New Technique Could Harvest More of the Sun's Energy

As solar panels become less expensive and capable of generating more power, solar energy is becoming a more commercially viable alternative source of electricity. However, the photovoltaic cells now used to turn sunlight into electricity can only absorb and use a small fraction of that light, and that means a significant amount of solar energy goes untapped.

A new technology created by researchers from Caltech, and described in a paper published online in the October 30 issue of Science Express, represents a first step toward harnessing that lost energy.

Sunlight is composed of many wavelengths of light. In a traditional solar panel, silicon atoms are struck by sunlight and the atoms' outermost electrons absorb energy from some of these wavelengths of sunlight, causing the electrons to get excited. Once the excited electrons absorb enough energy to jump free from the silicon atoms, they can flow independently through the material to produce electricity. This is called the photovoltaic effect—a phenomenon that takes place in a solar panel's photovoltaic cells.

Although silicon-based photovoltaic cells can absorb light wavelengths that fall in the visible spectrum—light that is visible to the human eye—longer wavelengths such as infrared light pass through the silicon. These wavelengths of light pass right through the silicon and never get converted to electricity—and in the case of infrared, they are normally lost as unwanted heat.

"The silicon absorbs only a certain fraction of the spectrum, and it's transparent to the rest. If I put a photovoltaic module on my roof, the silicon absorbs that portion of the spectrum, and some of that light gets converted into power. But the rest of it ends up just heating up my roof," says Harry A. Atwater, the Howard Hughes Professor of Applied Physics and Materials Science; director, Resnick Sustainability Institute, who led the study.

Now, Atwater and his colleagues have found a way to absorb and make use of these infrared waves with a structure composed not of silicon, but entirely of metal.

The new technique they've developed is based on a phenomenon observed in metallic structures known as plasmon resonance. Plasmons are coordinated waves, or ripples, of electrons that exist on the surfaces of metals at the point where the metal meets the air.

While the plasmon resonances of metals are predetermined in nature, Atwater and his colleagues found that those resonances are capable of being tuned to other wavelengths when the metals are made into tiny nanostructures in the lab.

"Normally in a metal like silver or copper or gold, the density of electrons in that metal is fixed; it's just a property of the material," Atwater says. "But in the lab, I can add electrons to the atoms of metal nanostructures and charge them up. And when I do that, the resonance frequency will change."

"We've demonstrated that these resonantly excited metal surfaces can produce a potential"—an effect very similar to rubbing a glass rod with a piece of fur: you deposit electrons on the glass rod. "You charge it up, or build up an electrostatic charge that can be discharged as a mild shock," he says. "So similarly, exciting these metal nanostructures near their resonance charges up those metal structures, producing an electrostatic potential that you can measure."

This electrostatic potential is a first step in the creation of electricity, Atwater says. "If we can develop a way to produce a steady-state current, this could potentially be a power source. He envisions a solar cell using the plasmoelectric effect someday being used in tandem with photovoltaic cells to harness both visible and infrared light for the creation of electricity.

Although such solar cells are still on the horizon, the new technique could even now be incorporated into new types of sensors that detect light based on the electrostatic potential.

"Like all such inventions or discoveries, the path of this technology is unpredictable," Atwater says. "But any time you can demonstrate a new effect to create a sensor for light, that finding has almost always yielded some kind of new product."

This work was published in a paper titled, "Plasmoelectric Potentials in Metal Nanostructures." Other coauthors include first author Matthew T. Sheldon, a former postdoctoral scholar at Caltech; Ana M. Brown, an applied physics graduate student at Caltech; and Jorik van de Groep and Albert Polman from the FOM Institute AMOLF in Amsterdam. The study was funded by the Department of Energy, the Netherlands Organization for Scientific Research, and an NSF Graduate Research Fellowship.

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New Center Supports Data-Driven Research

With the advanced capabilities of today's computer technologies, researchers can now collect vast amounts of information with unprecedented speed. However, gathering information is only one half of a scientific discovery, as the data also need to be analyzed and interpreted. A new center on campus aims to hasten such data-driven discoveries by making expertise and advanced computational tools available to Caltech researchers in many disciplines within the sciences and the humanities.

The new Center for Data-Driven Discovery (CD3), which became operational this fall, is a hub for researchers to apply advanced data exploration and analysis tools to their work in fields such as biology, environmental science, physics, astronomy, chemistry, engineering, and the humanities.

The Caltech center will also complement the resources available at JPL's Center for Data Science and Technology, says director of CD3 and professor of astronomy George Djorgovski.

