Caltech and Dow Chemical Team Up in Solar Materials Effort

Collaboration includes the creation of Dow Graduate Fellowship in Chemical Sciences and Engineering

PASADENA, Calif.—The California Institute of Technology (Caltech) and the Dow Chemical Company today announced a new solar-research collaboration aimed at developing the use of semiconductor materials that are less expensive and more abundant than those used in many of today's solar cells.

In addition, they announced the creation of the Dow Chemical Company Graduate Fellowship in Chemical Sciences and Engineering.

The fellowship will be granted to a second- or third-year doctoral student who shows excellence in research, leadership, and interpersonal effectiveness, and whose research program aligns with broad areas of interest to Dow, such as alternative energy sources, the development of novel specialty chemicals, and the investigation of new polymer systems. Dow's $500,000 gift will be matched by $250,000 in funds from the Gordon and Betty Moore Matching Program.

Each recipient will be selected by the chair of the Division of Chemistry and Chemical Engineering, and will hold the fellowship for up to two years.

"We are pleased that Dow and Caltech are building this relationship to support innovative research as a basis for new technologies," says Jacqueline K. Barton, the division's current chair.

The solar-research collaboration will be a four-year, $4.2 million effort to explore earth-abundant materials for solar-energy applications. The project is led by applied physicist Harry Atwater, Caltech's Howard Hughes Professor, and chemist Nate Lewis, Caltech's George L. Argyros Professor.

"In combining the R&D strengths of Dow and Caltech, we have created a powerful alliance for innovation in the field of photovoltaics," says Bill Banholzer, executive vice president and chief technology officer of Dow. "This alliance will allow the best scientists the opportunity to work together to achieve the kinds of breakthrough technologies that will be game-changing in solar-energy capture."

The new Dow/Caltech solar-research initiative is one of the company's largest externally funded research agreements, Banholzer notes.

As part of the agreement, Atwater, Lewis, and their team will develop new mineral-like electronic materials suitable for use in thin-film solar-energy-conversion devices. 

"Development of materials that are abundant in the earth's crust will enable solar-energy technologies to ultimately scale to large volumes at low cost without concern about the materials' availability," says Atwater.

Most solar cells today are made with silicon, which is itself an abundant material. Still, silicon solar technology has a relatively higher cost than that of current thin-film solar materials like cadmium telluride and copper indium diselenide. But these inexpensive semiconductors pose a problem of their own: they contain materials too scarce to ultimately meet the demands of full-scale solar-energy technologies.

That's why Atwater and Lewis are turning their attention to semiconductors found in the earth's crust.

"Use of earth-abundant materials can provide new technology options and could open new areas of design space," Lewis notes. "But it also brings new challenges. This project will develop the science and technology base for thin-film solar-energy conversion using these widely available materials."

"This is an example of industry stepping up to the plate with a long-term vision that acknowledges the importance of supporting research in its most fundamental forms," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

"Dow understands that high-quality research is occurring in both industrial and academic laboratories. We believe that partnerships like this one are crucial to our success in the development of efficient, affordable energy solutions," says Banholzer.

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Lori Oliwenstein
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Caltech Scientists Develop DNA Origami Nanoscale Breadboards for Carbon Nanotube Circuits

PASADENA, Calif.—In work that someday may lead to the development of novel types of nanoscale electronic devices, an interdisciplinary team of researchers at the California Institute of Technology (Caltech) has combined DNA's talent for self-assembly with the remarkable electronic properties of carbon nanotubes, thereby suggesting a solution to the long-standing problem of organizing carbon nanotubes into nanoscale electronic circuits.

A paper about the work appeared November 8 in the early online edition of Nature Nanotechnology

"This project is one of those great 'Where else but at Caltech?' stories," says Erik Winfree, associate professor of computer science, computation and neural systems, and bioengineering at Caltech, and one of four faculty members supervising the project.

Both the initial idea for the project and its eventual execution came from three students: Hareem T. Maune, a graduate student studying carbon nanotube physics in the laboratory of Marc Bockrath (then Caltech assistant professor of applied physics, now at the University of California, Riverside); Si-ping Han, a theorist in materials science, investigating the interactions between carbon nanotubes and DNA in the Caltech laboratory of William A. Goddard III, Charles and Mary Ferkel Professor of Chemistry, Materials Science, and Applied Physics; and Robert D. Barish, an undergraduate majoring in computer science who was working on complex DNA self-assembly in Winfree's lab.

The project began in 2005, shortly after Paul W. K. Rothemund invented his revolutionary DNA origami technique. At the time, Rothemund was a postdoctoral scholar in Winfree's laboratory; today, he is a senior research associate in bioengineering, computer science, and computation and neural systems.

Rothemund's work gave Maune, Han, and Barish the idea to use DNA origami to build carbon nanotube circuits. 

DNA origami is a type of self-assembled structure made from DNA that can be programmed to form nearly limitless shapes and patterns (such as smiley faces or maps of the Western Hemisphere or even electrical diagrams). Exploiting the sequence-recognition properties of DNA base pairing, DNA origami are created from a long single strand of viral DNA and a mixture of different short synthetic DNA strands that bind to and "staple" the viral DNA into the desired shape, typically about 100 nanometers (nm) on a side.

Single-wall carbon nanotubes are molecular tubes composed of rolled-up hexagonal mesh of carbon atoms. With diameters measuring less than 2 nm and yet with lengths of many microns, they have a reputation as some of the strongest, most heat-conductive, and most electronically interesting materials that are known. For years, researchers have been trying to harness their unique properties in nanoscale devices, but precisely arranging them into desirable geometric patterns has been a major stumbling block. 

"After hearing Paul's talk, Hareem got excited about the idea of putting nanotubes on origami," Winfree recalls. "Meanwhile, Rob had been talking to his friend Si-ping, and they independently had become excited about the same idea."

Underlying the students' excitement was the hope that DNA origami could be used as 100 nm by 100 nm molecular breadboards—construction bases for prototyping electronic circuits—on which researchers could build sophisticated devices simply by designing the sequences in the origami so that specific nanotubes would attach in preassigned positions. 

"Before talking with these students," Winfree continues, "I had zero interest in working with carbon nanotubes or applying our lab's DNA-engineering expertise toward such practical ends. But, seemingly out of nowhere, a team had self-assembled with a remarkable spectrum of skills and a lot of enthusiasm. Even Si-Ping, a consummate theorist, went into the lab to help make the idea become reality."

