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If you chill fermions enough, they can pair up to form bosons and settle into a single collective ground state, a Bose–Einstein condensate. In the case of helium-3 atoms, the resulting BEC is a superfluid that flows without dissipation—provided the flow is not so energetic that it breaks the pairs apart or destroys the ground state's coherence. Until now, theorists could characterize placid flows in fermionic superfluids, but not the vigorous turbulence that results from shaking or stirring. Aurel Bulgac of the University of Washington in Seattle and his colleagues have adapted density functional theory—a computational approach originally devised to calculate molecular energy levels—and applied its time-dependent extension to model turbulent fermionic superfluids. Although the underlying quantum mechanical equations are straightforward, solving them required the use of one of the world's most powerful supercomputers, Jaguar at Oak Ridge National Laboratory in Tennessee. In their simulations, Bulgac and his colleagues agitated a fermionic superfluid by shooting spherical projectiles through it or by stirring it with a laser beam. Turbulent superfluids are known to harbor tubes of quantized vorticity. As the figure below shows, the simulation could track how two vortex tubes (marked a and b) joined to form a ring, which then opens in a manner reminiscent of the unzipping of a DNA molecule during transcription. Bulgac's model could help astronomers understand another agitated superfluid: the interior of a rapidly spinning neutron star. (A. Bulgac et al., Science 332, 1288, 2011.)—Charles Day

In 1805 Thomas Young derived the relationship between the static forces of a liquid droplet at rest on a solid substrate. As shown in the schematic, the droplet’s contact angle, θ, can be determined by balancing the horizontal solid–liquid and solid–vapor forces and the horizontal component of the liquid–vapor force, determined by the surface tension, γ/v (see Physics Today, February 2007, page 84). However, for more than two centuries, theories yielded unrealistic singularities for stress and strain at the three-phase contact line. That’s because Young’s equation does not account for the vertical, out-of-plane force pulling on the solid substrate, which naturally should be balanced by the substrate’s elastic response. Now, researchers at Yale University and at consumer products manufacturer Unilever have experimentally and theoretically resolved the out-of-plane contributions. Using a confocal fluorescence microscope, the researchers, led by Yale’s Eric Dufresne, laced a 20-micron-thick film of silicone gel with fluorescent beads and measured the deformation due to a water droplet. At equilibrium, a one-micron-high ridge, illustrated in the inset, formed in the gel at the contact line. When the researchers factored the gel’s surface tension and thickness into a linear elastic model, they arrived at a nonsingular theoretical solution for stress that closely fit their experimental data. Their model, however, underestimates the deformations in the solid-liquid contact plane, which they believe are caused by pinning or viscous drag. (E. Jerison et al., Phys. Rev. Lett., in press.)—Jermey N. A. Matthews

Plutons are mountain-sized formations of igneous rock that poke through Earth's surface. Their origin as solidified eruptions of magma is straightforward to explain. What's puzzling is why plutons are so homogeneous on large scales, despite their immense size and despite being inhomogeneous in their mineral composition on small scales. Alain Burgisser of the Institute of Earth Sciences in Orléans, France, and George Bergantz of the University of Washington in Seattle have proposed an answer. In their model, a mass of viscous, semisolid magma—"mush" is the technical term—lies beneath the surface, hemmed in by walls of more solid rock. Pluton formation begins when the slow churning of the mantle below happens to bring a body of hot magma into contact with the bottom surface of the cooler mush. Over the ensuing decades, the heat rises slowly via conduction, making the mush less viscous and—crucially—less dense. In a process that Burgisser and Bergantz call unzipping, the gradually thickening layer of hot, light mush abruptly undergoes a Rayleigh–Taylor instability, which sends convective plumes of hot mush upward through the nascent pluton. Within a few months, the first plumes reach the top of the nascent pluton, cool, then sink. Only a few successive cycles of overturning suffice to homogenize the mush on plutonic scales. Burgisser and Bergantz's model can plausibly account for the speed with which three real plutons formed, including the one left by the 1991 eruption of Mount Pinatubo in the Philippines. (A. Burgisser, G. W. Bergantz, Nature 471, 212, 2011.)—Charles Day