"Bringing together the research, technical expertise, and respective disciplines of the two centers to form this joint initiative creates a wonderful synergy that will allow us opportunities to explore and innovate new capabilities in data-driven science for many of our sponsors," adds Daniel Crichton, director of the Center for Data Science and Technology at JPL.

At the core of the Caltech center are staff members who specialize in both computational methodology and various domains of science, such as biology, chemistry, and physics. Faculty-led research groups from each of Caltech's six divisions and JPL will be able to collaborate with center staff to find new ways to get the most from their research data. Resources at CD3 will range from data storage and cataloguing that meet the highest "housekeeping" standards, to custom data-analysis methods that combine statistics with machine learning—the development of algorithms that can "learn" from data. The staff will also help develop new research projects that could benefit from large amounts of existing data.

"The volume, quality, and complexity of data are growing such that the tools that we used to use—on our desktops or even on serious computing machines—10 years ago are no longer adequate. These are not problems that can be solved by just buying a bigger computer or better software; we need to actually invent new methods that allow us to make discoveries from these data sets," says Djorgovski.

Rather than turning to off-the-shelf data-analysis methods, Caltech researchers can now collaborate with CD3 staff to develop new customized computational methods and tools that are specialized for their unique goals. For example, astronomers like Djorgovski can use data-driven computing in the development of new ways to quickly scan large digital sky surveys for rare or interesting targets, such as distant quasars or new kinds of supernova explosions—targets that can be examined more closely with telescopes, such as those at the W. M. Keck Observatory, he says.

Mary Kennedy, the Allen and Lenabelle Davis Professor of Biology and a coleader of CD3, says that the center will serve as a bridge between the laboratory-science and computer-science communities at Caltech. In addition to matching up Caltech faculty members with the expertise they will need to analyze their data, the center will also minimize the gap between those communities by providing educational opportunities for undergraduate and graduate students.

"Scientific development has moved so quickly that the education of most experimental scientists has not included the techniques one needs to synthesize or mine large data sets efficiently," Kennedy says. "Another way to say this is that 'domain' sciences—biology, engineering, astronomy, geology, chemistry, sociology, etc.—have developed in isolation from theoretical computer science and mathematics aimed at analysis of high-dimensional data. The goal of the new center is to provide a link between the two."

Work in Kennedy's laboratory focuses on understanding what takes place at the molecular level in the brain when neuronal synapses are altered to store information during learning. She says that methods and tools developed at the new center will assist her group in creating computer simulations that can help them understand how synapses are regulated by enzymes during learning.

"The ability to simulate molecular mechanisms in detail and then test predictions of the simulations with experiments will revolutionize our understanding of highly interconnected control mechanisms in cells," she says. "To some, this seems like science fiction, but it won't stay fictional for long. Caltech needs to lead in these endeavors."

Assistant Professor of Biology Mitchell Guttman says that the center will also be an asset to groups like his that are trying to make sense out of big sets of genomic data. "Biology is becoming a big-data science—genome sequences are available at an unprecedented pace. Whereas it took more than $1 billion to sequence the first genome, it now costs less than $1,000," he says. "Making sense of all this data is a challenge, but it is the future of biomedical research."

In his own work, Guttman studies the genetic code of lncRNAs, a new class of gene that he discovered, largely through computational methods like those available at the new center. "I am excited about the new CD3 center because it represents an opportunity to leverage the best ideas and approaches across disciplines to solve a major challenge in our own research," he says.

But the most valuable findings from the center could be those that stem not from a single project, but from the multidisciplinary collaborations that CD3 will enable, Djorgovski says. "To me, the most interesting outcome is to have successful methodology transfers between different fields—for example, to see if a solution developed in astronomy can be used in biology," he says.

In fact, one such crossover method has already been identified, says Matthew Graham, a computational scientist at the center. "One of the challenges in data-rich science is dealing with very heterogeneous data—data of different types from different instruments," says Graham. "Using the experience and the methods we developed in astronomy for the Virtual Observatory, I worked with biologists to develop a smart data-management system for a collection of expression and gene-integration data for genetic lines in zebrafish. We are now starting a project along similar methodology transfer lines with Professor Barbara Wold's group on RNA genomics."

And, through the discovery of more tools and methods like these, "the center could really develop new projects that bridge the boundaries between different traditional fields through new collaborations," Djorgovski says.

