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.

Writer: 
Exclude from News Hub: 
No
News Type: 
Research News

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.

Writer: 
Exclude from News Hub: 
No
News Type: 
In Our Community

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.

Writer: 
Kimm Fesenmaier
Listing Title: 
How To Study High-Speed Flows
Writer: 
Exclude from News Hub: 
No
Short Title: 
How To Study High-Speed Flows
News Type: 
In Our Community
Friday, February 13, 2015
Center for Student Services 360 (Workshop Space)

Backpocket Barnburner: A Lightning Quick Overview of Educational Theory

The Sky Is the Limit: $7.8 Million Gift to Caltech Will Support Aerospace Innovation

Through three gifts to Caltech's Division of Engineering and Applied Science (EAS), investor and philanthropist Foster Stanback and his wife, Coco, aim to help the Institute advance innovation in space exploration, with the attendant benefits of an educated workforce, skilled jobs, and spinoff technologies.

The suite of gifts totals $7.8 million: $3 million to create an endowment for space innovation, $3 million for a fund that will support four graduate student fellowships, and $1.8 million to launch a new outreach program.

According to Foster Stanback, these contributions represent "the equivalent of what investing in big sailing ships was many years ago." He explains: "When Portugal made those investments and Vasco de Gama rounded the coast of Africa with that first load of pepper from India, the world changed.

"There are opportunities waiting out there that are going to advance civilization. We've got to do the hard work, the science, in order to make that possible. It's not going to be as easy as building ships out of wood, but Caltech has the people who can do this, working with nanotechnology and advanced materials science, new propulsion systems, atomic energy, and more."

Caltech President Thomas Rosenbaum, holder of the Sonja and William Davidow Presidential Chair and a professor of physics, anticipates that the impact of the Stanbacks' gifts will be substantial.

"Foster and Coco's insightful philanthropy is empowering Caltech to make world-changing discoveries," he says, noting that the Stanbacks have advanced an array of Institute priorities with more than $24 million in support over the past decade. "Through this new gift to EAS, they are positioning the Institute to attract the most outstanding scholars and to educate future generations of leaders, on campus and off. We are deeply grateful for their remarkable vision."

Stanback was inspired to make the gifts after participating in the inaugural meeting of Caltech's Space Innovation Council in April 2014. The group, chaired by filmmaker James Cameron and Charles Elachi, director of the Jet Propulsion Laboratory and a professor of electrical engineering and planetary science at Caltech, aims to advance space science and promote technological creativity.

Stanback established the space innovation fund to help novel technical projects get off the ground. One example: electrically powered aircraft that can be recharged in flight by energy beamed from space. "This sounds like not just science fiction, but crazy science fiction," Stanback says, "but they're working on it!"

Says Ares Rosakis, holder of the Otis Booth Leadership Chair in the Division of Engineering and Applied Science and Caltech's Theodore von Kármán Professor of Aeronautics and Mechanical Engineering: "The Stanback gifts contribute vitally to the EAS strategy of attracting the best faculty and students, then giving them the resources, acknowledgement, and support to shine. For space engineering, these gifts will allow us to perpetually fund bold seed projects—many of which will lead to spectacular inventions and technologies."

The innovation fund will give EAS additional resources to do just that, and the new fellowships will strengthen the division's ability to offer financial assistance to top graduate students.

"Fellowships are dear to my heart," says Ravi Ravichandran, the John E. Goode, Jr., Professor of Aerospace and a professor of mechanical engineering. Ravichandran directs the Graduate Aerospace Laboratories at Caltech (GALCIT), which will be the primary program to benefit from the new fellowships.

"We want to produce critical thinkers who can transform academia and industry, and define future directions in aerospace," Ravichandran adds. "For the nation to continue to provide leadership in this area, we need to train extraordinary people. The new fellowships will help us attract the best people in the world."

The outreach program will engage these extraordinary young scientists in training the next-generation workforce. Caltech will collaborate with a network of community colleges to identify students from disadvantaged backgrounds who have high technical aptitude and interest. These young people will team up with GALCIT students who will mentor them and teach them topics in aerospace engineering. Stanback envisions this program inspiring students from community colleges to pursue education and careers in aerospace.

For his part, Ravichandran is excited to offer GALCIT students systematic opportunities for outreach. "This will make them better able to teach, to convey the excitement of aerospace, and to work in teams," he says.

Teamwork is already a strength for Caltech, in Stanback's view. "Caltech has something unique—I think there's a community and a culture that leads to interaction and sharing of ideas. The students have an idea and they share it with the professor, and oftentimes the professor says, 'Well, let's do it.' Then the administration says, 'Well, let's figure out how to support this.' Everyone is working together to support the mission."

