# News this Week

Science  14 Mar 2014:
Vol. 343, Issue 6176, pp. 1182
1. # Around the World

1 - Juba
Cholera Vaccine Stash Deployed
2 - Cheonan, South Korea
Avian Flu Claims Research Flock
3 - Geneva, Switzerland
WHO Proposes Halving Recommended Sugar Intake
4 - Washington, D.C.
House Science Panel Takes Up Controversial NSF Bill

## Juba

### Cholera Vaccine Stash Deployed

The World Health Organization (WHO) is putting its new stockpile of oral cholera vaccines to use for the first time. Some 140,000 people in refugee camps in wartorn South Sudan are expected to receive the vaccine by the end of March. There is no cholera in South Sudan at the moment, but the impoverished country has been considered at risk since fighting broke out between the government and opposition forces in December. The vaccines will be administered by Doctors Without Borders.

Just how useful cholera vaccines are has been hotly debated; some have argued they are impractical and distract from the need to provide clean water and sanitation (Science, 17 August 2012, p. 785). But in 2012, WHO decided to assemble a 2-million-dose stockpile for use in emergency situations. The vaccine will first be offered in relatively secure refugee camps in Juba, the capital, and in the centrally located Lakes state, but the program may be expanded to other areas later.

## Cheonan, South Korea

### Avian Flu Claims Research Flock

Korea's premier poultry research center was forced to cull its 11,000 hens and 5000 ducks last week after an outbreak of the deadly new avian influenza strain H5N8. Researchers at the National Institute of Animal Science (NIAS) made every effort to keep the Cheonan campus, 85 kilometers south of Seoul, virus-free. But on 3 March, 30 dead ducks were confirmed to be infected, and staff immediately culled the rest. Researchers predict that it will take nearly 2 years to reconstitute the flocks—used to study breed improvement and husbandry techniques—from birds kept at other facilities.

The incident highlights the difficulty of protecting poultry farms from circulating avian influenza. Officials are investigating three possible routes the virus could have taken onto campus: wild birds, NIAS vehicles, and supply deliveries, said Lee Jun-Won, deputy agriculture minister, at a press conference, where he promised "to hold those responsible accountable."

Since the strain emerged in central South Korea in January, it has spread virtually nationwide and outbreaks are still being reported. There have been no reports of human infections. http://scim.ag/_H5N8

## Geneva, Switzerland

### WHO Proposes Halving Recommended Sugar Intake

The World Health Organization (WHO) last week released draft guidelines that halve the maximum amount of sugar it recommends people consume. The current recommendation states that sugars should make up less than 10% of daily energy intake. That includes both sugar that manufacturers or consumers add to food and sugar naturally present in honey or fruit juices. The draft suggests a further reduction to less than 5% of total energy—about 25 grams per day for an adult with a normal body mass index. But it calls this a "conditional recommendation," and says that there is "a need for substantial debate and involvement of stakeholders before this recommendation can be adopted as policy."

WHO will accept comments on the draft until 31 March, and many food companies are likely to voice strong opposition. (A single can of sugar-sweetened soda would exceed the daily recommended intake.) "If pressure comes to the organization, we are very well equipped to resist that type of pressure," said Francesco Branca, WHO's director of nutrition for health and development, at a press conference.

## Washington, D.C.

### House Science Panel Takes Up Controversial NSF Bill

A highly partisan bill affecting research and education programs at the National Science Foundation (NSF) and other federal agencies is advancing in the U.S. House of Representatives. Introduced this week by Representative Lamar Smith (R–TX), who chairs the House science committee, the legislation would alter NSF's peer-review process, reduce funding for social science research, and create a new office within NSF to oversee federal science education programs. "To remain globally competitive, we need to make sure our priorities are funded and that taxpayer dollars are spent wisely," Smith said in a statement before his panel took up the measure, titled the Frontiers in Innovation, Research, Science, and Technology (FIRST) Act.