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Converting Data Into Knowledge: An Interview with Yisong Yue

When a movie-streaming service recommends a new film you might like, sometimes that recommendation becomes a new favorite; other times, the computer's suggestion really misses the mark. Yisong Yue, assistant professor of computing and mathematical sciences, is interested in how systems like these can better "learn" from human behavior as they turn raw data into actionable knowledge—a concept called machine learning.

Yue joined the Division of Engineering and Applied Science at Caltech in September after spending a year as a research scientist at Disney Research. Born in Beijing and raised in Chicago, Yue completed a bachelor's degree at the University of Illinois in 2005, a doctorate at Cornell University in 2010, and an appointment as a postdoctoral researcher at Carnegie-Mellon in 2013.

Recently he spoke with us about his research interests, his hobbies, and what he is looking forward to here at Caltech.


What is your main area of research?

My main research interests are in machine learning. Machine learning is the study of how computers can take raw data or annotated data and convert that into knowledge and actionable items, ideally in a fully automated way—because it's one thing to just have a lot of data, but it's another thing to have knowledge that you can derive from that data.


Is machine learning a general concept that can be applied to many different fields?

That's right. Machine learning is becoming a more and more general tool as we become a more digital society. In the past, some of my research has been applied to applications such as data-driven animation, sports analytics, personalized recommender systems, and adaptive urban transportation systems.


What application of this work are you most excited about right now?

This is tough because I'm excited about all of them, really, but if I had to just pick one, it would be human-in-the-loop machine learning. The idea is that although we would love to have computers that can derive knowledge from data in a fully automated way, oftentimes the problem is too difficult or it would take too long. So machine learning with humans in the loop acknowledges that we can learn from how humans behave in a system.

I think that we are entering a society where we depend on digital systems for basically everything we do. And that means we have an opportunity to learn from humans how to optimize our daily lives. Because human interaction with digital systems is so ubiquitous, I think learning with humans in the loop is a very compelling research agenda moving forward.


Can you give an example of humans-in-the-loop machine learning that we experience on a daily basis?

One example of humans-in-the-loop that we experience fairly regularly is a personalized recommender system. Many websites have a recommendation system built into them, and the system would like to provide personalized recommendations to maximize feedback and engagement of that user with the system. However, when there is a brand-new user, the system doesn't really understand their interests. What the system can do is recommend some stuff and see if the user likes it or not, and their response—thumbs up, thumbs down, or whatever—is an indicator of the topics or content this user is interested in. You see this sort of closed loop between a machine learning system that's trying to learn how best to personalize to a user and a user that's using the system and providing feedback on the fly.


You also mentioned animation. How is your work applied in that field?

Before I came to Caltech, I spent one year as a research scientist at Disney Research. I worked on both sports analytics and data-driven animation. With regard to the animation, the basic idea is as follows: you take data about how humans talk in a natural sentence-speaking setting, and then you try to automatically generate natural lip movements or facial movements that correspond to the types of sentences that people would normally say. This is something that people at Disney Research have been working on for a while, so they have a lot of expertise here.

One of the things that you notice many times with animation is that either the character's lip movements are fairly unrealistic—like their mouths just open and close—or in the big-budget movies, it takes a team of artists to manually animate the character's lips. An interesting in-between technology would be to have fairly realistic automatically generated lip movements and facial movements to any type of sentence.


What are you looking forward to now that you're at Caltech?

Here I have a combination of research independence, talented colleagues, and support for my research endeavor—and a great culture for intellectual curiosity.

It's such a tight-knit community. It's one of the smallest institutions that I'm familiar with, and what that implies is that basically everyone knows everyone else. The great thing about that is that if you have a question about something that you may not be so knowledgeable about, it's really not that big of a deal to go down the block to talk to someone who works in that field, and you can get information and insight from that person.


Have you already begun collaborating with any of your new colleagues?

I'm starting a collaboration with Professor Pietro Perona [Allen E. Puckett Professor of Electrical Engineering] from electrical engineering and Professor Frederick Eberhardt [Professor of Philosophy]. In that collaboration, we'll be addressing a problem that biologists and neuroscientists at Caltech face in assessing how genes affect behavior. These researchers modify the genes of animals—such as fruit flies—and then they video the animal's resulting behaviors. The problem is that researchers don't have time to manually inspect hours upon hours of video to find the particular behavior they're interested in. Professor Perona has been working on this challenge in the past few years, and I was recently brought in to become a part of this collaboration because I work on machine learning and big-data analysis.