 "Caltech is a place where the sky is the limit," Ravichandran explains. "When we recruit students, what I tell them is that this is the place where you can imagine anything and you can do anything. Coco and Foster Stanback's transformative gifts will certainly help us in achieving this."

Writer: 
Stacey Hong and Ann Motrunich
Frontpage Title: 
Bold Space Projects Get Big Boost
Listing Title: 
Bold Space Projects Get Big Boost
Writer: 
Exclude from News Hub: 
No
Short Title: 
Bold Space Projects Get Boost
News Type: 
In Our Community

Remembering Fredric Raichlen

1932 – 2014

Fredric ("Fred") Raichlen, professor emeritus of civil and mechanical engineering in Caltech's Division of Engineering and Applied Science, passed away on December 13, 2014. He was 82 years old. Raichlen was an expert in coastal engineering whose pioneering studies of tsunami mechanics have led to standards for designing tsunami-resistant structures that have saved lives around the world.

Ordinary waves are wind-driven and propagate at and just below the ocean's surface. A tsunami, however, is driven by a displacement in the earth's crust, such as an underwater earthquake or a volcanic eruption. The entire depth of the water column is set in motion from seafloor to surface. In the open ocean, the waves are hardly noticeable—the peaks are a few feet high at most, and the interval between successive waves can be several hours. But as the tsunami approaches land, the transition to shallow water concentrates the wave's energy. This rising wall of water, focused by local topography, can flood many miles inland.

That much was known when Raichlen entered the field, says his graduate student Costas Synolakis (BS '78, MS '79, PhD '86), now a professor of civil and environmental engineering at USC and the director of USC's Tsunami Research Center. But, as Synolakis says, "There were several theories and hypotheses, but there was no laboratory validation of any of them. Further, there were very few field observations. Scientists did not even know what a tsunami looked like."

This was at least partly because funding for tsunami research was hard to come by. Tsunamis were seen as a threat to other shorelines, not American ones. "Tsunamis were not trendy," Synolakis says, "and their study was considered humdrum. For almost a decade, Fred was the only professor in the U.S. working on tsunami hydrodynamics. But the students he trained, trained others. And by the time it was realized how important tsunamis are, there were knowledgeable scientists who could rise to the challenge."

Upon arriving at Caltech in 1962, Raichlen built a set of wave tanks to analyze how tsunamis originate, how they propagate through the open ocean, and what happens when they run up on shore. The data from these experiments enabled him to develop a comprehensive, three-dimensional computer model of tsunami behavior. The first part of the model described the waves' motions through the deep sea, while the second part of the model described the waves' behavior within the harbor. The two models were fused at the harbor's entrance, with the connecting region modifying the incoming tsunami's waves as they entered the harbor.

"The work he supervised remains the world standard," Synolakis says. "Nobody else before or since has done laboratory experiments of such precision and quality. Fred believed that answers could only be mined in the laboratory and that the only numerical models that could be trusted were the ones that had been benchmarked with laboratory experiments."

Previous models had represented harbors as simple geometric shapes. This model, however, re-created the harbor's interior in great detail, rendering its basins, jetties, islands, and channels as collections of line segments. The waves' reflections off of each line segment were easy to calculate when each segment was handled individually, and the tsunami's actual behavior was derived by superimposing all the reflected waves on the incoming ones to map out where they would reinforce one another and where they would damp each other out. This approach reduced the computation to a straightforward exercise in matrix algebra that could be solved on Caltech's IBM 360/75 mainframe computer—the fastest, most sophisticated machine of its day.

In 1965, Raichlen built a 31-by-15-foot wave tank instrumented to measure wave heights and water velocities anywhere within its walls. Graduate student Jiin-Jen Lee (PhD '70), also now a professor of civil and environmental engineering at the University of Southern California and the director of USC's Foundation for Cross-Connection Control and Hydraulic Research, used the tank to verify the model's predictions of wave behavior in idealized circular and rectangular harbors. He then built a scale model of the east and west basins of the port of Long Beach, California, out of 15 sheets of quarter-inch-thick Lucite. The waves created by Lee's physical model in the wave tank were well described by the mathematical model in the computer. Says Lee, "Fred wanted a theory and the numerical analysis to go with it, but he also wanted them verified against a physical model. A lot of people would just say, 'OK, I did this, and now I'll move on.' Fred was very careful to make sure that the theory could actually be checked out."

Raichlen continued to refine and expand the model. A third section was added to reproduce the different types of seabed motions that could give a wave its initial impetus. Other experiments considered a tsunami's interactions with objects floating in the harbor, such as ships and mooring platforms, or measured how fast different regions within a wave moved as the wave broke, which allowed the force of the wave's impact to be calculated.

