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(Chem) (Earth) Mixing and Mixtures: Earth's Turbulence

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PostPosted: Sat Mar 04, 2006 7:20 am    Post subject: (Chem) (Earth) Mixing and Mixtures: Earth's Turbulence Reply with quote

Earth's turbulence stirs things up slower than expected
Cornell University News Service
3 March 2006

In a simple world rivers would flow in straight lines, every airplane ride would be smooth, and we would know the daily weather 10 years into the future. But the world is not simple -- it is turbulent.

That's good news, since turbulence helps drive natural processes essential for life. Unfortunately it also means we are never 100 percent sure it won't rain on Saturday.

"Turbulence is the last major unsolved problem of classical physics," explains Eberhard Bodenschatz, professor of physics who studies turbulence with his research group at Cornell and the Max Planck Institute (MPI) for Dynamics and Self-Organization, Germany.

The group recently moved closer to a solution by measuring how two tiny polystyrene spheres in turbulent water separate based on how far apart they initially are from each other. The results were published in the Feb. 10 issue of Science.

The findings suggest that, for almost every turbulent flow on Earth, including violent volcanic eruptions, particles separate more slowly than expected. This discovery could help improve models of dispersion of pollutants and bioagents and even help explain how crustaceans find food, mates and predators by sensing odors in the ocean depths.

Turbulence occurs when a gas or fluid, like air or water, is pushed at high speeds or on large scales, and is characterized by chaotic, seemingly random, flow patterns. Because of its complexity, turbulence is very efficient at mixing: a solution of two liquids, such as cream and coffee, will mix much more quickly if the flow is turbulent than if it is not.

As a white-water rafter might toss a stick into rapids to observe its behavior before jumping in, physicists watch particles in turbulence to understand the flow. A key measurement is how quickly two particles will separate, or "pair dispersion."

In the 1920s, British scientist L.F. Richardson predicted that pair dispersion should grow quickly, as time multiplied by itself twice (time cubed), independent of the initial separation of the pair -- a statement known as the Richardson-Obukhov law. In the 1950s, Australian-born Cambridge mathematician G.K. Batchelor added the amendment that for short timescales, pair dispersion is not independent of initial separation and should grow more slowly, as time multiplied by itself (time squared).

Until recently, the difficulty of photographing tiny particles at high speeds made direct measurements of these predictions impossible.

"When we first planned these experiments, fast enough cameras didn't exist," said Cornell graduate student Nicholas Ouellette, a co-author of the Science article. The final experiment used three high-tech digital cameras able to record up to 27,000 pictures per second of several hundred polystyrene spheres simultaneously in 8 cubic inches of water. The diameter of the spheres was about one-fourth the thickness of a human hair -- a thickness needed because it matched the smallest eddies in the turbulent water.

The experiment showed that when the initial separation of the spheres is large relative to the turnover time of the eddies, they will obey Batchelor dispersion, independent of the turbulence's severity. However, if the initial separation is smaller, then the particles will only exhibit Batchelor dispersion initially before transitioning to behavior consistent with the Richardson-Obukhov law.

"Right now new technology -- like our fast cameras -- is making experiments possible that just 10 years ago were considered impossible. It's a very exciting time to be in the field," Ouellette said.

The other authors of the Science paper are Haitao Xu, Cornell and MPI for Dynamics and Self-Organization, lead author Mickal Bourgoin, Laboratoire des Écoulenments Géophysiques et Industriels, France, and Jacob Berg, Ris National Laboratory, Denmark.


Questions to explore further this topic:

An e-textbook on chemistry

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A Gallery of Mixing

What are mixtures?

An example of topics involving mixtures

What is the difference between a compound and a mixture?

What is diffusion?

What are solutions?

What are colloids?;pNid=0

An animation of dissolution

An animation of precipitation

What are emulsions?

Are there solid mixtures?

What are the diffferent types of mixtures?

Mixtures and solutions for the biosciences

How do we quantify solutions?

What is molarity?

What is percentage concentration?

What is normality?

How are mixtures separated?

Stoichiometry and Mixtures

How does mixing occur in the oceans?


How does mixing occur in the atmosphere?

What is magma mixing?

Do wet and dry conditions play a role in mixing solids?

How do immiscible liquids mix?

What is turbulence?