The goal is to develop a way to take raw video data of animals under various conditions and try to automatically digest, process, and summarize the significant behaviors in that video data, such as an aggressive attack or attempt to mate.


Tell us a little bit about your background.

It is a bit all over the place. I was born in Beijing. I moved to Chicago when I was fairly young, and I spent most of my childhood in Chicago and the surrounding areas. But my parents actually moved out of Chicago after my sister and I left for college, and so I really don't have any relatives or strong ties to Chicago anymore. Where I call home is … I don't really know where I call home. I guess Pasadena is my home.


Do you have any hobbies outside of your research?

I like hiking and photography, and I'm really excited to try some of the hiking trails in the area and to bring my camera and my tripod with me.

I have a few other hobbies, although I don't really have the time to do them as much now. I was part of an improv group in high school, and I did a fair amount of comedic acting. I wasn't very good at it, so it's not something I can really brag about, but it was fun. I am also an avid eSports fan. For instance, I love watching and playing StarCraft.

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Heat Transfer Sets the Noise Floor for Ultrasensitive Electronics

A team of engineers and scientists has identified a source of electronic noise that could affect the functioning of instruments operating at very low temperatures, such as devices used in radio telescopes and advanced physics experiments.

The findings, detailed in the November 10 issue of the journal Nature Materials, could have implications for the future design of transistors and other electronic components.

The electronic noise the team identified is related to the temperature of the electrons in a given device, which in turn is governed by heat transfer due to packets of vibrational energy, called phonons, that are present in all crystals. "A phonon is similar to a photon, which is a discrete packet of light," says Austin Minnich, an assistant professor of mechanical engineering and applied physics in Caltech's Division of Engineering and Applied Science and corresponding author of the new paper. "In many crystals, from ordinary table salt to the indium phosphide crystals used to make transistors, heat is carried mostly by phonons."

Phonons are important for electronics because they help carry away the thermal energy that is injected into devices in the form of electrons. How swiftly and efficiently phonons ferry away heat is partly dependent on the temperature at which the device is operated: at high temperatures, phonons collide with one another and with imperfections in the crystal in a phenomenon called scattering, and this creates phonon traffic jams that result in a temperature rise.

One way that engineers have traditionally reduced phonon scattering is to use high-quality materials that contain as few defects as possible. "The fewer defects you have, the fewer 'road blocks' there are for the moving phonons," Minnich says.

A more common solution, however, is to operate electronics in extremely cold conditions because scattering drops off dramatically when the temperature dips below about 50 kelvins, or about –370 degrees Fahrenheit. "As a result, the main strategy for reducing noise is to operate the devices at colder and colder temperatures," Minnich says.

But the new findings by Minnich's team suggest that while this strategy is effective, another phonon transfer mechanism comes into play at extremely low temperatures and severely restricts the heat transfer away from a device.

Using a combination of computer simulations and real-world experiments, Minnich and his team showed that at around 20 kelvins, or –424 degrees Fahrenheit, the high-energy phonons that are most efficient at transporting heat away quickly are unlikely to be present in a crystal. "At 20 kelvins, many phonon modes become deactivated, and the crystal has only low-energy phonons that don't have enough energy to carry away the heat," Minnich says. "As a result, the transistor heats up until the temperature has increased enough that high-energy phonons become available again."

As an analogy, Minnich says to imagine an object that is heated until it is white hot. "When something is white hot, the full spectrum of photons, from red to blue, contribute to the heat transfer, and we know from everyday experience that something white hot is extremely hot," he says. "When something is not as hot it glows red, and in this case heat is only carried by red photons with low energy. The physics for phonons is exactly the same—even the equations are the same."

The electronic noise that the team identified has been known about for many years, but until now it was not thought to play an important role at low temperatures. That discovery happened because of a chance encounter between Minnich and Joel Schleeh, a postdoctoral scholar from Chalmers University of Technology in Sweden and first author of the new study, who was at Caltech visiting the lab of Sander Weinreb, a senior faculty associate in electrical engineering.

Schleeh had noticed that the noise he was measuring in an amplifier was higher than what theory predicted. Schleeh mentioned the problem to Weinreb, and Weinreb recommended he connect with Minnich, whose lab studies heat transfer by phonons. "At another university, I don't think I would have had this chance," Minnich says. "Neither of us would have had the chance to interact like we did here. Caltech is a small campus, so when you talk to someone, almost by definition they're outside of your field."

The pair's findings could have implications for numerous fields of science that rely on superchilled instruments to make sensitive measurements. "In radio astronomy, you're trying to detect very weak electromagnetic waves from space, so you need the lowest noise possible," Minnich says.