Science lobbyists and Democrats have sharply criticized the bill, saying its authorized spending levels are too low and its approach to managing research unwise. "[T]he bill does little to close this nation's innovation deficit, [and] it also does some things to widen it," said the Association of American Universities, a group of 62 research-intensive institutions. http://scim.ag/_FIRST

2. # Random Sample

## Noted

Serbian government officials prompted scientific outrage with plans to relocate Nikola Tesla's ashes from a museum in Belgrade to a burial site at the city's Church of Saint Sava. Critics of the move, planned for July, say the prolific inventor was not a believer and that his remains belong at the Nikola Tesla Museum—their home since 1957—rather than the world's largest Orthodox church. The Facebook page "Leave Tesla Alone" has garnered more than 37,000 "likes."

## They Said It

"I already knew I wanted to become a scientist, but that afternoon I learned from Carl the kind of person I wanted to become."

—Astrophysicist and science communicator Neil deGrasse Tyson, on his first meeting at age 17 with Carl Sagan, during the debut episode of the 13-part update to Sagan's classic 1980 documentary series Cosmos.

4. # Talking Back to Madness

1. Michael Balter

As the search for genes and new drugs for schizophrenia stalls, psychotherapies are getting new attention.

NEW YORK CITY AND NEWCASTLE, U.K.—Terry was 13, a lonely African-American boy growing up in a troubled home in Detroit, when he first heard the voices. They were ugly and mean. The voices said he was no good, that no one loved him, and that he should kill himself. So he tried his best: When he was 15, he took 30 Valium pills and had to have his stomach pumped. Then the voices commanded him to kill his father. They told him exactly how to do it—put rat poison in his food. Fortunately, some other, gentler voices intervened and told him not to.

After high school, Terry began attending university in Detroit, but that didn't last long. Still haunted by the voices, he was soon addicted to heroin, and his marriage ended in divorce. In 1980, he moved to New York, looking for a new start. He got a job at a doughnut shop, then at a community center, but eventually the voices got worse and so did his drug habit. He found another woman to be with, but she was also taking drugs, and eventually abandoned Terry and their two daughters.

Terry (not his real name), now 60, is telling me his story over lunch at a restaurant on 42nd Street, across from Grand Central Terminal. He's tall and stocky, with kind eyes and a gentle sense of humor that mask his tortured soul. But things are better for Terry now. About 14 years ago, he met the psychotherapist he credits with saving his life. During a drug-fueled crisis, with the ugly voices raging in his head, his eldest daughter checked him into New York Methodist Hospital in Brooklyn, where psychologist Jessica Arenella was working. "I was there 6 weeks," Terry says. "She would sit by my bedside, listening to me rambling on."

Four years later he was hospitalized again, just when Arenella was about to go into private practice. She suggested that he start seeing her. "I said, 'You're a white bitch, how the hell can you help me?' " Terry recalls. "She said, 'I may be a white bitch, but I can back my play with you.' She was tough."

Terry has been seeing Arenella for psychotherapy sessions for the past decade. The voices haven't entirely gone away, he says, but she has taught him how to live with them, and how to follow the gentle voices and ignore the nasty ones. "Without Jessica, I wouldn't have made it," Terry says.

Terry is suffering from schizoaffective disorder, one of a number of so-called schizophrenia spectrum disorders. By treating his psychosis with "talk" psychotherapy, Arenella, along with a small number of other psychologists and psychiatrists, is bucking a decades-old trend, in which antipsychotic drugs have long been seen as the first line of defense against the illnesses. In a radical departure, Arenella and other advocates of psychological approaches are engaging with patients' symptoms, such as hearing voices or experiencing hallucinations or paranoid fantasies, and taking them seriously rather than dismissing them or relying on medication to stamp them out.