"This collaborative research project is evidence of how we at Caltech select the top students in science and engineering and place them in an environment where their creativity and imagination can thrive," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech and Theodore von Kármán Professor of Aeronautics and Professor of Mechanical Engineering.

Bringing the students' ideas to fruition wasn't easy. "Carbon nanotube chemistry is notoriously difficult and messy—the things are entirely carbon, after all, so it's extremely difficult to make a reaction happen at one chosen carbon atom and not at all the others," Winfree explains.

"This difficulty with chemically grabbing a nanotube at a well-defined 'handle' is the essence of the problem when you're trying to place nanotubes where you want them so you can build complex devices and circuits," he says.

The scientists' ingenious solution was to exploit the stickiness of single-stranded DNA to create those missing handles. It's this stickiness that unites the two strands that make up a DNA helix, through the pairing of DNA's nucleotide bases (A, T, C, and G) with those that have complementary sequences (A with T, C with G). 

"DNA is the perfect molecule for recognizing other strands of DNA, and single-stranded DNA also just happens to like sticking to carbon nanotubes," says Han, "so we mix bare nanotubes with DNA molecules in salt water, and they stick all over the nanotubes' surfaces. However, we make sure that a little bit of each DNA molecule is protected, so that that little portion doesn't stick to the nanotube, and we can use it to recognize DNA attached to the DNA origami instead."

The scientists created two batches of carbon nanotubes labeled by DNA with different sequences, which they called "red" and "blue."

"Metaphorically, we dipped one batch of nanotubes in red DNA paint, and dipped another batch of nanotubes in blue DNA paint," Winfree says. Remarkably, this DNA paint acts like color-specific Velcro.

"These DNA molecules served as handles because a pair of single-stranded DNA molecules with complementary sequences will wrap around each other to form a double helix. Thus," he says, "'red' can bind strongly to 'anti-red,' and 'blue' with 'anti-blue.'"

"Consequently," he adds, "if we draw a stripe of anti-red DNA on a surface, and pour the red-coated nanotubes over it, the nanotubes will stick on the line. But the blue-coated nanotubes won't stick, because they only stick to an anti-blue line."

To make nanometer-scale electronic circuits out of carbon nanotubes requires the ability to draw nanometer-scale stripes of DNA. Previously, this would have been an impossible task.

Rothemund's invention of DNA origami, however, made it possible.

"A standard DNA origami is a rectangle about 100 nm in size, with over 200 'pixel' positions where arbitrary DNA strands can be attached," Winfree says. To integrate the carbon nanotubes into this system, the scientists colored some of those pixels anti-red, and others anti-blue, effectively marking the positions where they wanted the color-matched nanotubes to stick. They then designed the origami so that the red-labeled nanotubes would cross perpendicular to the blue nanotubes, making what is known as a field-effect transistor (FET), one of the most basic devices for building semiconductor circuits.

Although their process is conceptually simple, the researchers had to work out many kinks, such as separating the bundles of carbon nanotubes into individual molecules and attaching the single-stranded DNA; finding the right protection for these DNA strands so they remained able to recognize their partners on the origami; and finding the right chemical conditions for self-assembly.

After about a year, the team had successfully placed crossed nanotubes on the origami; they were able to see the crossing via atomic force microscopy. These systems were removed from solution and placed on a surface, after which leads were attached to measure the device's electrical properties. When the team's simple device was wired up to electrodes, it indeed behaved like a field-effect transistor.  The "field effect" is useful because "the two components of the transistor, the channel and the gate, don't actually have to touch for there to be a switching effect," Rothemund explains. "One carbon nanotube can switch the conductivity of the other due only to the electric field that forms when a voltage is applied to it."

At this point, the researchers were confident that they had created a method that could construct a device from a mixture of nanotubes and origami.

"It worked," Winfree says. "I can't say perfectly—there's lots of room for improvement. But it was sufficient to demonstrate the controlled construction of a simple device, a cross-junction of a pair of carbon nanotubes."

"We expect that our approach can be improved and extended to reliably construct more complex circuits involving carbon nanotubes and perhaps other elements including electrodes and wiring," Goddard says, "which we anticipate will provide new ways to probe the behavior and properties of these remarkable molecules."

The real benefit of the approach, he points out, is that self-assembly doesn't just make one device at a time. "This is a scalable technology. That is, one can design the origami to construct complex logic units, and to do this for thousands or millions or billions of units that self-assemble in parallel."

The work in the paper, "Self-assembly of carbon nanotubes into two-dimensional geometries using DNA origami templates," was supported by the National Science Foundation, the Office of Naval Research, and the Center on Functional Engineered Nano Architectonics.

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Kathy Svitil
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James. K. Knowles, 78

James K. Knowles, William J. Keenan Jr. Professor of Applied Mechanics, Emeritus, at the California Institute of Technology (Caltech), passed away November 1. He was 78 years old.

Knowles made fundamental research contributions to the theory of nonlinear elasticity and the mathematical theories of materials and structures.

His work provided important insight into how various materials and structures behave and enabled him and others to develop predictive theories.

Born in Cleveland, Ohio, on April 14, 1931, Knowles grew up in Phoenix, Arizona. He entered the Massachusetts Institute of Technology (MIT) in the fall of 1948, earning his bachelor's and doctoral degrees, both in mathematics, in 1952 and 1957, respectively. He then stayed at MIT for an additional year, as an instructor in mathematics.

Knowles joined the faculty at Caltech in 1958 as assistant professor of applied mechanics; he was named associate professor in 1961, followed by full professor in 1965. He spent the remainder of his academic career at Caltech, becoming professor emeritus in 1997.

Considered a remarkable teacher and mentor, Knowles inspired and influenced generations of students and scholars through classes in mathematics and mechanics. A visionary thinker, he recruited and mentored a number of junior colleagues who took Caltech in new and fruitful research directions. He had a deep affection for Caltech and served in various administrative capacities.

"Jim was the greatest mentor I ever had. He held my hand when I first came to Caltech as an assistant professor. He also taught me how to teach," says Ares Rosakis, chair of the Division of Engineering and Applied Science, and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech. "He would look for the spark in people's eyes and help them make their dreams a reality. As we at Caltech seek to create the best mentoring opportunities for our young faculty, we should be guided by Jim's example."