Electronic noise poses a similar problem for quantum-physics experiments. "Here at Caltech, we have physicists trying to observe certain quantum-physics effects. The signal that they're looking for is very tiny, and it's essential to use the lowest-noise electronics possible," Minnich says.

The news is not all gloomy, however, because the team's findings also suggest that it may be possible to develop engineering strategies to make phonon heat transfer more efficient at low temperatures. For example, one possibility might be to change the design of transistors so that phonon generation takes place over a broader volume. "If you can make the phonon generation more spread out, then in principle you could reduce the temperature rise that occurs," Minnich says.

"We don't know what the precise strategy will be yet, but now we know the direction we should be going. That's an improvement."

In addition to Minnich and Schleeh, the other coauthors of the paper, "Phonon blackbody radiation limit for heat dissipation in electronics," are Javier Mateos and Ignacio Iñiguez-de-la-Torre of the Universidad de Salamanca in Salamanca, Spain; Niklas Wadefalk of the Low Noise Factory AB in Mölndal, Sweden; and Per A. Nilsson and Jan Grahn of Chalmers University of Technology. Minnich's work on the project at Caltech was funded by a Caltech start-up fund and by the National Science Foundation.

Written by Ker Than

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Making Hotter Engines and Lasting Artwork: An Interview with Katherine Faber

Ceramics are extremely versatile materials. Because they can be formed into a variety of shapes and serve as effective insulators from heat, they're used in thousands of applications ranging from dainty porcelain teacups to hardy barrier coatings in engines. However, the characteristic brittle nature of ceramics can often be the material's Achilles' heel. New faculty member Katherine Faber, Simon Ramo Professor of Materials Science in Caltech's Division of Engineering and Applied Science, studies the reasons why brittle ceramics fracture—and how these materials can be made stronger and tougher in the future.

Faber, who comes to Caltech from Northwestern University, received her bachelor's degree in ceramic engineering from Alfred University, her master's degree in ceramic science from Penn State, and a doctorate in materials science from UC Berkeley.

Recently, she spoke about her work, her background, and how her research interests have been applied to sustainability and the arts.


What will you be working on in your laboratory at Caltech?

My training is in ceramic materials, studying the fracture of brittle solids. If we understand enough about how brittle materials fail, we can then design new materials that are more robust. In particular, I am interested in high-temperature materials—those desirable for energy-related applications. This includes, for example, the ceramic coatings that are used in power-generation applications, as thermal-barrier coatings in engines. These very-high-temperature materials provide insulation, protecting underlying metallic materials, so that an engine can run hotter and hence more efficiently. Right now, we are characterizing new coating systems for next-generation engines.


Does your laboratory also make brand-new materials?

Yes we do. One thing that I have found through the years is that often times the materials I want to study are ones that don't exist. That has moved my research into ceramic processing. Currently we are looking at strategies to make ceramic materials that are filled with pores. Such ceramics might be used as filters, in fuel cells, or as biological scaffolds into which cells can grow. Beyond processing, it is essential to characterize the pores. It's not just the pore size, or what fraction of the material is porous, but how tortuous the pathway through the porous network is. This will determine the ease of flow or filtration capabilities Each application may require a different level of connectivity and tortuosity. Here, we rely on three-dimensional imaging to illustrate these features.


Your work also touches on art conservation. How did that come about?

Yes, that's the other part of my work. It all happened rather serendipitously more than a decade ago. When I was chair of my department at Northwestern, the Art Institute of Chicago received funding to hire its very first PhD-level conservation scientist. Museum staff approached me to see if our department of materials science and engineering might be interested in collaborating with their scientist in order to provide a scholarly community and possible opportunities for research. We gladly forged the partnership. My personal involvement included projects on jades and porcelains. But I also became the matchmaker, finding the right people within the university for the right problems at the Art Institute.

Art conservation is not what I was trained to do, but involvement in museum work has become an important part of my career. It's still materials science and engineering, it's just that the "materials" in the museum projects happen to be valuable works of art.

We've been able to involve students through the years, and they, too, see this as a thrilling opportunity to take their training in materials science and engineering to one of the great museums of the world.


Do you also enjoy the arts in your spare time?

I've always loved going to art museums, and I'm really looking forward to exploring the museums here in Los Angeles. To be within walking distance of the Huntington Library is just extraordinary. I also love the theater, so I'm ready to discover the Pasadena Playhouse and beyond. We already have tickets for a couple of shows.