A number of clinical trials of these techniques have shown modest but measurable effects on symptoms such as hallucinations and delusions. One of these, a short-term approach called cognitive behavioral therapy (CBT), has been recommended since 2002 by health authorities in the United Kingdom for all new cases of schizophrenia, and long-term psychotherapy has been adopted as standard treatment in a number of Scandinavian communities. It's generally combined with traditional drug treatment, but one study, published earlier this year, suggests that CBT could substitute for antipsychotic drugs in some cases. "There is a strong possibility that psychological treatments are likely to be at least as effective as drugs, and they are certainly preferred by patients," says Peter Tyrer, a psychiatrist at Imperial College London.

Nevertheless, the idea that schizophrenia, long regarded as a disease of the brain, can be treated psychologically remains very controversial, and some are not swayed by the recent clinical trials. "These studies have no more credibility than studies of homeopathy," says Keith Laws, a psychologist at the University of Hertfordshire in Hatfield, U.K., and co-author of a recent meta-analysis concluding that CBT has only a very small effect on psychotic symptoms.

## Stress and vulnerability

About 1% of people worldwide fall victim to schizophrenia or a related disorder over their lifetimes. They may suffer both "positive" symptoms, such as hallucinations and delusions, and "negative" symptoms, such as emotional withdrawal and severe inability to focus on daily tasks.

Most schizophrenia experts subscribe to the stress-vulnerability model of the disorder, in which some individuals have a greater predisposition—either because of genes, childhood trauma, or environmental factors—to psychosis than others. In vulnerable people, psychotic episodes are often set off by some sort of stressful event, usually in the late teens or early adulthood.

But past psychological approaches, such as psychoanalysis, have shown limited success in treating the disease. Sigmund Freud, the founder of psychoanalysis, eventually gave up on using it to treat psychotic patients, although a number of later post-Freudian psychiatrists continued to use it with sporadic success. When antipsychotic drugs arrived in the 1950s, with their clear ability to dampen the worst psychotic symptoms, psychotherapy became increasingly marginalized.

Drugs have serious side effects, however, and at least 50% of patients either refuse or fail to take them, according to recent studies. Moreover, the search for genes behind schizophrenia and other mental illnesses, which might lead to new drug therapies, has failed to produce any smoking guns and has led only to the discovery of a large number of genetic variants, each conferring a very small additional risk. "We're trying to fix something, but we don't know what's broken," says Brian Koehler, a psychologist at New York University in New York City who also sees schizophrenia patients in private practice.

Now, psychological treatments are gaining ground again. Most advocates of psychotherapies insist they are not claiming that schizophrenia is purely a psychological malady caused by a dysfunctional family background. "We're looking for a much more nuanced form of psychiatry that doesn't reject biology, but that is able to situate the biology within the realm of lived human experience, which is socially and culturally determined," says psychiatrist Pat Bracken, director of mental health at Bantry General Hospital in Ireland.

Today's psychotherapists use two main approaches to treat schizophrenia. The first, called psychodynamic therapy, is derived from earlier psychoanalytic techniques but discards older Freudian ideas that sexual repression is behind psychosis. Instead, it focuses on both childhood experiences and the way that psychotic symptoms unconsciously serve a useful function for the patient, for example, by masking unbearably painful thoughts and feelings.

Psychodynamic sessions typically go on for many years, as in Terry's case, and scientific evidence for their benefits is limited. Although anecdotal stories of success abound, advocates of psychodynamic therapy increasingly recognize the importance of rigorous trials. "We live in an evidence-based era, we can't duck out of that," says Brian Martindale, a U.K.-based psychiatrist and chair of the International Society for Psychological and Social Approaches to Psychosis.

The gold standard for medical evidence is the randomized controlled trial, and these have been difficult to design for psychodynamic treatment. For one, the treatment is lengthy and costly, and few patients receive it—thus making adequate sample sizes difficult to assemble. But one influential study, led by psychiatrist Bent Rosenbaum of the University of Copenhagen and published in the journal Psychiatry in 2012, did find signs that it is effective. Rosenbaum's study compared 150 patients receiving what is often called treatment as usual (TAU)—including meetings, education about their condition, and low doses of antipsychotic medication—with 119 patients who also received intense psychodynamic therapy. After 2 years, both groups had improved, but the psychodynamic cohort achieved markedly greater reductions in psychotic symptoms.