Knowles' research was primarily focused on mathematical problems in structural mechanics, and in particular on linear and nonlinear elasticity. In 1960, he provided the first solution for a dynamical problem in finite elasticity and in 1966, he published what would turn out to be a seminal paper concerning the foundations of Saint-Venant's principle in linear elasticity theory.

His later papers on the influence of nonlinearity on point singularities, such as those found at the tip of a crack, demonstrated how they could lead to new phenomena.

In 1979, Knowles published a paper concerning the dissipation of mechanical energy during quasi-static motions of elastic bodies. This led to his later work on the evolution of metastable states of equilibrium, which had applications in phase transformations.

Knowles' contributions are described in more than one hundred journal publications. In 1998, he authored a textbook for graduate students entitled Linear Vector Spaces and Cartesian Tensors (Oxford University Press).

In 1991, he was made an honorary member of the Caltech Alumni Association in recognition of his distinguished service. That same year, the Journal of Elasticity dedicated an issue to Knowles on the occasion of his 60th birthday for "seminal contributions made to the field of elasticity."

"He set an example of scholarship and fundamental thought, both broad and deep, that challenged students as well as researchers," says Roger Fosdick, editor-in-chief of the Journal of Elasticity. "He was highly inquisitive, deeply thoughtful, masterfully insightful and always seeking an explanation. He made indelible marks of value during his life both personally and professionally, and he will most certainly be missed."

Knowles' contributions were also recognized by the Society of Engineering Science with the Eringen Medal, and by the American Society of Mechanical Engineers with the Koiter Medal.

Knowles was a fellow of the American Academy of Mechanics, the American Society of Mechanical Engineers, and the American Association for the Advancement of Science, and was associate editor for the Journal of Applied Mechanics. From 1985 to 1986, he served as president of the American Academy of Mechanics.

Knowles was known outside the classroom for his paintings and baritone voice.

He leaves behind a wife, Jacqueline, and sons John, Jeff, and James, and their families.

A graveside service is scheduled for Saturday, November 14, at 10:30 a.m. at Sierra Madre Pioneer Cemetery, 553 East Sierra Madre Boulevard (at the corner of Coburn Avenue and Sierra Madre Boulevard), in Sierra Madre.

The Division of Engineering and Applied Science has established a memorial fund in honor of Knowles. The fund will support the James K. Knowles Lecture in Solid Mechanics at Caltech, to be delivered annually by an internationally recognized scholar chosen by the faculty. The lecture will be followed by a workshop on solid mechanics, which will be a daylong event of talks by selected current Caltech graduate students and postdoctoral scholars in the area of solid mechanics. The Knowles lecture and workshop will commemorate Knowles' contributions to solid mechanics, his love for Caltech, and his encouragement of young researchers.

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Jon Weiner
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Two Caltech Researchers Receive DARPA Young Faculty Awards

PASADENA, Calif.—The Defense Advanced Research Projects Agency (DARPA) has selected two researchers from the California Institute of Technology (Caltech) to participate in its Young Faculty Award (YFA) program.

Julia R. Greer, assistant professor of materials science, and Doris Tsao, assistant professor of biology, are among the 33 "rising stars" from 24 U.S. universities who each will receive grants of approximately $300,000 to develop and validate their research ideas over the next 24 months.

Greer joined the Caltech faculty in the Division of Engineering and Applied Science (EAS) in 2007 after receiving her PhD from Stanford University in 2005. In 2008, Greer made Technology Review's list of the world's top innovators under the age of 35 for her work with materials at the nanoscale level. In 2008, she received a Faculty Early Career Development award from the National Science Foundation.

Greer's YFA project is aimed at understanding and subsequently mimicking the superior mechanical robustness and strength of naturally occurring protective layers—such as nacre, or mother of pearl, a composite produced by some mollusks to line their inner shell—to create strong, ductile, damage-tolerant materials that maintain a relatively low density.

"Drawing inspiration from hard biological systems will allow us to gain insight into new physical phenomena operating in these materials, and to subsequently create innovative material systems with greatly amplified mechanical properties dictated by the choice of individual components, specific geometries, and microstructure in a truly across-scales fashion," says Greer.

One key objective of the work will be to fabricate a "brick-and-mortar" architecture using tiny plates of a metallic glass and ultrafine-grained ductile metal with nanoscale dimensions; this hierarchical architecture could then be used to fabricate new engineering composites with amplified strength and ductility.

"Greer's nature-inspired work exemplifies the cutting-edge research being carried out in the division," says Ares Rosakis, chair of EAS and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

Tsao received her PhD from Harvard University in 2002, and came to Caltech in 2009 from the University of Bremen in Germany. She was named on Technology Review's 2007 list of top young innovators; in 2009, she became a John Merck Scholar, a Searle Scholar, and a Klingenstein Scholar.

Tsao uses functional magnetic resonance imaging, electrical recordings from single neurons, anatomical measurements, and mathematical modeling to understand how the brain identifies objects and reconstructs the three-dimensional world. Specifically, her proposed work will attempt to decipher the neural machinery underlying spatial navigation. 

"Navigation, which is the purposeful movement through space guided by sensory feedback and memory, is a defining behavior in all animals," says Tsao. Understanding the brain mechanisms responsible for navigation, she says, "constitutes a critical step toward designing artificial systems capable of human-like autonomous navigation. Such systems may be used to explore dangerous terrain and to perform tasks, such as clearing land mines, that could risk the loss of human life."

The objective of the DARPA YFA program is to identify and engage rising research stars in junior faculty positions in academia.  The YFA program provides funding mentoring, and industry and Department of Defense (DOD) contacts to these faculty members early in their careers, so that they can develop their research ideas in the context of the needs of the DOD. DARPA's long-term goal for this program is to develop the next generation of academic scientists, engineers, and mathematicians in key disciplines who will focus a significant portion of their careers on DOD and National Security issues.

The YFA awardees were chosen though a competitive selection process. Applicants were required to be untenured faculty at U.S. institutions within six years of appointment to a tenure-track position. Nearly 300 proposals were reviewed for the 2009 awards.