Are there any other reasons that you're excited to have made the move from Chicago to Southern California?

Well, given the winter that we had in Chicago last year, I am very excited that I don't have to shovel snow this winter. To wake up in the morning and go outside your door to grab a fresh grapefruit off of a tree—that's pretty cool.


What else are you looking forward to about being at Caltech?

One of the best things about moving to a new institution later in one's career is that it provides an opportunity to make new connections and work on new problems. That's what I'm most excited about. I suspect that as I meet people across the campus and at JPL I'll learn about a host of new research problems that will intrigue me.

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Wednesday, October 29, 2014
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Caltech Awards International Aerospace Honor to Industry Leader

John Tracy, chief technology officer and senior vice president of engineering, operations, and technology for the Boeing Company, is the 2014 recipient of the International von Kármán Wings Award. The honor—bestowed annually by the Aerospace Historical Society, which is part of the Graduate Aerospace Laboratories of the California Institute of Technology (GALCIT)—acknowledges outstanding contributions by international innovators, leaders, and pioneers in aerospace.

The award recognizes Tracy for his technology and leadership contributions to technical and engineering excellence in the aerospace industry and for leadership in advanced structural and material technologies for commercial aviation, defense programs, and space systems.

"It is an honor for GALCIT and the Aerospace Historical Society to give the International von Kármán Wings Award to John Tracy, whose pioneering work with Boeing continues to transform the aerospace industry. His technical and functional excellence has resulted in transformative structural and material technologies for commercial and defense applications, which have significantly enhanced the reliability of the aerospace systems," says Guruswami (Ravi) Ravichandran, chair of the Aerospace Historical Society, director of GALCIT and the John E. Goode, Jr., Professor of Aerospace and professor of mechanical engineering at Caltech.

In his current position, Tracy oversees the development and implementation of Boeing's technology investment strategy and provides strategic direction to functions, business organizations, and initiatives involving more than 100,000 employees. He is a member of the National Academy of Engineering and a fellow of the American Institute of Aeronautics and Astronautics, the Royal Aeronautical Society, and the American Society of Mechanical Engineers.

"John is passionate about everything aerospace; he loves the intellectual challenge of making it all work," says Boeing chairman and chief executive officer Jim McNerney. "The aerospace industry is stronger because John has been in it."

Ravichandran presented the Wings Award to Tracy at a gala banquet and awards ceremony on October 20 at the Caltech Athenaeum. Also recognized during the banquet was Morgane Grivel, a graduate student in Caltech's aeronautics program, who received the Shirley Thomas Academic Scholarship. The scholarship, which is named in honor of the Aerospace Historical Society's founder, has been awarded annually since 2010.

The Wings Award has annually honored aerospace pioneers for their achievements and role in shaping the aerospace industry for the last 30 years.

Recent recipients of the award include Neal Blue, chairman and CEO of General Atomics; Sir Martin Sweeting, executive chairman of Surrey Satellite Technology Ltd.; and David Thompson, chairman, president, and CEO of Orbital Sciences Corporation.

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How to Grip an Asteroid

On Saturday, October 18, hundreds of undergraduate students shared the results of their projects during SURF Seminar Day. The event provides students with the opportunity to discuss and explain their research to individuals with a wide-range of expertise and interests.

For someone like Edward Fouad, a junior at Caltech who has always been interested in robotics and mechanical engineering, it was an ideal project: help develop robotic technology that could one day fly on a NASA mission to visit and sample an asteroid.

Fouad spent 10 weeks this summer as part of the Summer Undergraduate Research Fellowship (SURF) program working in the lab of Aaron Parness, a group leader at JPL, where researchers are designing, prototyping, and refining technology for a device called a microspine gripper. Looking something like a robotic circular foot with many toes extending radially outward, such a gripper has the ability to grab onto a rocky surface and cling to it even when hanging upside down.

That makes it a good candidate to be included in the robotic capture phase of NASA's Asteroid Redirect Mission, which aims to capture an asteroid and haul it into lunar orbit where robotic and manned missions could study it more easily. One of two concepts that NASA is currently considering for that mission involves using robotic arms to grab a boulder for return from a much larger asteroid. Microspine gripper technology is being evaluated for use on these robotic arms.