Still, questions remain about whether such improvements last after the treatment ends, and whether they are really due to the treatment or, as psychiatrist Richard Warner of the University of Colorado, Boulder, puts it, "because they had contact with a human being who was kind and interested in them."

The second approach, CBT, is a shorter, more pragmatic method that takes patients through a series of guided steps designed to explore alternative interpretations of what he or she is experiencing, with the goal of changing both outlook and behavior. CBT, which has proven effective for depression and anxiety disorders, typically takes months rather than years, and it has shown more clear-cut effectiveness.

"There's always a little bit of truth at the heart of the delusion," explains Douglas Turkington, a CBT pioneer at Newcastle University in the United Kingdom. "If someone has a funny idea we call a delusion, you have to talk about it and put it on the table," says Ross Tappen, a psychologist at the Manhattan Psychiatric Center in New York City.

And if delusions are taken seriously, Tappen adds, they can often be treated. "A delusion is the psychological equivalent of an inoperable tumor that is out of control and takes over your normal functioning," he says. "What therapy does, at its best, is to shrink the psychological tumor."

## Sandy's CBT

An invisible companion, named John, had been tormenting Sandy (a pseudonym) since he was 10. John would talk and sing loudly, often during the night, keeping him awake. Once, John told Sandy to put the wrong answer on a school exam, and he obeyed. When Sandy, who lives in Britain's Greater Manchester area, was 18, doctors referred him to the Psychosis Research Unit in Manchester, a joint program of the University of Manchester and local mental health services. There he came under the care of psychologist Paul Hutton.

Sandy was convinced that John was real and had nearly complete control over his life. He declined to take medication, but did agree to undergo a series of CBT sessions. Hutton, now at the University of Edinburgh, was able to figure out that John made Sandy feel less lonely, and also that John was helpful in some situations, taking his side during Sandy's frequent arguments with his parents. But having John in his life convinced Sandy that he was "weird."

Hutton encouraged Sandy to avoid trying to push John away and instead let him come and go as he pleased. Sandy was also taught to test how much control John really had over him with so-called mindfulness exercises in which he remained detached during John's exhortations. Meanwhile, Hutton gave Sandy educational materials indicating that having invisible friends was normal, and that he was not really weird at all. Each week, Sandy was asked to rate how convinced he was that John was real, how often John appeared, and for how long.

With these numbers steadily dropping, by week 4 Sandy agreed to get rid of John entirely. After week 11, he had done so, and the psychotic episode seemed to be over—at least for the time being, as Hutton described in a case study first published online in 2011 in Behavioural and Cognitive Psychotherapy.

Hutton concedes that Sandy is "at the positive end of the spectrum" of CBT successes, because he was fairly young and his hallucinations were "very amenable … to the sort of well-tested approaches we use." But he adds that he often sees "fairly dramatic responses" to CBT.

As early as 2000, Turkington and others published a study of 90 patients in the Archives of General Psychiatry showing that while 9 months of either CBT or a sympathetic support technique called befriending could improve both positive and negative schizophrenia symptoms, only the CBT group maintained its improvement 9 months after the trial had ended.

In 2012, another team confirmed that CBT could be effective for so-called negative symptoms of schizophrenia, such as emotional distance, apathy, and social withdrawal, which are usually much harder to treat.

And the most recent CBT trial, published last month in The Lancet, concludes that CBT might serve as a substitute for antipsychotic drugs in some cases, rather than just an adjunct to them as in most clinical studies (see ScienceNOW, http://scim.ag/schizCBT). In this study, 74 schizophrenia spectrum patients who were being treated in Manchester and Newcastle, and who had declined to take drugs, were randomized by computer into two groups, one receiving TAU and the other TAU plus CBT.

After 18 months, the CBT group showed moderately better scores on various tests for psychotic symptoms; indeed, CBT performed about as well as antipsychotic drugs do when compared with placebos, meaning that CBT could substitute for drugs in some situations—especially those in which patients are refusing to take them anyway.

