The Buttefly Solution

Everywhere Ioannis Miaoulis looks, he sees life's answers to technology's problems--they're in the innocent beat of a butterfly's wing, in the gentle undulation of a sea anemone.

By David H. Freedman|Tuesday, April 01, 1997
For sheer entertainment value, it’s hard to beat Ioannis Miaoulis’s comparative biology lab at Tufts University in Medford, Massachusetts. Between classes, students routinely line up outside the lab’s corridor window to watch a) a sea anemone riding back and forth on a miniature train at the bottom of a tank, b) tropical fish swimming furiously just to stand still in an artificial current, and c) (dead) butterflies wired to electrodes slowly roasting under a lamp.

These attractions, it must be noted, are research, and research that has begun to pay off handsomely. Recently Miaoulis and his students have discovered that the ultrathin layered construction of a butterfly’s wings--the same construction that makes some butterflies shimmer colorfully via the phenomenon of iridescence--also enables the insect to neatly control how much heat energy is absorbed by its wings and how much is reflected away.

Admittedly, that discovery is of only modest interest to lepidopterists. Miaoulis, however, isn’t a lepidopterist. He isn’t even formally a biologist; he is the dean of Tufts’s college of engineering. But then, his interest in butterflies isn’t a biological concern. As it happens, the workings of a butterfly’s wings are helping solve a problem that has vexed materials scientists for years: how to heat microelectronics chips evenly enough during production to avoid melting parts of their thin- film surfaces. Likewise, all the other lab denizens--the anemones, the beetles, the crabs, the fish--are unwittingly toiling in the potential service of technology. Miaoulis is out to prove that engineering and biology are in some ways not all that different. The tools of conventional engineering, he says, can help us better understand biology; and those biological insights, in turn, can lead to better technological designs. Animals have had millions and millions of years to deal with different challenges, he explains. That means some of the very difficult problems we face in technology have already been solved by evolution.

Most researchers would see little in common between bugs (of the literal variety) and chips. But the 35-year-old Miaoulis is extraordinarily well equipped to make such serendipitous connections. He is a sort of scientific gigolo--a man who never met a specialty he didn’t love and who has achieved fluency in subjects as diverse as economics, optics, and materials science. As a teenager, snorkeling in the warm waters of the Bay of Corinth in his native Greece, Miaoulis developed a downright slavish devotion to marine biology. It was a pure love, and it lasted right up to the day he flunked physics in tenth grade. Miaoulis responded to this failure by henceforth throwing himself into the subject. He even took on an extra-credit physics project, in which he designed and built a miniature solar-powered desalinator. As sunlight shone through a two-foot-tall glass pyramid, it evaporated a cup of salt water, leaving the salt behind; pure water condensed on the inside of the plastic walls and ran down gutters into collecting cups. I had never had the experience of studying something and then being able to go out and apply it by building something, says Miaoulis. It was a very exciting feeling. By his senior year in high school, he had dedicated himself to a life of design engineering. I was leaning toward aeronautical engineering, he says, because it impressed girls at parties.

Once he got to Tufts, the aeronautical spin lasted for about a week, until his adviser, Lloyd Trefethen, explained his own specialty: fluid mechanics, the study of the way liquids behave under the influence of various forces. Trefethen showed Miaoulis a slow-motion film of a plunging water droplet. You’d think it would be teardrop-shaped, wouldn’t you? says Miaoulis. But it’s spherical. Isn’t that surprising? Soon Miaoulis was spending every spare minute on fluid-related engineering projects. With one professor, he designed and constructed a device that used water and chemicals to capture and store heat energy from the sun--eventually arousing the interest of the U.S. Army, which wanted to keep its diesel engines warm in cold weather so they’d be easier to start. He worked with another professor on the problem of measuring the flow of liquid through a tube. It’s a tricky dilemma, it turns out, because if you use rotating wheels as a yardstick, some portion of the liquid tends to slip by without turning the wheels. Miaoulis solved the problem by placing a metal screen in the flow, which imparts static electricity to the screen. Then he simply measured the charge accumulated.