Researchers at JPL have been working on this technology for almost five years. The latest version of the gripper is made entirely of metal and consists of two concentric rings of carriages—the toe-like appendages that stick out from the gripper. Each of those carriages is in turn made up of a number of "microspines" with steel hooks at their tips. When the gripper makes contact with a rocky surface, the carriages extend downward onto the rock and then pull inward toward the gripper's center. Because the carriages and microspines all move independently, the gripper is able to conform well to the rock's nooks and crannies.

For his SURF project, Fouad helped with the construction of the latest gripper prototype and worked on improving the design of the microspines for the next generation. In particular, his goal was to design a metal microspine that could conform to a rocky surface and stretch as needed toward the center of the gripper. One of the key elements in such a design is a compliant flexure, a material that can bend and flex, allowing each hook to move independently of its neighbors, to grab onto the crags of an uneven surface. In the past, elastic polymers and metal extension springs have been used for this purpose, but elastic polymers cannot stand up to the extreme temperatures of space, and the springs greatly increase the complexity of the gripper's design and complicate the manufacturing and assembly processes. A different metal option was needed.

"I started by brainstorming many different flexure designs, modeling them on the computer with CAD software, and laser cutting them out of acrylic to test their compliant properties," Fouad says. After repeating that process and improving the designs over several weeks, Fouad and Parness settled on two designs to prototype in metal and test on different rock types. In the end, one of Fouad's designs worked so well in bench-top tests that Parness's group is now incorporating it into their new gripper design.

"Edward did a great job this summer," says Parness. "The SURF program provides a great balance; it ensures an educational experience for the student but also provides a lot of value to the projects and mentors. I always try to work with the students before the summer so that the SURF projects provide some autonomy but give the students a chance to work toward something that could make a long-term contribution to the main project. Edward's project was a good example."

Fouad says he went into the SURF project with a lot of relevant experience. A statics and material mechanics course (ME 35—now ME 12) had provided him with the background he needed to understand how the microspine toes of a particular geometry would deform under different loading conditions. A mechanical design and fabrication class (ME 14) taught him important design skills. And, he says, "The experience I have gained leading the mechanical subgroup of the Caltech Robotics Team was invaluable for my work this summer. Through designing and constructing an autonomous underwater vehicle over the past year, I have acquired a great deal of design and machining techniques as well as the skills necessary to collaborate with others on a large group project."

Fouad says he loved working in Parness's lab and enjoyed having the freedom to pursue the design paths that he found most interesting and promising. And he says that he will now strongly consider pursuing a future career at JPL. "It is an incredible environment for someone looking for exciting robotics opportunities."

Kimm Fesenmaier
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Sweeping Air Devices For Greener Planes

The large amount of jet fuel required to fly an airplane from point A to point B can have negative impacts on the environment and—as higher fuel costs contribute to rising ticket prices—a traveler's wallet. With funding from NASA and the Boeing Company, engineers from the Division of Engineering and Applied Science at Caltech and their collaborators from the University of Arizona have developed a device that lets planes fly with much smaller tails, reducing the planes' overall size and weight, thus increasing fuel efficiency.

On October 8, the researchers—including Emilio Graff, research project manager in aerospace at Caltech and a leader on the project—were presented with a NASA Group Achievement Award "for exceptional achievement executing a full-scale wind-tunnel test, proving the flight feasibility of active flow control."

An airplane's tail forms a critical part of the control system that helps steer the plane during flying. During flight, air rushes around the vertical tail with great force and is deflected by the tail's rudder—a moveable flap at the rear of the tail that can steer the plane by angling air to the left or right. By moving the rudder left or right, a pilot can move the air in one direction or the other, helping to keep the plane flying straight during a strong crosswind.

During the high speeds of flight, the air flow around the tail is so strong that the rudder can control the plane's path with minimal movement. However, during the lower speeds of takeoff and landing, larger rudder deflections are required to maneuver the plane. And in the case of engine failure in a multiengine airplane, the vertical tail must generate enough force to keep the plane going straight by turning "against" the working engine. Airplane manufacturers deal with this challenge by fitting planes with very large vertical tails that can deflect enough air and generate enough force to control the plane—even at low speeds.

"But this means that the planes have a tail that's too big 99 percent of the time," says Emilio Graff, research project manager in aerospace at Caltech and a leader on the project, "because you only need a tail that big if you lose an engine during takeoff or landing. Imagine if the only way you could have airbags in your car was to tow them in a big trailer behind your car, just in case there was an accident. It ends up sucking up a lot of fuel."