Clinical psychologist Anthony Morrison of the University of Manchester, who led the study, stresses that a drug-free approach might be appropriate only for patients who are relatively high-functioning and have not shown any risk to themselves or others. Nevertheless, the results are "utterly convincing," says Max Birchwood, a psychologist at the University of Warwick in Coventry, U.K.

Other researchers, however, are deeply skeptical of the claims for CBT. In January, a team led by Laws and psychiatrist Peter McKenna, now at the University of Barcelona, concluded in a meta-analysis in The British Journal of Psychiatry that past trials of CBT for schizophrenia were seriously flawed. The study found that the differences between treatment and control groups were very small, and that these were reduced further when sources of bias—such as inadequate blinding or masking—were controlled for. "The UK government's continued vigorous advocacy of this form of treatment … might be considered puzzling," the authors wrote, adding that "claims that CBT is effective against these symptoms of the disorder are no longer tenable."

Arenella, who treats Terry and some of her other patients with a combination of psychodynamic and CBT approaches, says that in the end it doesn't matter whether talk therapies work because of the theory behind them or just because someone is taking the patient and their symptoms seriously. "It may be a placebo effect, but I will go for all the placebo effect I can get," she says. "I'll take it."

In the end, the spread of talk therapies for psychosis could be limited by a scarcity of resources, and of therapists willing to try them. Treating such clients is very stressful and seldom financially rewarding. "A lot of people don't want to take these patients," Arenella says. "Working with them is scary. People get violent, people get hurt, computers get thrown to the ground, ceiling tiles get pulled out." And Martindale says that "contact with madness is very disturbing; it conjures up all sorts of feelings."

Government agencies and insurance companies can help by covering such treatments, even though they are more expensive in the long run than drugs, say Arenella and others. They are worth trying, Bracken says. "I have a lot of patients whom I would say recovered from psychosis. I see people who move on with their lives, get their quality of life back, are able to live independently." Indeed, the popular notion that a schizophrenia diagnosis is a life sentence of mental illness is not borne out by the statistics: In one typical study, published in the American Journal of Psychiatry in 2004, researchers found that nearly 50% of first-episode schizophrenia or schizoaffective disorder patients were symptom-free after 5 years.

"But many people don't get there no matter what we do," Bracken says, "until that spark in them finally says, 'I want my life back.' "

My lunch with Terry was coming to an end, so I pulled out my American Express card to pay the bill. Terry was still smiling, although he looked very tired from telling me his story over the previous 2 hours. As I paid up, I told him about a meeting I had just attended in San Francisco on psychological approaches to psychosis, as part of my reporting for this story.

"I'd like to fly to San Francisco and take people out to lunch with my own American Express card," Terry said. "I'd like to get married again, or have a girlfriend. I'm going to get all that. It's going to happen because, like I told Jessica, I'm not going to settle for anything less."

5. # Water's Tough Skin

1. Elizabeth Pennisi

Surface tension is a force to be reckoned with, especially if you are small.

An ant could step off a cliff and land unharmed. But if it dips a leg in a raindrop, the insect can be caught in a life-threatening morass, all because of the surface tension of water. The result of polar interactions among water molecules, surface tension is what draws a water droplet into a sphere. It creates an elastic surface that can deform without breaking—think of a water strider oaring its way across the surface of a pond. At the same time, it enables water to cling like quicksand to an ant unlucky enough to blunder in. Creatures of our size barely acknowledge surface tension's existence, but for the tiny, "it becomes a dominant force," says David Hu, a mechanical engineer at the Georgia Institute of Technology in Atlanta.

Because of surface tension, a rat can't pee a steady stream, but instead must slowly push out urine drop by drop. (It can take 10 minutes for a single drop to fall.) Surface tension thwarts juvenile flying fish: When they try to escape into the air like the larger adults, they sometimes bounce off the underside of the water's surface layer. Dew forming on a mosquito's wing will cause the wing to fold up, grounding the insect until the wing dries out.