After blowing through Tufts in three years, he hopped three miles over to Cambridge to pursue a master’s in engineering at mit. His adviser there happened to focus on the ways in which heat moves through a thin film. Within a month, so did Miaoulis. It was a natural transition from fluid mechanics, he felt, since heat often flows according to the same laws that govern the flow of a liquid. Miaoulis picked up his master’s in a year, at which point Tufts, having decided this was one engineering prodigy it didn’t want to surrender to mit, wooed him back for his Ph.D. work. During the two and a half years it took him to finish his doctorate, Miaoulis also grabbed a master’s in economics. He had acquired 11 years’ worth of degrees in little more than six years.

Tufts hired miaoulis onto its faculty immediately upon his graduation in 1987, at the age of 26. He quickly set to work putting together a materials-processing thermodynamics lab. Attending conferences on materials processing, Miaoulis was struck by a single theme that seemed to run through all the meetings. In session after session, materials- processing researchers were complaining about problems caused by heat, he recalls. I realized that the people who did materials processing weren’t studying the work done by heat transfer researchers, and heat transfer people weren’t applying their work to materials processing.

This observation was confirmed a few weeks later, when a local manufacturer of optical fiber, those long strands of glass that carry light signals in computer and communications networks, called Miaoulis to help troubleshoot one of its machines that was on the blink. As he stood in front of the machine, watching it heat up the glass, pull it like taffy into long strands, and let the strands cool into optical fibers, the engineers told him all they knew: that one day the contraption inexplicably started producing substandard fibers. Suddenly, Miaoulis turned his back on them and peered around the factory floor. Not far away he saw an open doorway. Had that door always been open? he asked. Only since a new furnace had been installed in an adjoining room a few weeks ago, he was told. There’s your problem, he declared; a slight breeze through the doorway was causing one side of the fibers to cool faster than the other, distorting the fibers. He left the factory with a commitment for a research grant of $25,000.

The industry most desperate for a heat-related breakthrough in materials processing, though, was the semiconductor business. Computer chips are made en masse from wafers of silicon several inches in diameter, and a thin film of silicon oxide, a mere hundred-thousandth of an inch thick, covers each wafer surface. The film is then etched with the tiny bumps and gullies that make up the detailed circuitry of a chip--the transistors and so forth that provide the chip’s functionality by directing a stream of electrons one way or another. Before the final step--when the wafer is cut into 50 or so chips--most wafers have to be heated to extremely high temperatures during some stage of their production. In certain cases this comes during the process of laying down a coat of hot metallic vapor on the chip’s surface to produce an extremely thin film; in other cases, heat is needed to melt a layer of silicon sandwiched in the wafer so that it can smoothly freeze into crystalline form with the desired electrical properties. But for some reason never quite understood by the materials-processing engineers who develop chip-making techniques, many wafers exposed to high temperatures refuse to heat evenly. Instead, they develop acute hot spots, tiny regions that get much hotter than the rest of the wafer. They then warp or crack, drastically reducing the number of chips the wafer yields.

Miaoulis, in thinking about the problem, found himself baffled as well. The appearance of hot spots seemed related to the thickness of the film, but in a bizarre way. A thicker film should in theory be better insulated against hot spots, but in the lab the spots would often get markedly worse when the film’s thickness was increased--sometimes by as little as a few millionths of an inch. Miaoulis decided hot spots simply couldn’t be explained by conventional heat transfer processes. There had to be something funky going on here, he says. But what that something might be didn’t strike him until a couple of years later, when he visited former Tufts student Paul Zavracky, a specialist in optics who had gone on to head research at a local chip maker.

Physicists who study optics--how light rays are reflected, bent, or absorbed--are well acquainted with the ways in which light bouncing off a thin film can interfere with itself. A light ray that runs into a thin film has two chances to be reflected: the first is when it hits the top of the film; if it isn’t reflected there, it can pass through and be reflected by the bottom of the film, having traveled, by the time it reaches the surface again, an extra distance of twice the film’s thickness.