The system—designed by Graff and his colleagues in the laboratory of Mory Gharib, Hans W. Liepmann Professor of Aeronautics and Bioinspired Engineering—would allow airplanes to be designed with smaller tails by helping to increase the tail's steering effect at low speeds. The work was done in collaboration with Israel Wygnanski, a professor at the University of Arizona.

In their new approach, the researchers installed air-blowing devices called sweeping jet actuators under the outer skin of the tail along the tail's vertical length. The sweeping jet actuators deliver a strong, steady burst of sweeping air just along the rudder, equivalent to the amount of airflow that would normally be encountered by the tail and rudder at higher speeds. The engineers hypothesized that with the sweeping jets turned on, a smaller tail and rudder could straighten the path of the airplane, even at low speeds.

Graff says that, using these devices, airplane manufacturers could reduce the size of airplane tails by 20 percent, only needing to activate the sweeping jet actuators during the low speeds of takeoff and landing. "That means that most of the time when you're flying around normally, you're saving gas because you have a smaller, lighter tail. So even if this system itself uses a lot of energy, it's only on in emergencies," he says. "When you take off or land, the air jets will be on—just in case an engine fails. But on a 12-hour flight, if you're only using the system for 30 minutes, you're still saving gas during 11 hours and 30 minutes."

The fuel savings come not only from reduced drag due to the smaller size, but also from weight savings and structural advantages from having a shorter tail, Graff adds.

The researchers first tested this hypothesis in the approximately five-by-six-foot Lucas Wind Tunnel at Caltech, recording the effect of sweeping jet actuators on a small model—only 15 percent of the size of an actual airplane tail. Because the jets of air created by the device move back and forth, "sweeping" the air over the length of the tail rather than blasting a single, linear burst of air, the researchers discovered that they could increase air flow over the entire tail with just six of the sweeping jets. On the small-scale model, these six jets boosted the effectiveness of the rudder by over 20 percent.

Upon seeing the favorable results from this preliminary experiment, and as part of NASA's Environmentally Responsible Aviation program, Graff and his colleagues designed the system to test the effects of sweeping jet actuators on a full-sized airliner tail. However, since such tails are nearly 27 feet tall, the engineers had to move this stage of their experiment off campus, to the National Full-Scale Aerodynamics Complex at Moffett Field, California—home of the world's two largest wind tunnels.

After machining sized-up sweeping jet actuators at Caltech, the multi-institutional team, which also included engineers from Boeing Research and Technology and NASA's Langley Research Center, installed the devices on a refurbished Boeing 757 tail, found at an airplane parts salvage yard. The large wind tunnel allowed the researchers to simulate wind conditions that realistically would be experienced during takeoff and landing. Data from the full-scale test confirmed that sweeping jet actuators could sufficiently increase the air flow around the rudder to steer the plane in the event of an engine failure.

The technique used by sweeping jet actuators—called flow control—is not new; it has previously been used for quick takeoffs and landings in military applications, Graff says. But those existing systems are not energy-efficient, he adds, "and if you need a third engine to power the system, then you may as well use it to fly the plane." The system designed by Graff and his colleagues is small and efficient enough to be powered by an airliner's auxiliary power unit—the engine that powers the cabin's air conditioning and lights at the gate. "We were able to prove that a system like this can work at the scale of a commercial airliner, without having to add an extra engine," Graff says.

For the next phase of the project, collaborators at Boeing will test the sweeping jet actuators on their Boeing ecoDemonstrator 757, a plane used for testing innovations that could improve the environmental performance of their aircraft.

These findings could one day help Boeing and other manufacturers produce "greener" planes. However, Graff notes, there are still kinks to work out—for example, as currently designed, the sweeping jets could be noisy for passengers—and the adoption of any new features on an aircraft can be a lengthy process. But once adopted, the payoffs could be huge—and improving the tail is not the only goal, Graff says.

"This is only the beginning. The tail is a 'low risk' surface; modifying it puts engineers at ease compared to, for example, modifying wings," he says. "But the data shows that similar systems could be applied to wings to increase the cruise speed of airplanes and allow some maneuvers to be achieved without moving parts.

"I would be surprised if this ends up in the next line of airplanes—since the new planes are already probably years into the design stage—but some version of this device could be adopted in the near future," he says. And the researchers estimate that if all commercial airplanes were fitted with this device and used it for one year, the fuel savings would be the equivalent of taking a year's worth of traffic off of Southern California's notoriously crowded 405 freeway—a worthy goal.

The sweeping jet actuator was developed as part of NASA's Environmentally Responsible Aviation (ERA) project, which aims to reduce the impact of aviation on the environment.

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