Physicists have a pretty good understanding of how surface tension arises. The clingy water molecules attempt to minimize their connections with other types of molecules. So when something deforms the water surface, the displaced water molecules work to return to their minimum-energy configuration—unless the intruder itself attracts water molecules, in which case the water clings like glue. But biologists have tended to ignore the air-water interface, Hu says. His own eyes were opened a decade ago when he began to study how water striders skate so effortlessly along the surfaces of ponds. Using dyes and high-speed video, Hu and his colleagues found that by vigorously rowing along the surface, striders create swirls that help propel them forward, all without rupturing the water surface. "That work was the trigger of academic interest in surface tension in biology," says Ho-Young Kim, a mechanical engineer at Seoul National University.

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Hu has since looked at other phenomena involving air-water interfaces—how dogs shake to dry off, how mosquitoes cope with rain (Science, 8 June 2012, p. 1216), and how animals pee—disparate phenomena "linked by common equations and modeling ideas," he says. Since then, other researchers have recognized the power of surface tension to explain form and behavior on the small scale. And while humans, unlike ants, don't have to worry about being trapped because of surface tension, it's still relevant to our lives. For example, our lungs secrete a chemical inside their air-filled sacs to lower the surface tension there, which allows us to breathe without the sacs collapsing when we exhale. Surface tension also allows human and agricultural pathogens to travel long distances in tiny, lightweight droplets.

Some of the most eye-catching new findings were on display at "Shaking, dripping and drinking: surface-tension phenomena in organismal biology," a symposium that Hu helped organize at the annual meeting of the Society for Integrative and Comparative Biology in Austin in January. "The sheer dazzling diversity of biological phenomena to which surface tension is relevant is mind-blowing," says Steven Vogel, a biomechanist at Duke University in Durham, North Carolina.

## Giving plants "muscles"

Plants lack muscles, but findings presented at the symposium showed that for some, surface tension can substitute. "When there's a change in surface tension, you get motion," explains Rachel Levy, a mathematician at Harvey Mudd College in Claremont, California. "It creates motion in ways you don't expect."

Consider Erodium, a group of flowers whose fruit resemble a bird beak. Inside that beak, each seed develops a centimeter-long awn—a rodlike "tail" that serves two purposes. Initially, inside the fruit, the awn is stretched out. When the fruit dries and cracks open, the freed awn spontaneously coils, releasing its stored energy and sending the seed a half-meter from the parent plant. After the seed lands, the awn winds up during the day and unwinds at night, screwing the seed into the ground bit by bit—a millimeter or so a day.

Kim has found that surface tension drives the process. Normally, surface tension causes water droplets to ball up to minimize the air-water interface, he explains. But when those droplets meet a surface that has a greater attraction for the molecules of water than water itself, they will spread out and wet it. He found that the awns of Erodium and of Pelargonium, another group of plants with self-burying seeds, consist mostly of fibers of lignin and pectin, both water-loving molecules. At times of day when humidity is high, the fibers quickly absorb moisture. "The wet tissues swell and become straight from [an] initially dry, coiled configuration," Kim explains. When humidity drops, the fibers dry out, the tissues shrink, and the awn coils up again.

Kim mounted seeds from both groups of plants onto a force sensor and increased the humidity to measure the force they generated as they tried to uncoil. He also tested the burying potential of the force by watching awns drive their seeds into "soils" of glass beads of different sizes. The force "is just enough to dig into the soil," Kim reported in January.

It might also be enough to propel a microrobot. Today's microrobots all require electrical tethers, because no battery is both sufficiently powerful and small enough to be carried on board. Eventually Kim wants to equip robots with humidity-driven "muscles" that won't require external power. "But first we need to know how the biology works," he says.