But light is a wave, with alternating peaks and valleys; the distance between two peaks in a light wave is, by definition, the light’s wavelength. If a thin film happens to be, say, half as thick as the light’s wavelength, then a light ray that bounces off the bottom of the film ends up traveling one wavelength farther than a ray that bounces directly off the top layer. When these two rays join up again after bouncing off the film, their peaks and valleys will still line up neatly, and the waves will reinforce each other, creating relatively bright light. If the film is three-quarters as thick as a wavelength, however, then the ray reflecting off the bottom travels 1.5 extra wavelengths, and its peaks will end up aligning with a top-reflecting wave’s valleys, and vice versa, with the result that the two waves will largely cancel each other out and create weak light.

The picture gets complicated when different colors of light--that is, light with different wavelengths--are involved, and even more so when the light is bouncing off the film at different angles. But the bottom line is this: light reflecting off a film whose thickness is in the ballpark of the light’s average wavelength tends to break into different brightnesses and colors as waves bouncing off the bottom of the film variously reinforce or cancel out waves bouncing off the top. This is iridescence, and it is what causes thin oil slicks floating on puddles of water to take on complex swirls of rainbow colors, even though there is no pigment in the oil.

While listening to Zavracky talk about optics, Miaoulis had a thought: Could the hot spots on chips be an optical effect? Heat, after all, though invisible, can radiate light--infrared light, to be exact, with a wavelength some ten times longer than that of visible light. Was it possible that heat radiation during chip production was reflecting off the top and bottom of the chip’s thin-film coating and then canceling out in some spots and reinforcing in others? It might make sense, if the thickness of the chip’s film was by coincidence roughly equal to the wavelength of heat radiation; in that case, even the microscopic bumps and gullies on the film’s surface would make a fantastically tiny but all-important difference in thickness. Some quick back-of-the-envelope calculations informed Miaoulis that this was indeed typically the case. He became convinced they had identified the source of the problem. No one had thought about applying optical interference effects to heat, says Miaoulis. But of course it works exactly the same way as it does with visible light.

Experiments in the thin-film lab confirmed the theory. Miaoulis picked up a Presidential Young Investigator Award--he was 30--and he suddenly became a hot item on the materials processing lecture circuit. Too bad the only advice he could offer was obvious: Avoid making thin films the thickness of the wavelength of infrared light. Unfortunately, given the characteristics of thin films and the heat etching process, this excellent prescription isn’t always practical. Miaoulis suspected there might be a way to produce thin films that were less vulnerable to melting, even when exposed to heat of the forbidden wavelength, but he couldn’t find anything in the fields of materials processing, heat transfer, or optics to suggest what it might be.

In 1991, however, he was invited to present his thin-film heat interference work at a conference in Sydney, Australia. Miaoulis got the speechifying out of the way, then took off for Heron Island in the Great Barrier Reef, one of the world’s more spectacular snorkeling spots. Drifting languidly among the coral and tropical fish, hearing only the sounds of exotic birds overhead and of the air rushing in and out of his snorkel, Miaoulis couldn’t help but think . . . what an interesting engineering problem he saw staring up at him from the ocean floor.

The object of his attention was a sea anemone. These fist-size pinkish blobs have little locomotive ability; mostly they sit on the ocean floor and let the underwater currents carry plankton and the like into their open maws. Obviously their ideal strategy, foodwise, is to present the largest possible bodily cross section to the current so that the maximum amount of water--and hence food--will sweep into them. Except that the anemone needs to avoid being swept away by the current, which means that the ideal strategy, stability-wise, is to show the smallest possible cross section to the current. Apparently, evolution has enabled the anemone to meet both needs--enough cross section to bring in an adequate amount of food, but a sufficiently streamlined shape so that the animal isn’t swept away. It’s like holding your hand out the window of a speeding car on a rainy day: if you cup your hand toward the wind, you catch a lot of rain but the force of the wind is enormous; if you make a fist, you get hit by almost as much water but the wind fairly slips off.