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Kathy Svitil
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Caltech Scientists First to Trap Light and Sound Vibrations Together in Nanocrystal

Optomechanical crystals could be used in information processing, as supersensitive biosensors, and more

PASADENA, Calif.—Researchers at the California Institute of Technology (Caltech) have created a nanoscale crystal device that, for the first time, allows scientists to confine both light and sound vibrations in the same tiny space.

"This is a whole new concept," notes Oskar Painter, associate professor of applied physics at Caltech. Painter is the principal investigator on the paper describing the work, which was published in the online edition of the journal Nature. "People have known how to manipulate light, and they've known how to manipulate sound. But they hadn't realized that we can manipulate both at the same time, and that the waves will interact very strongly within this single structure."

Indeed, Painter points out, the interactions between sound and light in this device—dubbed an optomechanical crystal—can result in mechanical vibrations with frequencies as high as tens of gigahertz, or 10 billion cycles per second. Being able to achieve such frequencies, he explains, gives these devices the ability to send large amounts of information, and opens up a wide array of potential applications—everything from lightwave communication systems to biosensors capable of detecting (or weighing) a single macromolecule. It could also, Painter says, be used as a research tool by scientists studying nanomechanics. "These structures would give a mass sensitivity that would rival conventional nanoelectromechanical systems because light in these structures is more sensitive to motion than a conventional electrical system is."

"And all of this," he adds, "can be done on a silicon microchip."

Optomechanical crystals focus on the most basic units—or quanta—of light and sound. (These are called photons and phonons, respectively.) As Painter notes, there has been a rich history of research into both photonic and phononic crystals, which use tiny energy traps called bandgaps to capture quanta of light or sound within their structures.

What hadn't been done before was to put those two types of crystals together and see what they are capable of doing. That is what the Caltech team has done.

"We now have the ability to manipulate sound and light in the same nanoplatform, and are able to interconvert energy between the two systems," says Painter. "And we can engineer these in nearly limitless ways."

The volume in which the light and sound are simultaneously confined is more than 100,000 times smaller than that of a human cell, notes Caltech graduate student Matt Eichenfield, the paper's first author. "This does two things," he says. "First, the interactions of the light and sound get stronger as the volume to which they are confined decreases. Second, the amount of mass that has to move to create the sound wave gets smaller as the volume decreases. We made the volume in which the light and sound live so small that the mass that vibrates to make the sound is about ten times less than a trillionth of a gram."

Eichenfield points out that, in addition to measuring high-frequency sound waves, the team demonstrated that it's actually possible to produce these waves using only light. "We can now convert light waves into microwave-frequency sound waves on the surface of a silicon microchip," he says.

These sound waves, he adds, are analogous to the light waves of a laser. "The way we have designed the system makes it possible to use these sound waves by routing them around on the chip, and making them interact with other on-chip systems. And, of course, we can then detect all these interactions again by using the light. Essentially, optomechanical crystals provide a whole new on-chip architecture in which light can generate, interact with, and detect high-frequency sound waves."

These optomechanical crystals were created as an offshoot of previous work done by Painter and colleagues on a nanoscale "zipper cavity," in which the mechanical properties of light and its interactions with motion were strengthened and enhanced. (That release can be found at http://media.caltech.edu/press_releases/13263.)

Like the zipper cavity, optomechanical crystals trap light; the difference is that the crystals trap—and intensify—sound waves, as well. Similarly, while the zipper cavities worked by funneling the light into the gap between two nanobeams—allowing the researchers to detect the beams' motion relative to one another—optomechanical crystals work on an even tinier scale, trapping both light and sound within a single nanobeam.

"Here we can actually see very small vibrations of sound trapped well inside a single 'string,' using the light trapped inside that string," says Eichenfield. "Importantly, although the method of sensing the motion is very different, we didn't lose the exquisite sensitivity to motion that the zipper had. We were able to keep the sensitivity to motion high while making another huge leap down in mass."

"As a technology, optomechanical crystals provide a platform on which to create planar circuits of sound and light," says Kerry Vahala, the Ted and Ginger Jenkins Professor of Information Science and Technology and professor of applied physics, and coauthor on the Nature paper. "These circuits can include an array of functions for generation, detection, and control. Moreover," he says, "optomechanical crystal structures are fabricated using materials and tools that are similar to those found in the semiconductor and photonics industries. Collectively, this means that phonons have joined photons and electrons as possible ways to manipulate and process information on a chip."

And these information-processing possibilities are well within reach, notes Painter. "It's not one plus one equals two, but one plus one equals ten in terms of what you can do with these things. All of these applications are much closer than they were before."

"This novel approach to bringing both light and sound together and letting them play off of each other exemplifies the forward-thinking work being done by the Engineering and Applied Science (EAS) division," says Ares Rosakis, chair of EAS and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech.

Other authors on the Nature paper, "Optomechanical crystals," include Caltech graduate student Jasper Chan and postdoctoral scholar Ryan Camacho. Funding for their work was provided by a Defense Advanced Research Projects Agency seed grant and by grants from the National Science Foundation.

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Lori Oliwenstein
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Caltech Scientists Solve Decade-Long Mystery of Nanopillar Formations

Research paves way for new 3-D lithography method

Pasadena, Calif.—Scientists at the California Institute of Technology (Caltech) have uncovered the physical mechanism by which arrays of nanoscale (billionths-of-a-meter) pillars can be grown on polymer films with very high precision, in potentially limitless patterns.

This nanofluidic process—developed by Sandra Troian, professor of applied physics, aeronautics, and mechanical engineering at Caltech, and described in a recent article in the journal Physical Review Letters—could someday replace conventional lithographic patterning techniques now used to build three-dimensional nano- and microscale structures for use in optical, photonic, and biofluidic devices.

The fabrication of high-resolution, large-area nanoarrays relies heavily on conventional photolithographic patterning techniques, which involve treatments using ultraviolet light and harsh chemicals that alternately dissolve and etch silicon wafers and other materials. Photolithography is used to fabricate integrated circuits and microelectromechanical devices, for example.

However, the repeated cycles of dissolution and etching cause a significant amount of surface roughness in the nanostructures, ultimately limiting their performance.

"This process is also inherently two-dimensional, and thus three-dimensional structures must be patterned layer by layer," says Troian.

In an effort to reduce cost, processing time, and roughness, researchers have been exploring alternative techniques whereby molten films can be patterned and solidified in situ, and in a single step.

