Caltech Professors Named Fellows of the American Physical Society

John Dabiri and Maria Spiropulu have been named fellows of the American Physical Society (APS) for their exceptional contributions to physics.

The APS Division of Fluid Dynamics nominated Dabiri, professor of aeronautics and bioengineering, for his contributions to "vortex dynamics and biological propulsion, and for pioneering new concepts in wind energy."

Dabiri, the director of the Center for Bioinspired Engineering, studies the mechanics and dynamics of biological propulsion—particularly using jellyfish as a model. His group aims to discover biologically inspired design principles that can be applied in engineering systems.

In addition, Dabiri oversees the Caltech Field Laboratory for Optimized Wind Energy (FLOWE), an experimental wind farm for testing the energy-generating efficiency of various configurations of vertical-axis wind turbines. By optimizing the placement of the wind turbines based on observations of schools of fish, Dabiri and his group demonstrated that power output can be increased tenfold.

Professor of Physics Maria Spiropulu is an experimental particle physicist. She has worked with particle accelerators and detectors for the past 22 years and has pioneered new methods of data analysis in order to learn about the physics of the universe at both astrophysical and atomic scales. She was nominated by the APS Division of Particles and Fields for her work searching for evidence of supersymmetry (a theory that says that every fundamental particle has a supersymmetric partner) and extra dimensions at the Tevatron, a proton-antiproton collider at Fermilab in Illinois. Spiropulu was also noted for her work on the characterization of the Higgs boson—a long-sought fundamental particle thought to give other particles their mass—at the Large Hadron Collider (LHC) in Geneva, Switzerland.

In addition to Dabiri and Spiropulu, 39 other Caltech faculty and researchers have been elected as fellows of the APS since the program began in 1980.

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Rosakis Elected to Academia Europaea

Ares J. Rosakis, the Theodore von Kármán Professor of Aeronautics and Mechanical Engineering and Otis Booth Leadership Chair of the Division of Engineering and Applied Science, has been elected to the Academia Europaea in the section of Physics and Engineering Sciences.

The academy, similar to the National Academy of Sciences, aims to promote a wider appreciation of European scholarship and research, and to encourage the highest possible standards of scholarship. Members of the academy represent a diverse range of fields in the sciences and humanities.

A native of Greece, Rosakis earned a BA and MA in engineering science from Oxford University and a PhD in solid mechanics from Brown University. He joined the Caltech faculty in 1982. Rosakis has conducted research in aerospace, solid mechanics, and the mechanics of earthquake seismology. He is a leading expert in the area of dynamic failure of solid materials.

As chair of the Division of Engineering and Applied Science at Caltech, Rosakis has spearheaded multiple collaborations with European academic and research institutions. These include establishing programs with École Polytechnique and Institut supérieur de l'aéronautique et de l'espace in France as well as with University of Seville in Spain. Rosakis has received numerous awards, including the Commandeur de l'Ordre des Palmes Académiques, a decoration created by Napoleon to honor educators and scholars. He is a fellow of the National Academy of Engineering and the American Academy of Arts and Science. In addition to the Academia Europaea, he is a fellow of two other European academies, the European Academy of Sciences and Arts and the Academy of Athens.

The election took place in July 2014 at the 26th Annual Conference of the Academia Europaea. Rosakis joins Caltech Provost Edward Stolper, the Carl and Shirley Larson Provostial Chair and William E. Leonhard Professor of Geology; President Emeritus and Robert Andrews Millikan Professor of Biology David Baltimore; and Howard and Gwen Laurie Smits Professor of Cell Biology Alexander Varshavsky as members of the academy.

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The Secret Life of a Snowflake

Faraon Receives Air Force Grant

Andrei Faraon (BS '04) has been awarded a research grant from the Air Force's Young Investigator Research Program. The award, given to scientists and engineers who have received their PhD in the last five years, is intended to foster creative research in science and engineering areas of interest to the Air Force.

Faraon builds devices that will interface atoms and photons (the elementary particles of light) at the quantum level. In the future, such devices could be used to interconnect future quantum computers and to create optical quantum memories, an interface between light and matter allowing for storage and retrieval of photonic quantum information. These quantum memories can operate in telecommunications wavelengths that can be directly coupled to the optical fibers that wire the Internet. Work on these devices will be funded by the Air Force grant.