The more Miaoulis thought about it, the more intriguing the anemone’s situation seemed. Nature, apparently, was capable of solving a thorny challenge of exactly the sort that engineers faced in the lab every day. The anemone was facing a straightforward optimization problem, he explains. It had to solve for two equations simultaneously--one describing its stability against the current, the other its volume of food.

Upon his return to Medford, Miaoulis dug up some research on anemones. Sure enough, at least one marine biologist had studied the question of how an anemone’s shape copes with its conflicting cross- sectional needs. But the results weren’t thorough enough for Miaoulis’s taste. They didn’t provide a detailed analysis of the water flow around the anemone, nor of how the anemone might dynamically alter its shape as water rushed past it. In short, no one had brought an engineer’s perspective to the problem.

Miaoulis resolved to fill in the blanks himself. To do so, he’d need to construct a device that would duplicate the flow of water back and forth over the anemone. In fact, marine biologists have a standard tool for doing just that: a wave tank, in which motorized pumps at each end create an adjustable flow through the tank. But when Miaoulis saw the price tag for such a tank--about $15,000 for a decent small one--he blanched. Last time he had checked, no one was standing in line to finance thin-film engineers who wanted to study marine creatures.

Well, fine. When engineers are faced with a prohibitively expensive experiment, they turn to a standard tool of their own: simulation. After all, aeronautical engineers don’t launch prototypes of new jet designs through the air to measure their drag; they put them in a wind tunnel and let the air rush over the stationary jets. Physics doesn’t care who stands still and who moves. It’s the relative motion that counts. In that same spirit, Miaoulis decided he could save money by building an anemone train--a small cart pulled by a cheap electric motor that towed the anemone back and forth across the bottom of a plain old aquarium at the same rate as the current created by a wave. The cost: $300.

It worked. (Well, there was one glitch. Every so often, for reasons no one has quite understood, an anemone would slowly roll itself over the front edge of the train, instantly if inadvertently converting the train into a sushi cart. To stem the carnage, Miaoulis had his students install an emergency stop button--a button to which many an anemone now owes its life.) Using both real anemones and pliable fake anemones to test different shapes, Miaoulis was able to confirm that evolution really had found the ideal size and shape for a creature that had to have as much water as possible pass through it without being itself carried away. He and his students presented a well-received paper on their findings at a biology conference.

Buoyed by his success with the anemone, Miaoulis started looking at the aquadynamics of other sea creatures. He constructed a water version of a wind tunnel and put fish, crabs, starfish, mussels, and sea urchins in it so that they could swim and scuttle and creep in place, making observation easy. To make use of his primary expertise, he started studying how the various animals warm and cool themselves by transferring heat between their bodies and the surrounding water.

Soon Miaoulis had four large tanks in what was quickly becoming a genuine bio lab: one for creatures from the warm Caribbean, one for those of the colder Massachusetts waters, one for Pacific organisms, and one for freshwater animals. Arrayed in ever-growing variegation around the tank was a garden of probes, sensors, meters, video cameras, and computer equipment, all acquired or built on the cheap with an engineer’s resourcefulness and demand for precision. When engineering students started to drop by the lab for a little relief from the equations, Miaoulis had half the lab wall abutting the corridor knocked down and replaced with a long window. Video monitors near the window provided gawkers with close-up views of experiments in progress. The whole building came by to watch barnacles feed on video microscope, he beams. Normally barnacles extend their arm and scoop food in, but in a current they just hold it up and catch the food as it comes by.

What did all this have to do with his job as a professor of mechanical engineering? Perhaps it was simply a matter of broadening engineering students’ perspectives. Or maybe the point was to contribute his engineer’s eye and logic to biology. But the truth was, in the back of his mind, Miaoulis was hoping for a reciprocal payoff--that somehow nature’s ingenuity in solving its children’s problems via evolution would cast light on the problems he faced as a mechanical engineer. I believe that if you do interesting research, you will find interesting applications, he says. Sometimes it’s just a matter of posing the right question. Posing questions is harder than finding answers.