Last edited by adedios on Sat Jan 27, 2007 3:56 pm; edited 2 times in total
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PostPosted: Thu Sep 21, 2006 8:07 pm    Post subject: Swarms of Small Creatures Stir the Sea Reply with quote

Swarms of Small Creatures Stir the Sea

By Charles Q. Choi
Special to LiveScience
posted: 21 September 2006
02:01 pm ET

Swarms of tiny shrimp-like crustaceans known as krill could have a big impact on ocean life, by churning the waters and bringing nutrients from the depths up to the surface.

For the full article and images:
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PostPosted: Wed May 16, 2007 9:02 am    Post subject: From ink to optics, study of particle mixtures yields fundam Reply with quote

Princeton University, Engineering School
16 May 2007

From ink to optics, study of particle mixtures yields fundamental insights

Since the invention of ink over 3,000 years ago, people have exploited the unique properties of colloids, in which particles of one substance are suspended in another. Now, Princeton University chemical engineers have answered a fundamental question about these mixtures in work that may have wide-ranging practical applications, including the manufacturing of medicines and optical fibers.

Researchers have long tried to use computer simulations to determine the conditions under which colloids will exist in the solid, liquid and gas states -- knowledge that is necessary to capitalize fully on colloidal properties. Charting the freezing, melting and boiling points in "phase diagrams" has been difficult, however, because colloids contain an overwhelmingly large number of charged particles that attract and repel each other in very unusual ways. The problem was particularly difficult when the particles had very high static electric charges, which commonly occurs in practical situations.

Athanassios Panagiotopoulos, the Susan Dod Brown Professor of Chemical Engineering at Princeton, and postdoctoral research associate Antti-Pekka Hynninen overcame this hurdle by realizing that they could accurately represent the system with just one to four colloidal particles and their oppositely charged counterparts, called counterions. Their result -- a complete phase diagram that is correct even when the particles have very high electric charges -- was published as an "Editors' Selection" in the May 11 issue of the journal Physical Review Letters.

"This is the closure of a long-standing, fundamental problem in statistical mechanics," said Panagiotopoulos, who credits Hynninen with the "elegant and powerful idea" to consider only a very small part of the colloidal system.

With the newfound ability to calculate accurate phase diagrams, researchers may be able to improve the fabrication of artificial opal crystals -- colloids essential for high-speed transmission of data given the way in which they diffract optical signals. In medicine, an enhanced understanding of colloids that contain proteins may enable the design and production of pharmaceutical formulations that are far more stable than those in existence today.

"This is truly pioneering work," said Sanat Kumar, a professor of chemical engineering at Columbia University, who was not affiliated with the research. "Before, the most elementary questions on this difficult, but highly relevant, topic were open. This work has really opened up the field."

Their work also revealed a surprising "love/hate relationship" among highly charged colloidal particles, which causes them to transition directly from a gas to a solid without ever going through a liquid phase. This strange behavior results from the previously observed fact that like-charged particles attract one another at close distances instead of the usual repulsion. At a certain intermediate distance, similarly charged particles switch from being strongly repulsed by one another to incredibly attracted to each other, which causes the phase transition direct from diffuse gas to dense solid.

Panagiotopoulos and Kumar predict that similar simulations of small numbers of particles to calculate the behavior of entire systems will allow researchers to explore a wide variety of fundamental and applied topics in physics, chemistry and biology. For example, the technique could be used in astrophysics to understand phase transitions of dense plasmas or in biology to explore the associations between proteins and DNA, which is key information for understanding the interactions that take place in living cells.

Disappearance of the Gas-Liquid Phase Transition for Highly Charged Colloids
A.-P. Hynninen and A. Z. Panagiotopoulos
Department of Chemical Engineering, Princeton University, Princeton, New Jersey 08544, USA

We calculate the full phase diagram of spherical charged colloidal particles using Monte Carlo free energy calculations. The system is described using the primitive model, consisting of explicit hard-sphere colloids and point counterions in a uniform dielectric continuum. We show that the gas-liquid critical point becomes metastable with respect to a gas-solid phase separation at colloid charges Q 20 times the counterion charge. Approximate free energy calculations with only one and four particles in the fluid and solid phases, respectively, are used to determine the critical line for highly charged colloids up to Q=2000. We propose the scaling law Tc*~Q1/2 for this critical temperature.
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