"I received this award with great enthusiasm because we can start working on quantum technologies that can be directly interfaced with the current Internet infrastructure, allowing for more secure communications." says Faraon. "Quantum memories will play a critical role in realizing quantum networks that will be used to send information over the Internet with absolute security."

Faraon joined the Caltech faculty in 2012 as an assistant professor of applied physics and material science.

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How Iron Feels the Heat

As you heat up a piece of iron, the arrangement of the iron atoms changes several times before melting. This unusual behavior is one reason why steel, in which iron plays a starring role, is so sturdy and ubiquitous in everything from teapots to skyscrapers. But the details of just how and why iron takes on so many different forms have remained a mystery. Recent work at Caltech in the Division of Engineering and Applied Science, however, provides evidence for how iron's magnetism plays a role in this curious property—an understanding that could help researchers develop better and stronger steel.

"Humans have been working with regular old iron for thousands of years, but this is a piece about its thermodynamics that no one has ever really understood," says Brent Fultz, the Barbara and Stanley R. Rawn, Jr., Professor of Materials Science and Applied Physics.

The laws of thermodynamics govern the natural behavior of materials, such as the temperature at which water boils and the timing of chemical reactions. These same principles also determine how atoms in solids are arranged, and in the case of iron, nature changes its mind several times at high temperatures. At room temperature, the iron atoms are in an unusual loosely packed open arrangement; as iron is heated past 912 degrees Celsius, the atoms become more closely packed before loosening again at 1,394 degrees Celsius and ultimately melting at 1,538 degrees Celsius.

Iron is magnetic at room temperature, and previous work predicted that iron's magnetism favors its open structure at low temperatures, but at 770 degrees Celsius iron loses its magnetism. However, iron maintains its open structure for more than a hundred degrees beyond this magnetic transition. This led the researchers to believe that there must be something else contributing to iron's unusual thermodynamic properties.

For this missing link, graduate student Lisa Mauger and her colleagues needed to turn up the heat. Solids store heat as small atomic vibrations—vibrations that create disorder, or entropy. At high temperatures, entropy dominates thermodynamics, and atomic vibrations are the largest source of entropy in iron. By studying how these vibrations change as the temperature goes up and magnetism is lost, the researchers hoped to learn more about what is driving these structural rearrangements.

To do this, the team took its samples of iron to the High Pressure Collaborative Access Team beamline of the Advanced Photon Source at Argonne National Laboratory in Argonne, Illinois. This synchrotron facility produces intense flashes of x-rays that can be tuned to detect the quantum particles of atomic vibration—called phonon excitations—in iron.

When coupling these vibrational measurements with previously known data about the magnetic behavior of iron at these temperatures, the researchers found that iron's vibrational entropy was much larger than originally suspected. In fact, the excess was similar to the entropy contribution from magnetism—suggesting that magnetism and atomic vibrations interact synergistically at moderate temperatures. This excess entropy increases the stability of the iron's open structure even as the sample is heated past the magnetic transition.

The technique allowed the researchers to conclude, experimentally and for the first time, that magnons—the quantum particles of electron spin (magnetism)—and phonons interact to increase iron's stability at high temperatures.

Because the Caltech group's measurements matched up with the theoretical calculations that were simultaneously being developed by collaborators in the laboratory of Jörg Neugebauer at the Max-Planck-Institut für Eisenforschung GmbH (MPIE), Mauger's results also contributed to the validation of a new computational model.

"It has long been speculated that the structural stability of iron is strongly related to an inherent coupling between magnetism and atomic motion," says Fritz Körmann, postdoctoral fellow at MPIE and the first author on the computational paper. "Actually finding this coupling, and that the data of our experimental colleagues and our own computational results are in such an excellent agreement, was indeed an exciting moment."

"Only by combining methods and expertise from various scientific fields such as quantum mechanics, statistical mechanics, and thermodynamics, and by using incredibly powerful supercomputers, it became possible to describe the complex dynamic phenomena taking place inside one of the technologically most used structural materials," says Neugebauer. "The newly gained insight of how thermodynamic stability is realized in iron will help to make the design of new steels more systematic."