About a decade ago, groups in Germany, China, and the United States encountered a bizarre phenomenon while using techniques involving thermal gradients. When molten polymer nanofilms were inserted within a slender gap separating two silicon wafers that were held at different temperatures, arrays of nanoscale pillars spontaneously developed.

These protrusions grew until they reached the top wafer; the resulting pillars were typically several hundred nanometers high and several microns apart.

These pillars sometimes merged, forming patterns that looked like bicycle chains when viewed from above; in other films, the pillars grew in evenly spaced, honeycomb-like arrays. Once the system was brought back down to room temperature, the structures solidified in place to produce self-organized features.

In 2002, researchers in Germany who had observed this phenomenon hypothesized that the pillars arise from infinitesimal—but very real—pressure fluctuations along the surface of an otherwise quiescent flat film. They proposed that the differences in surface pressure were caused by equally tiny variations in the way individual packets (or quanta) of vibrational energy, known as phonons, reflect from the film interfaces.

"In their model, the difference in acoustic impedance between the air and polymer is believed to generate an imbalance in phonon flux that causes a radiation pressure that destabilizes the film, allowing pillar formation," says Troian. "Their mechanism is the acoustic analogue of the Casimir force, which is quite familiar to physicists working at the nanoscale."

But Troian, who was familiar with thermal effects at small scales—and knew that the propagation of these phonons is actually unlikely in amorphous polymer melts, which lack internal periodic structure—immediately recognized that another mechanism might be lurking in this system.

To determine the actual cause of nanopillar formation, she and Caltech postdoctoral scholar Mathias Dietzel developed a fluid-dynamical model of the same type of thin, molten nanofilm in a thermal gradient.

Their model, Troian says, "exhibited a self-organizing instability that was able to reproduce the strange formations," and showed that nanopillars, in fact, form not via pressure fluctuations but through a simple physical process known as thermocapillary flow.

In capillary flow—or capillary action—the attractive force, or cohesion, between molecules of the same liquid (say, water) produces surface tension, the compressive force that is responsible for holding together a droplet of water. Since surface tension tends to minimize the surface area of a liquid, it often acts as a stabilizing mechanism against deformation caused by other forces. Differences in temperature along a liquid interface, however, generate differences in surface tension. In most liquids, cooler regions will have a higher surface tension than warmer ones—and this imbalance can cause the liquid to flow from warmer- to cooler-temperature regions, a process known as thermocapillary flow.

Previously, Troian has used such forces for microfluidic applications, to move droplets from one point to another.

"You can see this effect very nicely if you move an ice cube in a figure eight beneath a metal sheet coated with a liquid like glycerol," she says. "The liquid wells up above the cube as it traces out the figure. You can draw your name in this way, and, presto! You have got yourself a new form of thermocapillary lithography!"

In their Physical Review Letters paper, Troian and Dietzel showed how this effect can theoretically dominate all other forces at nanoscale dimensions, and also showed that the phenomenon is not peculiar to polymer films.

In the thermal-gradient experiments, they say, the tips of the tiny protrusions in the polymer film experience a slightly colder temperature than the surrounding liquid, because of their proximity to the cooler wafer.

"The surface tension at an evolving tip is just a little bit greater, and this sets up a very strong force oriented parallel to the air/polymer interface, which bootstraps the fluid toward the cooler wafer. The closer the tip gets to the wafer, the colder it becomes, leading to a self-reinforcing instability," Troian explains.

 

Upper: Schematic showing typical experimental setup. Lower: AFM image of 260 nm high nanopillars spaced 3.4 microns apart which formed in a polymer film.
Credit: Upper: Dietzel and Troian/Caltech; PRL. Lower: Chou and Zhuang, J. Vac. Sci. Technol. B 17, 3197 (1999).

Ultimately, she says, "you can end up with very long columnar structures. The only limit to the height of the column, or nanopillar, is the separation distance of the wafers."

In computer models, the researchers were able to use targeted variations in the temperature of the cooler substrate to control precisely the pattern replicated in the nanofilm. In one such model, they created a three-dimensional "nanorelief" of the Caltech logo.

Troian and her colleagues are now beginning experiments in the laboratory in which they hope to fabricate a diverse array of nanoscale optical and photonic elements. "We are shooting for nanostructures with specularly smooth surfaces—as smooth as you could ever make them—and 3-D shapes that are not easily attainable using conventional lithography," Troian says.

"This is an example of how basic understanding of the principles of physics and mechanics can lead to unexpected discoveries which may have far-reaching, practical implications," says Ares Rosakis, chair of the Division of Engineering and Applied Science (EAS) and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech. "This is the real strength of the EAS division."

The work in the paper, "Formation of Nanopillar Arrays in Ultrathin Viscous Films: The Critical Role of Thermocapillary Stresses," was funded by the Engineering Directorate of the National Science Foundation.

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Kathy Svitil
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Two International Leaders Receive Caltech Aerospace Award

Former president of India and France's top space administrator recognized for their achievements in the field

Pasadena, Calif., Sept. 15, 2009- Two distinguished aerospace leaders are the recipients of the 25th annual International von Kármán Wings Award. Receiving the honor this year are Abdul Kalam, the 11th president of India and distinguished professor at the Indian Institute of Technology, and Yannick d'Escatha, chairman and chief executive officer of the Centre National d'Etudes Spatiales (CNES), the agency responsible for shaping France's space policy.

"Along with their tremendous accomplishments in aerospace, this year's honorees are leaders in international collaboration, climate monitoring, and energy harvesting," says Ares J. Rosakis, chair of the Aerospace Historical Society, chair of the Division of Engineering and Applied Science, and Theodore von Kármán Professor of Aeronautics and Mechanical Engineering at Caltech.

This is the 25th year that the International von Kármán Wings Award has been given by the Aerospace Historical Society (AHS), which is now apart of the Graduate Aerospace Laboratories at Caltech (GALCIT). The award has a rich heritage in the preservation of world aerospace history and the recognition of renowned aerospace pioneers and luminaries.

"GALCIT is proud and privileged to be the home of the Aerospace Historical Society," says G. Ravi Ravichandran, director of the Graduate Aerospace Laboratories and the John E. Goode, Jr., Professor of Aeronautics and Mechanical Engineering. "It is an honor to give this award, named after the founding director of GALCIT and the founder of the Jet Propulsion Laboratory, to Abdul Kalam and Yannick d'Escatha."