Raichlen's model also provided the first mathematically sound explanation of how seiches, also known as "harbor waves," are created. Seiches can persist for days and are extremely damaging due to their height. They occur because every harbor has a set of resonant frequencies. Any waves of those frequencies will reverberate, amplifying themselves. Typical tsunamis have a frequency of one wave every several hours. Raichlen's model showed that many harbors also have a fundamental resonant frequency of one wave every several hours—an unfortunate frequency match that enables such a harbor to amplify a tsunami into a seiche. The model also resolved a long-standing paradox: Harbors with narrow mouths usually offer the best shelter, but those same harbors suffer the worst seiches. The model showed that as the harbor's mouth got narrower, the wave energy trapped within the harbor had less and less chance of escaping. The only way to dissipate the energy was by friction as the water sloshed back and forth.

Raichlen's commitment to his work was matched by his commitment to his students. Lee's thesis was published in the Journal of Fluid Mechanics, an unusual periodical for a civil engineer and one read by a much wider community. Says Lee, "Normally the professor and the student are coauthors, but Fred took his name out. He said, 'This is very important for your career. You should publish it as the sole author.' At first I thought that meant maybe the paper was not so good, and he didn't want his name on it. But he wanted that study to be identified with me, so he gave me all the credit. I was really moved, because it was a pretty important study. We could have published a hundred papers, each with a different-shaped harbor."

Raichlen was a hands-on adviser, spending time with each of his students every day, says Synolakis. "I will forever treasure how he trained me in the laboratory. I was a complete novice, and for several months, he stayed with me, making sure that I didn't run into trouble. He was always eager to explain what we were seeing. His attention to detail was legendary, and he could see things that nobody else could or can." 

Raichlen earned his bachelor's degree in engineering from the Johns Hopkins University in 1953 and his master's and doctoral degrees at MIT in 1955 and 1962.  He also served in the Air Force as an environmental health officer from 1956 to 1959. He came to Caltech as an assistant professor of civil engineering in 1962; he was promoted to associate professor in 1967 and to professor in 1972. In 1969, he became one of the founding faculty members of Caltech's doctoral program in environmental engineering science. He was appointed professor of civil and mechanical engineering in 1997 and professor emeritus in 2001.

Raichlen was inducted into the National Academy of Engineering in 1993, and in 1994 he received the John G. Moffatt–Frank E. Nichol Harbor and Coastal Engineering Award from the American Society of Civil Engineers (ASCE). In 2003, he was given the ASCE's International Coastal Engineering Award, the most prestigious honor in the international coastal engineering community.

In his retirement, Raichlen devoted his time to writing a book, Waves (MIT Press Essential Knowledge series, 2012). He also became an avid and prolific watercolor painter.

Raichlen is survived by his wife, Judy; his sons, Robert and David; their wives, Amy and Sarah (respectively); his sister, Linda Millison; his brother, Sonny; and two grandchildren. 

Writer: 
Douglas Smith
Frontpage Title: 
Remembering Fredric Raichlen
Listing Title: 
Remembering Fredric Raichlen
Writer: 
Exclude from News Hub: 
No
Short Title: 
Remembering Fredric Raichlen
News Type: 
In Our Community

Size Matters: The Importance of Building Small Things

Watson Lecture Preview

Strong materials, such as concrete, are usually heavy, and lightweight materials, such as rubber (for latex gloves) and paper, are usually weak and susceptible to tearing and damage. Julia R. Greer, professor of materials science and mechanics in Caltech's Division of Engineering and Applied Science, is helping to break that linkage. In Caltech's Beckman Auditorium at 8 p.m. on Wednesday, January 21, Greer will explain how we can give ordinary materials superpowers. Admission is free.

 

Q: What do you do?

A: I'm a materials scientist, and I work with materials whose dimensions are at the nanoscale. A nanometer is one-billionth of a meter, or about one-hundred-thousandth the diameter of a hair. At those dimensions, ordinary materials such as metals, ceramics, and glasses take on properties quite unlike their bulk-scale counterparts. Many materials become 10 or more times stronger. Some become damage-tolerant. Glass shatters very easily in our world, for example, but at the nanoscale, some glasses become deformable and less breakable. We're trying to harness these so-called size effects to create "meta-materials" that display these properties at scales we can see.

We can fabricate essentially any structure we like with the help of a special instrument that is like a tabletop microprinter, but uses laser pulses to "write" a three-dimensional structure into a tiny droplet of a polymer. The laser "sets" the polymer into our three-dimensional design, creating a minuscule plastic scaffold. We rinse away the unset polymer and put our scaffold in another machine that essentially wraps it in a very thin, nanometers-thick ribbon of the stuff we're actually interested in—a metal, a semiconductor, or a biocompatible material. Then we get rid of the plastic, leaving just the interwoven hollow tubular structure. The final structure is hollow, and it weighs nothing. It's 99.9 percent air.