It was a couple more years before the right question occurred to him. In 1994, while sitting around the Tufts cafeteria, he thought: Did any animal take advantage of the eccentric heat absorption properties of thin films? Imagine, he mused, a creature that had evolved a skin of some sort whose thickness was tuned to reflect the warmth of the sun--while perhaps a close cousin inhabiting cooler climes might have an almost identical skin mere millionths of an inch thicker or thinner, tuned instead to absorb that warmth.

It was such a clever solution to the problem of heating control, declared Miaoulis suddenly, that nature must already have hit upon it. But what might this thin-film-equipped animal be? Thinking about the question as he walked out of the cafeteria, Miaoulis looked around and noticed that it was butterfly season--the campus was swarming with them. One butterfly alighted on a large rock next to the path where he was standing and basked motionless in the warmth of the sun, but only after taking a moment to adjust its wings. Its brightly hued wings. Its brightly hued, and really, really thin wings.

A large plexiglas terrarium holds some 15 butterflies that flit about or bask on leaves without a care. The insects would be a little less blasé if they were able to recognize that nutty and slightly acrid smell wafting through the lab for what it is: roasting butterfly wings.

Manning the heat lamp today is Barnas Monteith, a tall and somewhat scraggly undergraduate who actually captured most of the butterflies single-handedly in the nearby woods of Blue Hills. Actually, one of them is a moth, he mumbles sheepishly, though he is quick to add that a couple of his legitimate butterfly specimens are fairly rare in Massachusetts.

The deceased butterfly getting the heat treatment today is mounted on a thin pole and has six electrodes glued to its shimmering green and aquamarine wings. Most of the color virtually disappears when the lamp is turned off, even though the room is still well lit. That’s because the colors are caused not by pigment but by iridescence. Butterfly wings are of multilayered thin-film construction--alternating layers of air and scales composed of a substance, common to insects, known as chitin--and individual chitin scales are on the order of a millionth of an inch thick, approximately the same thicknesses as the wavelengths of the various colors of light they reflect.

It has long been known to biologists that the iridescent colors are integral to a butterfly’s ability to attract mates and camouflage itself from predators (or in the case of some butterflies, to loudly advertise the threat of toxicity). What wasn’t known, however, is that the multiple layers of film in the wing make up a thicker (though still microscopically thin) film that enables the butterfly to absorb infrared light--that is, the radiation given off by hot objects--through interference effects. An unlayered chunk of chitin absorbs about 4 percent of the heat hitting it. The chitin-and-air scales that make up a butterfly wing can absorb as much as 96 percent.

Miaoulis isn’t sure how many butterflies he and his colleagues have had to roast to find this out. Maybe a few hundred. They’ve also heated imitation wings and even spray-coated real ones with aluminum to compare how substances of different reflectivities absorb heat. They’ve applied to butterfly wings the sensors and computer programs they developed for analyzing semiconductor thin films--chiefly to figure out how the butterfly can so radically vary the amount of heat its wings absorb merely by changing the angle at which they face the sun. They’ve measured the air temperature at different spots around the butterfly, to calculate the rate at which heat can be transferred from the wings, where it is captured, to the body, where it is needed. All this research has led to conclusive results: Thin-film absorption is the main way butterflies warm themselves by the sun, says Miaoulis. Their wings are little solar collectors. To turn up the heat, butterflies need only unfold their wings and angle them at the sun.