Rosakis described Adbul Kalam as an "international leader and humanitarian who is honored and admired by the next generation" and Yannick d'Escatha as a "visionary who is using space and technology to bring about collaboration and peace."  

One example of the honorees' collaborative efforts is the Megha-Tropiques weather satellite, a joint project of the Indian Space Research Organization (ISRO) and the French Centre National d'Etudes Spatiales (CNES).

The von Kármán Wings Awards will be handed out September 15 at a banquet on the Caltech campus and will be presented by Rosakis.

Previous recipients of the Wings Award include last year's winner, Northrop Grumman's chief technology officer Alexis Livanos; director of the NASA Jet Propulsion Laboratory Charles Elachi; Kent Kresa, chairman of Caltech's Board of Trustees; TRW cofounder Simon Ramo; aerospace engineer Burt Rutan; and astronaut Buzz Aldrin.

About Abdul Kalam:

Abdul Kalam, the 11th president of India, was born in 1931 in Rameswaram, in the Indian state of Tamil Nadu. He attended the Madras Institute of Technology, specializing in aeronautical engineering. Dr. Kalam was the project director of India's first indigenous Satellite Launch Vehicle (SLV-III), which successfully propelled the Rohini satellite into near-Earth orbit in July 1980 and made India a member of the exclusive "space club."

After working for two decades in the Indian Space Research Organization (ISRO) and mastering launch vehicle technologies, Dr. Kalam took up the responsibility of developing indigenous guided missiles at the Defense Research and Development Organization as the chief executive of the Integrated Guided Missile Development Program. He was responsible for the development and operations of AGNI and PRITHVI missiles and for building indigenous capability in critical technologies through networking with multiple institutions. Dr. Kalam was the scientific advisor to India's defense minister and secretary and boosted the country's self-reliance in defense systems by advancing multiple mission projects, such as the Light Combat Aircraft.

Dr. Kalam became the 11th president of India in July 2002 and served for five years. He led the country in arriving at Technology Vision 2020, giving a road map for transforming India from its present developing status to a developed nation.

Dr. Kalam is a distinguished professor at the Indian Institute of Technology and has also authored a number of books, including Wings of Fire, India 2020:A Vision for the New Millennium and Ignited Minds: Unleashing the Power Within India. These books have been translated into many Indian and foreign languages.

Being one of the most distinguished scientists of India, Dr. Kalam has received honorary doctorates from 36 universities and institutions in India and abroad. The Royal Society of the United Kingdom has awarded to him the King Charles II Medal for Science and Technology.

About Yannick d'Escatha:

Yannick d'Escatha was born in 1948 in Paris, France. He graduated from École des Mines and École Polytechnique, where he became a professor and was the chairman of the board of trustees. He was internationally recognized for his research in solid, structural, and fracture mechanics.

In 1973, he became an expert advisor to the minister of industry on nuclear regulatory and research issues. D'Escatha was the administrator general of the French Atomic Energy Commission (CEA) and chairman of the CEA Industrie Group. At CEA, he promoted astrophysics and global-change research and concentrated on the spin-off potential of the R&D activities.

Ares Rosakis (left) and Abdul Kalam
Credit: Bob Paz

He served as chief operating officer and vice president of Électricité de France from 2000 to 2003.

In 2003, d'Escatha was appointed chairman and chief executive officer of the CNES. He conducted an ambitious policy to restructure the French space agency. In addition, he secured the Ariane 5 launcher system and recently issued a report on future launch systems to the French prime minister. At CNES, he developed research and applications dedicated to global change. He led the CNES Automated Transfer Vehicle (ATV) Control Center to successfully dock the European ATV with the International Space Station.

D'Escatha is an advocate for international space cooperation. He is responsible for creating large partnerships with the top spacefaring nations in Europe and with other international partners. In Europe, he provides leadership for the new European Space Policy by encouraging strong partnership between ESA and European Union institutions. He also provided the road map for the European Space Council during the French presidency of the European Union (2008).

D'Escatha received two distinguished awards from the Académie des Sciences. He is a member of the Académie des Technologies and served on a variety of French Applied Science and Technology Councils. The French Republic awarded him both the Commandeur de l'Ordre National du Mérite and the Officier de la Légion d'Honneur decorations.

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About GALCIT:

The research at the Graduate Aerospace Laboratories of the California Institute of Technology (GALCIT) has evolved over the past three-quarters of a century to include aerospace and biosystems engineering. However, the tradition of integrating basic experiments, theory, and simulations over a broad range of spatial and temporal scales continues to characterize its approach.

GALCIT faculty are highly visible in their fields and continue to garner numerous awards. GALCIT contains unparalleled experimental facilities in solids, fluids, biomechanics, propulsion, combustion, and materials, as well as unique large-scale computational capabilities.

Its educational emphasis is on the fundamentals and advanced diagnostics, with a view toward the future, of biomechanics, biopropulsion, micro and nanomechanics, space science, and space technology. GALCIT takes an interdisciplinary view of mechanics-fluids, solids, and materials-and its graduate training reflects this focus.

About Caltech:

Caltech is recognized for its highly select student body of 900 undergraduates and 1,200 graduate students, and for its outstanding faculty. Since 1923, Caltech faculty and alumni have garnered 32 Nobel Prizes and five Crafoord Prizes.

In addition to its prestigious on-campus research programs, Caltech operates the Jet Propulsion Laboratory (JPL), the W. M. Keck Observatory in Mauna Kea, the Palomar Observatory, and the Laser Interferometer Gravitational-Wave Observatory (LIGO). Caltech is a private university in Pasadena, California. For more information, visit http://www.caltech.edu.

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Caltech and IBM Scientists Use Self-Assembled DNA Scaffolding to Build Tiny Circuit Boards

Nanotechnology advance could lead to smaller, faster, more energy-efficient computer chips

Pasadena, Calif.--Scientists at the California Institute of Technology (Caltech) and IBM's Almaden Research Center have developed a new technique to orient and position self-assembled DNA shapes and patterns--or "DNA origami"--on surfaces that are compatible with today's semiconductor manufacturing equipment. These precisely positioned DNA nanostructures, each no more than one one-thousandth the width of a human hair, can serve as scaffolds or miniature circuit boards for the precise assembly of computer-chip components.

