Undulating Underperformance: Swimming in Elastic Fluids

LB13132

-The main findings from this work is that fluid elasticity, the property that gives materials like silly putty, yogurt, gels, and human mucus their unusual and useful texture, hinders both the swimming speed and efficiency of live micro-organisms. This is a surprising result because many organisms live, move, feed, and reproduce in fluids possessing elasticity, and many biological process of vital importance take place in such media. Examples include the motion of spermatozoa in the female reproductive track (human reproduction), the beating of cilia in the respiratory track (removal of foreign agents), and the motion of worms in wet soil (soil aeration). We find that fluid elasticity decreases swimming speed of the nematode C. elegans up to 35% compared to ordinary fluids. The undulatory motion of C. elegans is typical of many limbless organisms of different sizes including eels, snakes, worms, cilia and flagellated eukaryotes. This is the first study that systematically investigates in experiments the role of fluid elasticity on swimming, which will help in developing and guiding theoretical models in the future.

Hot and Fast

LC13073

-When a drop of water falls onto a hot plate, the vapor layer between the drop and the plate provides levitation and lubrication that allows the drop to skate rapidly over the surface – an effect familiar to every chef. A novel application of this phenomenon, known as the Leidenfrost effect, has shown that a hot solid sphere under free fall in liquid can travel over twice as fast as a cold sphere by maintaining a continuous, robust, thin lubricating vapor layer around the sphere (see Figure). High-speed video imaging showed that the vapor layer reduced the adhesion between the liquid and the sphere surface resulting in a smoother liquid flow pattern and dramatic reduction in hydrodynamic drag (Videos available on EPAPS). These findings complement related lubricating vapor layer technologies such as using superhydrophobic surfaces, microbubbles injection, and supercavitation in the quest for efficient energy usage and reduced carbon emission in high speed under water propulsion applications.

Why chaos is warmer than order?

LB13044

Glasses are remarkably different from crystals at low temperature.
They accumulate more heat and conduct less. This anomaly is related
to a particular ensemble of atomic motions called the "boson peak",
which is universally observed for all glasses. Its nature, however,
remained unknown for more than 50 years. Because of this lengthy
research period, the boson peak has been called the last puzzle of
solid state physics. Most models explain the boson peak by additional
vibrational modes created by the chaos of the atomic positions,
while others attribute it to sound waves. We compared atomic motions
in a glass and a crystal using the nuclear inelastic scattering
technique which determines an exact number of vibrational states.
The results show that around the boson peak, the number of states
in a glass is exactly the same as the number of sound wave states
in the crystal. Furthermore, application of pressure causes a
gradual transformation of the boson peak towards a particular
(van Hove) singularity of the crystal created by sound waves with
a period equal to the crystal periodicity. These observations
unambiguously identify the boson peak with sound waves. Thus,
more heat can be stored in a glass not because chaos allows for
more vibrations, but because it changes sound waves.

Cosmic Dynamos—Coherent Motions, Not Turbulence, Generate Large-Scale Magnetic Fields

LY12726

From whence come the highly-organized, large-scale magnetic fields observed
around planets, stars, galaxies, AGN, and the giant radio lobes emanating from AGN.
According to many theories, dynamo action—the stretching, twisting, and folding of
magnetic flux needed to grow magnetic fields—should arise naturally from turbulent
motions in the molten metal cores of planets or the hot plasmas in the Sun, stars, and AGN. However, in the last few years, laboratory tests of that idea using liquid sodium have yielded negative results. Rather than enhancing the growth of magnetic fields, strong turbulence appears to diffuse magnetic flux and dissipate it away as fast as it’s generated.

Reported here for the first time, a dynamo experiment with liquid sodium has succeeded in shearing a radial magnetic field and wrapping it up in the toroidal direction to create a toroidal field 8 times larger that the original radial field. The secret was to keep the turbulence very low by creating a very rapidly rotating shear flow (Couette flow) stabilized against turbulence by the differential rotation (decrease in angular momentum) between a rapidly rotating inner cylinder and a less rapidly rotating outer cylinder. This type of stabilization occurs in the Keplerian flow of accretion disks around stars and super-massive black holes. Stabilization is achieved in the interior of stars by an entropy gradient at the base of the convective zone, and by viscosity in planets.

The new experiment has demonstrated the omega effect (amplification through stretching) of a classic alpha-omega dynamo. To demonstrate that this field could grow exponentially large from a small seed value, the experiment will add the coherent twisting motion of plumes (the alpha effect) to rotate a small fraction of the toroidal field back into the original radial field. In astrophysics such plumes occur naturally when a small number of early stars plunge back and forth through accretion discs, or convective plumes rise in the convective zone of stars or planets. Thus, the experimental results so far suggest that coherent flows, not turbulence, are the likely origin of the magnetic fields that produce the most dramatic effects of astrophysics—the Earth’s aurora, solar and stellar flares, massive magnetized jets from AGN, and ultra high energy cosmic rays.

Bubble formation in stout beers

EC10816

We show theoretically and experimentally that the same cellulose fibre
nucleation sites responsible for bubble formation in champagne can
also create bubbles in stouts beers, although at a substantially
slower rate. A rough calculation suggests that it may be possible to
replace the widgets of canned stout beers with a coating of cellulose
fibres on the inside of the can. We have extended a model of bubble
formation in champagne, a supersaturated solution of carbon dioxide,
to the case of stout beers, which are supersaturated solutions of
nitrogen and carbon dioxide. This model reveals that the low
solubility of nitrogen retards the rate of bubble formation within a
cellulose fibre, explaining why widgets are necessary to trigger
foaming in canned stout beers. However, the results suggest that a
coating of millions of cellulose fibres, covering an area the size of
a postage stamp might be able to generate the hundred million bubbles
needed to form the head of a pint of stout in the recommended pouring
time of thirty seconds.

Fluid mixing from viscous fingering

LZ11988

In this paper, we explain how the mixing of two fluids can be enhanced
by hydrodynamic instabilities when the mixing fluids have different
viscosities.

For flows at high velocities, turbulence creates chaotic flow
conditions that get a volume of fluid mixed very quickly (which is why
we stir our coffee to get the sugar dissolved quickly). At low
velocities and in small geometries, however, the flow is laminar and,
typically, mixing occurs very slowly. This is important in nature, for
instance, in flows through porous media, because biological activity
and chemical reactions are limited by how fast the fluids come into
contact. It is also important in engineering applications, especially
in the context of microfluidics, where it is difficult to get the
reactants to mix quickly. Many methods have been proposed to achieve
fast mixing in small devices, but all the strategies explored so far
assume that the fluids to be mixed have the same viscosity. In this
paper, we explain how mixing efficiency can be enhanced when the
fluids are of different viscosities. In that case, the flow is
unstable (through a phenomenon called 'viscous fingering'), which
creates disorder in the flow and leads to faster mixing.