We can even make structures nested within other structures. We recently started making hierarchical nanotrusses—trusses built from smaller trusses, like a fractal.

 

Q: How big can you make these things, and where might that lead us?

A: Right now, most of them are about 100 by 100 by 100 microns cubed. A micron is a millionth of a meter, so that is very small. And the unit cells, the individual building blocks, are very, very small—a few microns each. I recently asked my graduate students to create a demo big enough to be visible, so I could show it at seminars. They wrote me an object about 6 millimeters by 6 millimeters by about 100 microns tall. It took them about a week just to write the polymer, never mind the ribbon deposition and all the other steps.

The demo piece looks like a little white square from the top, until you hold it up to the light. Then a rainbow of colors play across its surface, and it looks like a fine opal. That's because the nanolattices and the opals are both photonic crystals, which means that their unit cells are the right size to interact with light. Synthetic three-dimensional photonic crystals are relatively hard to make, but they could be extremely useful as high-speed switches for fiber-optic networks.

Our goal is to figure out a way to mass produce nanostructures that are big enough to see. The possibilities are endless. You could make a soft contact lens that can't be torn, for example. Or a very lightweight, very safe biocompatible material that could go into someone's body as a scaffold on which to grow cells. Or you could use semiconductors to build 3-D logic circuits. We're working with Assistant Professor of Applied Physics and Materials Science Andrei Faraon [BS '04] to try to figure out how to simultaneously write a whole bunch of things that are all 1 centimeter by 1 centimeter.

 

Q: How did you get into this line of work? What got you started?

A: When I first got to Caltech, I was working on metallic nanopillars. That was my bread and butter. Nanopillars are about 50 nanometers to 1 micron in diameter, and about three times taller than their width. They were what we used to demonstrate, for example, that smaller becomes stronger—the pillars were stronger than the bulk metal by an order of magnitude, which is nothing to laugh at.

Nanopillars are awesome, but you can't build anything out of them. And so I always wondered if I could use something like them as nano-LEGOs and construct larger objects, like a nano-Eiffel Tower. The question I asked myself was if each individual component had that very, very high strength, would the whole structure be incredibly strong? That was always in the back of my mind. Then I met some people at DARPA (Defense Advanced Research Projects Agency) at HRL (formerly Hughes Research Laboratories) who were interested in some similar questions, specifically about using architecture in material design. My HRL colleagues were making microscale structures called micro-trusses, so we started a very successful DARPA-funded collaboration to make even smaller trusses with unit cells in the micron range. These structures were still far too big for my purposes, but they brought this work closer to reality.  

 

Named for the late Caltech professor Earnest C. Watson, who founded the series in 1922, the Watson Lectures present Caltech and JPL researchers describing their work to the public. Many past Watson Lectures are available online at Caltech's iTunes U site.

Writer: 
Douglas Smith
Writer: 
Exclude from News Hub: 
No
Short Title: 
Size Matters
News Type: 
Research News
Saturday, January 24, 2015
Center for Student Services 360 (Workshop Space)

The personal side of science

Wednesday, February 4, 2015
Center for Student Services 360 (Workshop Space)

Meet the Outreach Guys: James & Julius

Ravichandran Receives Award for Solid Mechanics

Guruswami (Ravi) Ravichandran, the John E. Goode Jr. Professor of aerospace and professor of mechanical engineering, has received the Warner T. Koiter Medal from the American Society of Mechanical Engineers (ASME).

The award, established in 1996, is given annually to individuals who make distinguished contributions to the field of solid mechanics.

Ravichandran, who is also the director of the Graduate Aerospace Laboratories of the California Institute of Technology (GALCIT), was noted for his "outstanding scientific, engineering, and mentoring contributions in the areas of ultra-high strain rate mechanics of ceramics and metals, and pioneering and innovative experiments to advance our understanding of coupled phenomena in the fields of smart materials and cellular mechanics."

"I am greatly honored to receive the Warner T. Koiter Medal from the ASME. I am pleased that my work is being recognized, but this would not be possible without the contributions of many talented students and postdoctoral scholars in my group," says Ravichandran. "I have also been very fortunate to collaborate with a number of colleagues on campus who have given me impetus to investigate interdisciplinary problems."

Ravichandran works to discover fundamental insights into the way that materials deform, are damaged, and fail. His primary aim is to develop new experimental methods to study these and other phenomena in solid mechanics.

Ravichandran received the award at the 2014 International Mechanical Engineering Congress and Exposition in Montreal, in November. There he gave his 2014 Koiter Lecture, titled "Peeling of Heterogeneous Adhesive Tapes."

Writer: 
Exclude from News Hub: 
No
News Type: 
In Our Community

Pages

Subscribe to RSS - EAS