The advance, described in the current issue of the journal Nature Nanotechnology, could allow the semiconductor industry to pack more power and speed into tiny computer chips, while making them more energy efficient and less expensive to manufacture than is possible today.  

DNA origami structures have been heralded as a potential breakthrough for the creation of nanoscale circuits and devices. In a process created by Caltech senior research associate Paul W. K. Rothemund and his colleagues, DNA molecules self-assemble in solution via a reaction between a long single strand of viral DNA and a mixture of different short synthetic DNA strands. These short segments act as staples that effectively fold the viral DNA into desired two-dimensional shapes through complementary base-pair binding.

In this way, DNA nanostructures such as squares, triangles, and stars can be prepared that measure 100 to 150 nanometers on an edge and are as thick as the DNA double helix is wide. 

One roadblock to the use of DNA origami, however, is that the structures are made in saltwater solution--whereas electronic circuits are created on surfaces, like a silicon wafer, so they can be integrated with other technologies.

DNA origami structures also adhere randomly to surfaces, which means that "if you just pour DNA origami over a surface to which they stick, they attach everywhere," explains Rothemund, who jointly led the project with IBM. "It's a little like taking a deck of playing cards and throwing it on the floor; they are scattered willy-nilly all over the place. Such random arrangements of DNA origami are not very useful. If they carry electronic circuits, for example, they are difficult to find and wire up into larger circuits."  

To eliminate these problems, Rothemund and his colleagues at the Almaden Research Center developed a way to precisely position DNA origami nanostructures on a surface, "to line them up like little ducks in a row," Rothemund says. "This knocks down one of the major roadblocks for the use of DNA origami in technology," he adds.

In a process developed by IBM scientists, electron-beam lithography and oxygen plasma etching, conventional semiconductor techniques, are used to make patterns on silicon wafers, creating lithographic templates of the proper size and shape to match those of individual triangular DNA origami structures created by Rothemund. The etched patches are negatively charged, as are DNA origami structures, and are therefore "sticky."

To connect the origami to the templates, magnesium ions are added to the saltwater solution containing the origami. The positively charged magnesium ions can stick to both the DNA origami and the negatively charged patches on the template. Thus, when the solution is poured over the template, a negative-positive-negative "sandwich" is formed, with the magnesium atoms acting as a glue to hold the origami to the sticky patches.

"The triangles bind strongly to the sticky patches, but also they can wiggle a bit, so they line up with the outline of the sticky patch. So not only can we put origami where we want them, but they can be oriented in the direction we want them," Rothemund says.

The positioned DNA nanostructures can then serve as scaffolds or miniature circuit boards for the precise assembly of components such as carbon nanotubes, nanowires, and nanoparticles at dimensions significantly smaller than possible with conventional semiconductor fabrication techniques. This opens up the possibility of creating functional devices that can be integrated into larger structures as well as enabling studies of arrays of nanostructures with known coordinates.

"The spacing between the components can be 6 nanometers, so the resolution of the process is roughly 10 times higher than the process we currently use to make computer chips," Rothemund says. "Then, if you want to design a really small electronic device, say, you just design DNA strands to create the pattern you want, attach little chemical 'fastening posts' to those DNA strands, assemble the pattern, and then assemble the components onto the pattern," he explains.

The process isn't limited to organizing things that are of interest to physical scientists and engineers, like electronic components, Rothemund adds. For example, he says, "Biologists studying how proteins interact can place them in patterns on top of DNA origami. This may be useful in the case of motor proteins, the little machines that power our muscles. They work in gangs, with multiple motors pulling together. To study how different configurations of motors cooperate, scientists may use DNA origami to organize the gangs."

"Rothemund and his colleagues have removed a key barrier to the improvement and advancement of computer chips. They accomplished this through the revolutionary approach of combining the building blocks for life with the building blocks for computing," says Ares Rosakis, Theodore von Kármán Professor of Aeronautics and Mechanical Engineering and chair of Caltech's Division of Engineering and Applied Science.

The paper, "Placement and orientation of individual DNA shapes on lithographically patterned surfaces," was published in the August 16 issue of Nature Nanotechnology. The work was supported by the National Science Foundation and the Focus Center Research Program.

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Shang-Li and Betty Huang create endowment to support Caltech graduate students

PASADENA, Calif.—Alumnus Shang-Li "S.L." Huang (MS '69, PhD '76) and his wife have pledged $1 million to Caltech to endow the Shang-Li and Betty Huang Endowed Graduate Fellowship Fund in Mechanical Engineering. The gift will be matched by an additional $500,000 from the Gordon and Betty Moore Matching Program.

"This generous gift will assist us in continuing to attract engineering graduate students with exceptional talent and ability. S.L. and Betty have changed lives far into the future with their support," says Ares Rosakis, chair of Caltech's Division of Engineering and Applied Science and the Theodore von Kármán Professor of Aeronautics and Mechanical Engineering.

"S.L. was my graduate student and did an outstanding PhD thesis back in the 1970s—a thesis whose results are still widely used in the rocket-engine design business," adds Chris Brennen, the Richard L. and Dorothy M. Hayman Professor of Mechanical Engineering at Caltech. "He and Betty are deeply interested in education—in particular, graduate education. They have been instrumental in rallying support for mechanical engineering at Caltech. We are most grateful for their generous help and advice."

For his part, S.L. believes that graduate school at Caltech shaped his life, laying a foundation for his success in areas beyond engineering and research.  Some of his deepest friendships began when he and a few other students and professors would surface from their labs to share meals, establishing a club they called the Meat and Wine Society. More than three decades later, they still meet. S.L. has also stayed involved with Caltech in more formal ways. He and Betty became life members of the Caltech Associates, have hosted Associates gatherings in Orange County, and have participated in Associates events ranging from dinners with faculty to a tour of Paris, Burgundy, and Lyon. S.L. serves on the board of the Associates and is a life member of the Caltech Alumni Association. He also serves on a special advisory committee for mechanical engineering at Caltech.

The Huangs' gift will make it possible for hundreds of future graduate students to study and live at Caltech, and, perhaps, to form meat and wine societies of their own.

"Betty and I are so glad to have this opportunity to endow this graduate fellowship fund," says S.L. "We have always believed that education is one of the most important tools that we can provide to our next generation. We have only done a small part, and we sincerely hope others will come forward and participate in creating additional endowments to benefit Caltech undergraduates and graduate students—especially in these difficult times."
 
