10-03-07

LB11409
Segregation, or un-mixing, of particles has been a topic of intense
research and industrial frustration for many decades, causing dramatic
revenue loss and product failure in a variety of industries. In this
paper we outline an elegant and robust method for eliminating
segregation that is generic for a huge class of particle flows,
specifically free-surface flows (one of the most popularly studied in
the literature). This paper will not only impact industrial practice in
fields as varied as ceramics, pharmaceuticals, mining, and agriculture,
but also change the way that academics think of attacking segregation
problems.

The crux of the technique relies on our identification of two critical
features of segregation: 1) that it has a preferred direction and 2) it
takes a finite amount of time. In order to exploit these two
observations we perturb a flow faster than a (theoretically identified)
critical frequency, essentially making segregation act as if it were "in
a hamster wheel", accomplishing nothing. Interestingly, mixing -- being
primarily random -- is not affected by these perturbations.

The attached figure shows experiments and simulations of unbaffled
mixers and mixers with baffles that perturb the flow as suggest in our
paper. Both density and size segregation are eliminated.

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LZ10210
Vortex core spontaneously deforms

Usually vortices in gases or fluids (e.g. air or water) are rotationally
symmetric. However, we observe a deformation of a density defect, which is a
signature of the vortex core, into a planer shape in a Bose-Einstein condensate
(BEC) of atomic gas. This deformation becomes possible because the original
vortex we create is "quadruply" charged. In other words, our vortex has four
quanta of rotational motion. The quadruply charged vortex splits into four
singly charged vortices, and they take linear alignment. As a result, the
original vortex seemingly deforms into a linear shape. The reason why they
choose the linear alignment is theoretically explained by excitation spectrum
analysis. Using the analysis, we predict that other kinds of vortex alignment
are also possible by controlling the number of atoms. The linear defect we
observe moves in a counter-intuitive manner. The motion is simply explained as a
combined motion of "rotation" and "precession" using the velocity field model.
This work shows both experimentally and theoretically that BEC of dilute atomic
gas is a highly suitable system to study the dynamics of multiply charged
vortices such as splitting and deformation.

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LF11419

Half-quantum vortices: Can the dream come true?

We have analyzed the possibility of finding half-quantum magnetic
vortices in Strontium Ruthenate (Sr2Ru04), an exotic
superconductor. Our theoretical work
can be considered as a first step towards an experimental observation
of half-quantum vortices.
In Strontium Ruthenate, a ceramic whose superconducting phase shares
many properties with
superfluidity in 3He, half-quantum vortices are expected to contain so
called Majorana-Fermion core states whose non-Abelian statistics would
protect them against environmental noise, but allow one to manipulate
them by moving one vortex around another. If half-quantum vortices
could be created and manipulated, they could potentially be used for
topologically protected quantum computation. Their properties would
mitigate the fast loss of information which so much troubles most
approaches to quantum information processing.
These unusual vortices, which carry only half as much
magnetic flux as ordinary vortices, are energetically forbidden to
appear in isolation in large samples. However, we have shown that
tightly
bound pairs of fractional vortices with a finite separation may be
stable or metastable. Furthermore, we find that it might be possible
to isolate them with present experimental techniques in
submicron-sized samples. Such an experiment which would be of great
fundamental and potential practical interest.

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LA11605
Physics of eukaryotic chemotaxis

Multicellular organisms would not exist if eukaryotic cells could not
move in a coordinated way following chemical signals. But which physics
allows the cell to chose the right direction? A puzzling aspect here is
that cells must be highly sensitive to small gradients of soluble
chemical attractants, while being indifferent to high, uniform levels of
the same substance. Physical modeling shows that a selforganized phase
ordering mechanism is at the heart of this biological function. Under
uniform stimulation, clusters of signaling molecules on the cell
membrane grow according to universal scaling laws. When the cell is
exposed to a slight stimulation gradient, an initial tuning regime
characterized by these same laws is followed by a faster growth
characterized by a higher scaling exponent. The crossover between the
two regimes takes place at a time $t_\epsilon$ which is inversely
proportional to the applied gradient. This simple physical picture
explains most of the observed phenomenology, in particular the
observation of a size-dependent threshold of detectable gradients. This
may provide the reason why spatial directional sensing is absent in
small bacteria but present in large eukaryotic cells: there was not
enough space for it.