For thousands of years, metallurgists have been working to make stronger steels in much the same way that you'd try to develop a recipe for the world's best cookie: guess and check. Steel begins with a base of standard ingredients—iron and carbon—much like a basic cookie batter begins with flour and butter. And just as you'd customize a cookie recipe by varying the amounts of other ingredients like spices and nuts, the properties of steel can be tuned by adding varying amounts of other elements, such as chromium and nickel.

With a better computational model for the thermodynamics of iron at different temperatures—one that takes into account the effects of both magnetism and atomic vibrations—metallurgists will now be able to more accurately predict the thermodynamic properties of iron alloys as they alter their recipes. 

The experimental work was published in a paper titled "Nonharmonic Phonons in α-Iron at High Temperatures," in the journal Physical Review B. In addition to Fultz and first author Mauger, other Caltech coauthors include Jorge Alberto Muñoz (PhD '13) and graduate student Sally June Tracy. The computational paper, "Temperature Dependent Magnon-Phonon Coupling in bcc Fe from Theory and Experiment," was coauthored by Fultz and Mauger, led by researchers at the Max Planck Institute, and published in the journal Physical Review Letters. Fultz's and Mauger's work was supported by funding from the U.S. Department of Energy.

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Six from Caltech Elected to National Academy of Engineering

Six members of the Caltech community—Caltech professors Harry Atwater, Mory Gharib (PhD '83), Robert Grubbs, and Guruswami (Ravi) Ravichandran, and JPL staff members Dan M. Goebel and Graeme L. Stephens—have been elected to the National Academy of Engineering (NAE), an honor considered among the highest professional distinctions accorded to an engineer. The academy welcomed 67 new American members and 12 foreign members this year. Included among the new class are four Caltech alumni, Dana Powers (BS '70, PhD '75), Michael Tsapatsis (MS '91, PhD '94), Vigor Yang (PhD '84), and Ajit Yoganathan (PhD '78).

Harry Atwater, the Howard Hughes Professor of Applied Physics and Materials Science and director of the Resnick Sustainability Institute, was cited for his contributions to plasmonics—the study of plasmons, coordinated waves of electrons on the surfaces of metals. Atwater is developing plasmonic devices for controlling light on a nanometer scale. Such devices could be important for the eventual creation of quantum computers and more efficient photovoltaic cells in solar panels.

Mory Gharib is the vice provost for research and the Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering. His election citation notes his contributions to fluid flow visualization techniques and the engineering of bioinspired medical devices. Gharib's biomechanical studies are often coupled with medical engineering; for example, by studying the fluid dynamics of the human cardiovascular system, he and his group are better able to develop new types of prosthetic heart valves.

Dan M. Goebel, a senior research scientist at JPL, was honored for his contributions to low-temperature plasma sources for thin-film manufacturing, plasma materials interactions, and electric propulsion.

Robert Grubbs, the Victor and Elizabeth Atkins Professor of Chemistry and corecipient of the 2005 Nobel Prize in Chemistry, was elected for the development of catalysts that have enabled commercial products. For example, Grubbs and his team developed a new method for synthesizing organosilanes—basic chemical building blocks. Normally these molecules are made with expensive and rare precious metals, but Grubbs's group has found a way to catalyze the reaction using a cheap and abundant potassium compound.

Guruswami (Ravi) Ravichandran is the John E. Goode, Jr., Professor of Aerospace, professor of mechanical engineering, and director of the Graduate Aerospace Laboratories (GALCIT). He is cited by the NAE for his contributions to the mechanics of dynamic deformation, damage, and failure of engineering materials. Ravichandran has studied the behavior of polymers under high pressures and strains, and how the peeling of an adhesive material—like Scotch Tape—may be modeled as a crack propagating in a medium.

Graeme L. Stephens, the director of JPL's Center for Climate Sciences, was elected by the Academy for the elucidation of Earth's cloud system and radiation balance.

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How To Study High-Speed Flows: An Interview With Joanna Austin

Joanna Austin (MS '98, PhD '03) does not just go with the flow. She picks it apart and analyzes it. One of the newest faculty members in Caltech's Division of Engineering and Applied Science, Austin studies the mechanics involved in compressible flows, those where gases reach such high speeds that the density of a fluid element changes drastically. These flows come into play in problems ranging from the logistics of a spacecraft's entry into a planet's atmosphere to the hows and whys of volcanic eruptions.