But the most exciting observation from Miaoulis’s point of view was one that no biologist would have noted. Even under intense heat, large areas of the wings always cook evenly. No hot spots. Yet the layers of butterfly wings aren’t uniformly thick. Indeed, they’re downright rough compared with man-made thin films. The chitin scales are a bit like hardened powder in texture; under a microscope each scale looks like a mountain range. And yet this variegated film boasts interference properties similar to those of the smoothest semiconductor film. We’ve always assumed that films should be smooth, Miaoulis explains. The macroscale experiences we have when we’re young convince us that flat things work better: a smoothly sanded table holds a glass better than one made out of rough-cut wood and doesn’t give us any splinters. The result is that we’ve developed an engineering instinct that equates flatness with functional and aesthetic superiority, and we’ve carried that instinct right into microelectronics.

The reason our instincts may be failing us when it comes to thin films, he contends, is that we have no intuitive grasp of the microscale effects of heat interference. Nature, on the other hand, doesn’t rely on intuition or instinct; it simply tries everything, by trial and error, eon after eon, and ends up with what works. Miaoulis believes his butterflies are trying to tell him something--namely, that rough films may work better than smooth films for manufacturing semiconductor chips. I always trust time, Miaoulis says. And time seems to have rejected flat films and optimized on rough ones.

How does roughness confer resistance to hot spots? Miaoulis believes that the butterfly’s roughness--a random and ubiquitous variation in scale thickness averaging about the wavelength of heat radiation--serves to break up some of the interference effects caused by any other larger, isolated variation in thickness. Think of it as static during a radio broadcast that makes it hard to discern a sudden and brief break in an opera singer’s libretto. Or, to use a closer analogy, consider how disruptive to a reflected image is a fist-size hole in a full-length mirror--but how the same hole might seem less distracting in a mirror made up of thousands of shards of mirrored glass glued together.

Miaoulis and his students are now trying to figure out how to make a rough-filmed semiconductor with all the properties of conventional chips but without the sensitivity to microscale variations in thickness. In theory, making a rough film should be far easier than making a smooth one, in the same way that anyone who can lay a neat, glossy coat of paint on a wall shouldn’t have trouble splattering the wall with paint. The real challenge, notes Miaoulis, is in etching in the transistors and other functional features. It may be easier to build a rough table than a smooth one, he points out, but it’s easier to carve neat designs into the smooth one.

Miaoulis hasn’t yet managed to build any chips with more-heat- resistant thin films, but if he succeeds in doing so, it could open the door to processing techniques that can’t even be considered now. The ability to apply higher temperatures without damaging the film would make it easier, for example, to melt and crystallize layers of materials sandwiched below the film. That in turn would pave the way for cheaper and more reliable films and ones that have brand-new properties. Not only would it be possible to make better semiconductor chips, but it would also be a shot in the arm to other technologies that use thin films, such as photographic materials, which employ thin-film coatings, and electrochromic windows, which have thin-film layers that shut out light when exposed to a small electric current.

The butterfly may yet provide other insights. Researchers in the semiconductor industry are eager to build multilayered chips, which could store electric charges in the empty space between the semiconductor layers. Some of the designers of these proposed new chips have turned to Miaoulis for help. We’re investigating their thermal properties now, says Miaoulis. We think we can improve the designs by applying some of the insights we’ve gained into how the butterfly’s multilayered structure handles heat. Such improvements might include adjusting the spacing between the layers, he notes, or imitating the structure of the chitin beams that run between layers to keep them separate and rigid.

Miaoulis is convinced that other work he’s doing in the bio lab will eventually pay off as well. Who knows? Maybe some day there’ll be a spacecraft shaped like an anemone that can gather large atmospheric samples without creating too much drag. Or underwater habitats covered with a tough, insulating artificial skin modeled after that of a starfish. Or a jacket that, at the push of a button, will absorb greater or lesser amounts of heat from the sun, just as some beetles apparently do by changing the thickness of their thin-film exoskeletons.

Although Miaoulis is optimistic about technological payoffs, he is quick to concede that the benefits are not likely to flow evenly in both directions. We’ll probably have more breakthroughs in helping biologists explain phenomena than in developing new devices, he says.

He doesn’t mean it in a defeatist way; he likes contributing to biology. And why shouldn’t he? Even to a gigolo, the first love is always special.
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