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LD11133

Shepherding drops on patterned surfaces

Studying and controlling wetting of surfaces is important in applications ranging from drug discovery to ink jet printing. These engineered surfaces are typically composed of patterns of two or more surfaces with different properties. On well-designed surfaces liquid drops march precisely along specific paths -- even uphill -- when subjected to random vibrations enabling controlled sequential chemical reactions required in many applications. In this paper we developed a model of wetting based on a theory of superconducting phase transitions which can help design such surfaces.

The theory of superconductors first proposed by Vitaly Ginzburg and Lev Landau in 1950 (leading to a Nobel Prize in Physics in 2003 for Ginzburg) is widely used to study transformations between solid phases. By viewing wetted and non-wetted areas of solid surfaces as distinct phases, we described the drop motion using the Ginzburg-Landau equation. This allows a comprehensive description of wetting and enables design of preferentially wettable surfaces without recourse to expensive experimental trials.

This model has been used to study the range of applicability of a simple theory due to A. B. D Cassie proposed in 1947. Cassie theory, currently being used by the scientific community to design such composite surfaces, suggests that the cosine of the contact angle of a drop on a composite surface is given by an area average of the cosine of the contact angles of the component surfaces. Phase field theory was used to study the contact angle on such composite surfaces and it was found that the drop gets pinned to a different contact angle than predicted by the Cassie theory. Wide deviations from Cassie theory were found when the difference in properties of the component surfaces became significant.

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LE11395
Balanced protein attractions help keep the eye lens clear

We have discovered that a fine balance of attractions between eye lens
proteins is needed to prevent clouding that could contribute to
cataract, the leading cause of blindness. Just as clouds in the sky
reflect water molecule attractions, forces between proteins cloud the
eye lens in cataract. Compared to the sky, though, the eye lens has a
richer set of ways to tilt the balance towards cataract, ways that are
still being discovered. Lens gamma crystallin proteins can gather
into droplets that cloud the lens. Alpha crystallins usually repel
each other, but can also form aggregates in cataract. Now, by
combining experiment and simulation, we find that mixtures of alpha
and gamma at high, realistic concentrations need finely tuned
interactions to avoid opacity. With too little alpha-gamma attraction
each protein can segregate with its own kind into domains that scatter
light (left panel). Too much attraction and the gammas glue the
alphas into clumps that scatter light (right). With the right
balance, however (center), either extreme is avoided. This new
potential reason for cataract adds to the framework for discovery of
specific molecular properties of alpha and gamma crystallin that
affect cataract, and possible ways to prevent the disease.

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LF11424

Heating up ice

Various materials have very different capabilities to store heat. In
fact, ordinary water is a very effective heat storage. This property
plays an important role in diverse natural phenomena even for global
warming but is also utilized for various everyday technical
applications. Recent novel experiments utilizing x rays have now
revealed how thermal energy is stored in solid water, i.e. in ice, on a
microscopic scale. The technique utilized is called Compton scattering,
in which very intensive x rays are shined on a small volume of the
sample and the backscattered radiation is measured. Although it has been
known for over 30 years that the technique in principle could provide
such information on the energy,
experimental limitations have so far hindered these studies. In the
present work it is shown that accurate experiments at synchrotron
radiation facilities, which can provide unforeseen intensive x-ray
beams, have now made these investigations feasible. The study has
revealed that in ice the bonds between the molecules change gradually to
accommodate part of the heat transferred to the sample, the rest of the
heat being stored in the vibrations of the molecules. This work opens up
new possibilities to study fundamental heat storage properties of
various materials.