is promising to control the acoustic waves in an acoustical system effectively and easily just like controlling the electric currents in an electric circuit. Potential practical applications can therefore be expected in the future for a variety of important situations that require special manipulations of acoustic energy flux, such as unidirectional sonic barriers and controlled destruction of kidney stones via ultrasonic lithotripsy etc.
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EF10540
Obstacles may speed Evacuation
We have discovered that we can evacuate faster when we put a certain
obstacle at a certain position in front of an exit both theoretically
and experimentally.
In recent years, evacuation dynamics are studied by many physicists
using physical models.
We have newly introduced the two new functions, which are the
frictional function and the turning function, to the model.
Frictional function represents the impact of conflicts among
pedestrians and the turning function represents the decrease in
walking speeds when pedestrians turn.
Frictional function tells us that when an obstacle is put in front of
an exit, it blocks the pedestrians moving to the exit at once and
weakens the impact of the conflict.
Thus, the total evacuation time theoretically decreases when an
obstacle is set up.
We have also verified this phenomenon by our experiments.
However, the obstacle does not always work well.
The turning function tells us that when an obstacle is put at the
center of the exit, pedestrians have to detour it, so that their
walking speeds decrease and the total evacuation time increases.
Therefore, the obstacle should be shifted from the center to decrease
the evacuation time according to our theory.
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BD11158
Towards perfectly defective diamonds
Perfect diamonds are boring; it is the color-center defects that give
diamonds their special colors. Some defects can also be very useful allowing
researchers to create tiny, exquisitely sensitive magnetometers. In this
paper, a UC-Berkeley led collaboration of researchers from several
institutions in US and Europe describe their analysis, using a battery of
modern techniques, aimed at understanding and optimization of various
stages of the preparation of diamond samples with nitrogen-vacancy (NV)
color centers, with a combination of high density and long spin-relaxation
times optimal for ensemble magnetometry.