## A raincoat for a leaf

A floating fern, Salvinia molesta, forms meter-thick beds at the surface of ponds and slow-moving rivers. Native to South America, it's become invasive around the world, clogging waterways and disrupting aquatic ecosystems. But what attracted Wilhelm Barthlott to this prolific plant was its ability to keep submerged leaves coated with a thin layer of air. The film of air gives the leaves a silvery sheen and enables the plant to carry out photosynthesis and gas exchange underwater. It also makes the fern buoyant, so it will quickly bob to the surface if an opening appears, filling in any gaps before other species can get established.

Engineers want to create similar air layers on the hulls of ships to reduce drag and fuel consumption. But to date, they have not been able to generate a layer that lasts. So Barthlott, a biologist emeritus at the University of Bonn in Germany, and his colleagues decided to see how Salvinia does it. Studying the microscopic structure of the leaf surface, they discovered hundreds of regularly spaced clumps of 2-millimeter-long hairs, four per clump. The clumps resemble eggbeaters: The hairs in each clump flare out midway up but reconnect at their tips. Along most of their length, the hairs are coated with hydrophobic, or water-repellent, wax, while the tips are waxless and hydrophilic—they attract water. Surface tension pins the air-water interface to the tips so that the air layer resists disruption by turbulence in water. The interface is "a bit like a tent where the hairs are the poles," said Matthias Mayser, a biologist with an engineering background at the University of Liège in Belgium who worked with Barthlott. "The water stays on top of the air."

The plant also repels rain, Mayser explained. "It would be impossible to establish an air layer upon submergence if the room in between the hairs was already filled by water from rain." By filming drops falling on the Salvinia leaf surface, he observed that "the [drop's] surface tension keeps the drop in a spherical form and prevents water from penetrating" between the hairs. That keeps the leaf's silvery sheath of air intact.

## Flying low among the lilies

Galerucella nymphaeae, or water lily beetles, spend most of their time munching water lily leaves. But as Manu Prakash watched them on a Massachusetts pond one day, he noticed a strange behavior. Flying from leaf to leaf, the beetles skimmed the water surface, never lifting off. Prakash, a physicist at Stanford University in California, wondered whether surface tension plays a role in this peculiar flight mode.

He and his graduate student Haripriya Mukundarajan filmed water lily beetles as they flew and took a close look at their anatomy with an electron microscope. They saw that each apple seed–sized beetle was covered with hairs. Further tests showed that the hairs made the insect superhydrophobic. Only the claws at the end of each leg were hairless and hydrophilic. Prakash suspected that the beetle's body and legs would be repelled from the water surface, but the claws would tend to stick to the water.

The film revealed that as the beetle flies, it drags the claws of four of its six legs in the water, raising only the middle two legs. "If there was no surface tension, the moment the wings generate lift, the beetle would pop off the water," Prakash explained at the meeting. But the claws tether the beetle to the water, while the rest of its body is repelled by the water surface. "So the beetle bounces up and down," he said.

Although erratic, skimming the water is a more efficient way for the beetle to travel from leaf to leaf than full-fledged flight would be. To take off into the air would be a waste of time and energy. The only downside is that if the beetle goes too fast—it normally flies about 0.5 meters per second—it catches up with the ripples it creates as it moves, which slows it down. But Prakash thinks the beetles, like water striders, are a textbook example of how evolution has put surface tension to work. "There's so much stuff that you see when you are sitting on a pond."

## Germs go airborne

Surface tension may keep some beetles water-bound, but it also helps pathogens take flight. Each cough or sneeze launches bacteria and viruses skyward in a cloud of droplets whose sizes are determined in part by surface tension, says Lydia Bourouiba. Through high-speed video, mathematical modeling, and lab experiments, this applied mathematician from the Massachusetts Institute of Technology in Cambridge is working out details of such airborne pathogens with an eye toward curbing the spread of disease.