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 About Caltech:
Caltech is recognized for its highly select student body of 900 undergraduates and 1,200 graduate students, and for its outstanding faculty. Since 1923, Caltech faculty and alumni have garnered 32 Nobel Prizes and five Crafoord Prizes. In addition to its prestigious on-campus research programs, Caltech operates the Jet Propulsion Laboratory (JPL), the W. M. Keck Observatory in Mauna Kea, the Palomar Observatory, and the Laser Interferometer Gravitational-Wave Observatory (LIGO). Caltech is a private university in Pasadena, California. For more information, visit http://www.caltech.edu.

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Jon Weiner
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Caltech Researchers Link Tiny Sea Creatures to Large-scale Ocean Mixing

Mechanism, which focuses on impact of drag and water viscosity, shows how swimming animals affect the ocean environment

PASADENA, Calif. —Using a combination of theoretical modeling, energy calculations, and field observations, researchers from the California Institute of Technology (Caltech) have for the first time described a mechanism that explains how some of the ocean's tiniest swimming animals can have a huge impact on large-scale ocean mixing.

Their findings are being published in the July 30 issue of the journal Nature.

"We've been studying swimming animals for quite some time," says John Dabiri, a Caltech assistant professor of aeronautics and bioengineering who, along with Caltech graduate student Kakani Katija, discovered the new mechanism. "The perspective we usually take is that of how the ocean—by its currents, temperature, and chemistry—is affecting the animals. But there have been increasing suggestions that the inverse is also important-how the animals themselves, via swimming, might impact the ocean environment."

Specifically, Dabiri says, scientists have increasingly been thinking about how and whether the animals in the ocean might play a role in larger-scale ocean mixing, the process by which various layers of water interact with one another to distribute heat, nutrients, and gasses throughout the oceans.

Dabiri notes that oceanographers have previously dismissed the idea that animals might have a significant effect on ocean mixing, saying that the viscosity of water would damp out any turbulence created, especially by small planktonic animals. "They said that there was no mechanism by which these animals could impact large-scale ocean mixing," he notes.

But Dabiri and Katija thought there might be a mechanism that had been overlooked—a mechanism they call Darwinian mixing, because it was first discovered and described by Charles Darwin. (No, not that Darwin; his grandson.)

"Darwin's grandson discovered a mechanism for mixing similar in principle to the idea of drafting in aerodynamics," Dabiri explains. "In this mechanism, an individual organism literally drags the surrounding water with it as it goes."

Using this idea as their basis, Dabiri and Katija did some mathematical simulations of what might happen if you had many small animals all moving at more or less the same time, in the same direction. After all, each day, billions of tiny krill and copepods migrate hundreds of meters from the depths of the ocean toward the surface. Darwin's mechanism would suggest that they drag some of the colder, heavier bottom water up with them toward the warmer, lighter water at the top. This would create instability, and eventually, the water would flip, mixing itself as it went.

What the Caltech researchers also found was that the water's viscosity enhances Darwin's mechanism and that the effects are magnified when you're dealing with such minuscule creatures as krill and copepods. "It's like a human swimming through honey," Dabiri explains. "What happens is that even more fluid ends up being carried up with a copepod, relatively speaking, than would be carried up by a whale."

"This research is truly reflective of the type of exciting, without-boundaries research at which Caltech engineering professors excel—in this case a deep analysis of the movement of fluid surrounding tiny ocean creatures leading to completely revelatory insights on possible mechanisms of global ocean mixing," says Ares Rosakis, chair of the Division of Engineering and Applied Science at Caltech.

To verify the findings from their simulations, Katija and collaborators Monty Graham (from the Dauphin Island Sea Laboratory), Jack Costello (from Providence College), and Mike Dawson (from the University of California, Merced) traveled to the island of Palau, where they studied this animal-led transport of water—otherwise known as induced drift—among jellyfish, which are the focus of much of Dabiri's work.

"From a fluid mechanics perspective, this study had less to do with the fact that they're jellyfish, and more to do with the fact that they're solid objects moving through water," Dabiri explains.

Katija's jellyfish experiments involved putting fluorescent dye in the water in front of the sea creatures, and then watching what happened to that dye—or, to be more specific, to the water that took up the dye—as the jellyfish swam. And, indeed, rather than being left behind the jellyfish—or being dissipated in turbulent eddies—the dye travelled right along with the swimming creatures, following them for long distances.

These findings verified that, yes, swimming animals are capable of carrying bottom water with them as they migrate upward, and that movement indeed creates an inversion that results in ocean mixing. But what the findings didn't address was just how much of an impact this type of ocean mixing—performed by impossibly tiny sea creatures—could have on a large scale.

After a series of calculations, Dabiri and Katija were able to estimate the impact of this so-called biogenic ocean mixing. And, Dabiri says, it's quite a significant impact.

"There are enough of these animals in the ocean," he notes, "that, on the whole, the global power input from this process is as much as a trillion watts of energy—comparable to that of wind forcing and tidal forcing."

In other words, the amount of power that copepods and krill put into ocean mixing is on the same scale as that of winds and tides, and thus their impact is expected to be on a similar scale as well.

And while these numbers are just estimates, Dabiri says, they are likely to be conservative estimates, having been "based on the fluid transport induced by individual animals swimming in isolation." In the ocean, these individual contributions to fluid transport may actually interact with one another, and amplify how far the ocean waters can be pulled upward.

In addition, says Dabiri, they have yet to consider the effects of such things as fecal pellets and marine snow (falling organic debris), which no doubt pull surface water with them as they drift downward. "This may have an impact on carbon sequestration on the ocean floor," says Dabiri. "It's something we need to look at in the future."

Dabiri says the next major question to answer is how these effects can be incorporated into computer models of the global ocean circulation. Such models are important for simulations of global climate-change scenarios.

The work described in the Nature paper, "A viscosity-enhanced mechanism for biogenic ocean mixing," was supported by grants from the National Science Foundation's Biological Oceanography, Ocean Technology, Fluid Dynamics, and Energy for Sustainability programs, and by the Office of Naval Research, the Department of Defense's National Science and Engineering Graduate Fellowship, and the Charles Lee Powell Foundation.

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