To simulate such high-speed, high-temperature conditions, Austin and her colleagues use pistons and explosives in large test tunnels to compress gases. Austin had her first experience with this kind of facility while an undergraduate at the University of Queensland, in her native Australia, where the Centre for Hypersonics houses the T4 shock tunnel. Later, as a graduate student at Caltech, Austin worked in the Explosion Dynamics Laboratory. While an assistant professor at the University of Illinois at Urbana-Champaign, she built another instrument for producing very-high-speed flows, known as the hypervelocity expansion tube (HET). In August 2014, she joined Caltech's faculty as a professor of aerospace. There, she will work with the T5 reflected shock tunnel, the next generation of the T4. She is having the HET transported to campus as well, which will create a full suite of complementary facilities.

We recently spoke with Austin about these tunnels, her research, and the challenges involved in studying high-speed flows.

 

What made you decide to come back to Caltech?

What it basically comes down to is that it is such an exciting and stimulating place to be, with the faculty, the students, and the facilities that are available. And I've always loved it here, so I was really thrilled to come back.

 

Your specialty is high-speed flows. What is your focus within that area?

My group investigates flows under conditions where the molecular processes in the gas couple with the fluid mechanics, the dynamics of the flow. That covers a range of topics, including hypervelocity flows. These are flows that are associated with objects—whether manmade or naturally occurring—entering a planet's atmosphere. We have a couple of different facilities for recreating and studying these kinds of flows.

That was another thing that was really exciting about coming back to Caltech—GALCIT already had some existing, fantastic facilities like T5. With the instrument that I built and am bringing from Illinois, the HET, we will have a really unique suite of experimental facilities.

 

Can you talk more about the T5 and the HET? What do they do?

Essentially what they do is produce a test gas that realistically simulates the flow over an object as it's entering an atmosphere. So if you're interested, for example, in a martian mission, we can make a model of a particular spacecraft configuration, place it in one of these two facilities, and then accelerate the gas to replicate the conditions that the model would actually experience during atmospheric entry.

Then we can make measurements and use various models to understand what happened under those conditions with regard to quantities such as heat flux, which is obviously critical to the survival of the vehicle. But we have just a one-millisecond or less window in which to make all of the measurements that we need.

 

What other kinds of studies do you conduct?

Another type of experiment we do involves probing the molecules themselves in a flow. So we can nonintrusively determine which molecular species are in the flow and what temperature they are at, and in that way we can inform models of the way those molecules interact.

 

Earlier, you provided the example of a martian mission. What work are you doing in that field?

For some of the larger vehicles that are being discussed for future Mars missions, we need to have much better models for predicting the heat flux that they will experience. For a smaller-scale vehicle, you might be able to get away with using a more protective, and therefore heavier, heat shield than you actually need. But with a larger vehicle, the mass penalty that you would pay for such a safety factor would be prohibitive. So we need to have better predictive models.

We've started working on that. I think our next step will be applying spectroscopic techniques to actually probe the molecules.

 

And back on Earth, what kinds of phenomena are you investigating?

We have some projects looking at bubble dynamics and the processes involved when bubbles or arrays of bubbles collapse. These come into play in various medical procedures where pulses generated by lasers selectively remove tissue but can also damage the surrounding tissue and cells.

We've also been looking at explosive volcanic eruptions. Most recently we've been interested in what happens if you send an explosive jet over different topographies, such as the side of Mount St. Helens. With 3-D printing, it's really fun, we can make physical models of the geometries of the different topographies you want to test and then run the experiment over the actual geometries.

 

Is there a topic within the field that most excites you?

I guess it's the umbrella topic of gas dynamics, and particularly looking at gas dynamics in reacting flows. That's the thing I really love. It's a very challenging, coupled, problem. As the fluid is going through the model that you're studying, you also have to account for the fact that the state of the fluid is changing—the gas is chemically reacting, so it's changing from reactants to products, or it's redistributing its energy states, or both. Understanding how best to model these processes, that's what excites me.

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