Some researchers believe respiratory viruses generally don't travel far after a cough or sneeze, arguing that they are mostly carried in large droplets that land within a meter or two. Others contend that it's the smaller droplets, which are airborne for much longer, that underlie transmission. But very few researchers have studied how pathogens are actually transported from place to place, says James Hughes, a medical epidemiologist at Emory University in Atlanta. "Even the sizes of the droplets emitted remain debated," Bourouiba says. Surface tension, by governing the shapes and sizes of drops and bubbles, as well as how quickly they burst, influences how far they travel.

By filming coughs, sneezes, and bursting bubbles and closely examining the cloud of droplets emitted, she has characterized their sizes and the flight distances. She used these observations to help come up with a mathematical model that assesses how buoyancy and momentum interact to determine how far the droplet cloud launched by a cough or sneeze can travel. The model shows that coughing can spread pathogens 200 times farther than other models had predicted, she reported at the meeting. Smaller droplets can waft up to 6 meters.

Temperature and humidity should also influence droplets' lifetimes, and Bourouiba says her model can account for their effects as well. That could prove useful for understanding what environmental conditions promote a pathogen's spread. "Here's somebody that comes from a totally different background and discipline who is applying her expertise and know-how to an important public health issue," Hughes says.

Bourouiba and her colleagues have put their approach to work to understand the spread of Clostridium difficile, which can cause severe and persistent diarrhea. The bacterial infection affects about a half-million people a year, particularly in hospitals or long-term care facilities. Outbreaks can be hard to stop because the bacterium produces spores that last for months, and hospitals find it everywhere, including suspended in the air. But how does it get airborne? High-pressure flush toilets, Bourouiba reported at the meeting and in a paper posted to the arXiv preprint server in October.

She mounted a camera to the sides of toilet seats; with special lighting, filming at up to 2000 frames per second, she was able to visualize water droplets erupting with each flush. While a substantial portion of the airborne droplets were large enough to sink back onto the toilet seat or other nearby surfaces, many were so small that they remained suspended, she said. As with the visualizations of coughs and sneezes, she used these observations to develop another mathematical model, one that predicts the range and spread of droplets spewing from the toilet.

Certain cleaning products actually worsen the problem, she says, by reducing the surface tension of the water in the toilet bowl, which allows more small droplets to escape. "This is important," she said, "because current mitigation strategies in hospitals only focus on bleaching surfaces, thus effectively ignoring the aerosol problem." The modeling may give "us enough information to design intervention strategies," she said, such as increasing surface tension with water additives.

Bourouiba and Tristan Gilet, a fluid dynamics engineer at the University of Liège, are also looking at how the properties of water can spread plant diseases. Agricultural experts have long known that plant diseases flourish after rain, among them wheat leaf rust, which threatens global wheat crops. Bourouiba and Gilet wondered whether the raindrops themselves help disperse the pathogens living on leaves. They again enlisted high-speed video, studying natural and artificial leaves with a range of sizes and other properties.

Because leaves are somewhat hydrophobic, an impacting raindrop tends to form a discrete puddle instead of a thin film. That standing water can absorb pathogens. The videos showed that when another raindrop lands right next to a puddle, it splashes, launching part or all of the puddle from the leaf, Gilet reported at the meeting. "The second raindrop expels [the first] in a very efficient way." How far that water travels depends on the size and flexibility, or compliance, of the leaf. Small leaves, like a tomato's, bend and absorb the impact of the raindrop, dampening any splash. Big leaves resist the impact, so the splash can travel much farther, he said.

"The idea of rain generating disease in plants is pretty new," Hu says. "It spreads [pathogens] in a way the pathogen couldn't do itself." The importance of the effect will vary depending on the concentration of pathogens and the size of the ejected drop. But the research suggests that a wider spacing between plants could slow the spread of some diseases.

From insect locomotion to a strategy for boosting global crop yields: A simple physical phenomenon lets us not only understand the world of the very small; it may also help us cope better with the world at large. And that's what appeals to Hu. "I love that when I study surface tension dynamics I can draw on ideas from many disciplines: mathematics, biology, chemistry, physics, computer science, and engineering," he says. "We need all of these perspectives to understand the many ways surface tension impacts the world."