To gather clues about the structure of a protein, DNA, or other chain-like biomolecule, researchers can tag it with two fluorescent dyes. In what's known as fluorescence resonance energy transfer (FRET), the excitation of one dye, the donor, can cause a nearby partner, the acceptor, to also fluoresce. The resulting two-color burst of photons hints at the biomolecule's shape: The larger the contribution of the acceptor to the total fluorescence, the closer the dyes. Now, two groups have paired single-molecule-resolution FRET with microfluidics to shed light on complex biophysical phenomena. Ashok Deniz (Scripps Research Institute, La Jolla, California), Alex Groisman (University of California, San Diego), and colleagues used a novel microfluidic design to quickly initiate folding of a tagged protein and then track the time evolution of the donor–acceptor distances. Their experiment revealed the three-step process, illustrated here, by which the protein α-synuclein folds from its intrinsic disordered state into an extended helix. Another group of researchers led by Shimon Weiss (UCLA) and Stephen Quake (Stanford University) devised a microscale network of flow channels, valves, and pumps to facilitate high-throughput FRET experiments. The team used the device to investigate effects of chemical environment on bacterial RNA transcription, which could be detected as changes in fluorescence when tagged DNA strands hybridized with matching RNA strands. (Y. Gambin et al., Nat. Methods 8, 239, 2011; S. Kim et al., Nat. Methods 8, 242, 2011.)—Ashley G. Smart

Stephen Hawking proposed in 1974 that black holes evaporate. In essence, vacuum fluctuations near a black hole's horizon produce particle–antiparticle pairs. One of each pair falls into the hole while the other escapes. Since the escaping particle has energy, the black hole must lose energy. A blackbody temperature inversely proportional to the black hole's mass can be assigned to the process, yet the temperature is so low—on the order of 100 nK for a solar mass—that the radiation is difficult to observe directly. But William Unruh (University of British Columbia) demonstrated an analogy between the behaviors of waves near the black hole and sound waves in moving fluids. Now, physicists Silke Weinfurtner, Matthew Penrice, and Unruh and engineers Edmund Tedford and Gregory Lawrence at UBC have used another analogous system, surface waves, to study the Hawking process. They put a streamlined object shaped like an airplane wing into a channel of flowing water to create a region of high-velocity flow. Long-wavelength surface waves created downstream could propagate upstream toward that region but were blocked by the obstacle and converted into short-wavelength waves. The figure shows the converted waves (bottom) and the interference between them and the incoming wave (top). The obstacle behaves like a so-called white hole, which, as a time-reversed black hole, lets no radiation in but does let radiation escape. The conversion is the analogue of stimulated emission, and the team's measurements of the amplitudes of the converted waves matched the expected thermal distribution. Moreover, despite the system's nonlinearities, turbulence, and viscosity, and along with prior numerical work by various groups, the new results demonstrate the generic nature of Hawking radiation. (S. Weinfurtner et al., Phys. Rev. Lett. 106, 021302, 2011.)—Richard J. Fitzgerald

Superplasticity is the ability of some crystalline materials to stretch up to several times their own length when heated. Although the minerals in Earth's mantle don't endure such large strains, circumstantial evidence suggests that superplasticity helps them respond to the subduction of continental plates and other tectonic processes. Now, a team led by Takehiko Hiraga of Tokyo University and Hidehiro Yoshida of Japan's National Institute for Materials Science has found direct evidence that mantle minerals are indeed superplastic. Like other superplastic materials—real or presumed—those in the mantle are polycrystalline aggregates. For their study, Hiraga, Yoshida, and their team sintered nanoscale powders to make two analogues of mantle minerals, both of which consisted mostly of forsterite (Mg₂SiO₄). In the absence of strain, a superplastic material is made up of nanoscale grains of the majority component interspersed with smaller grains of the minority component. When heated under strain, the majority and minority grains both grow by merging with their neighbors. That response ensures that grains continue to abut each other, forestalling failure of the bulk material. As the accompanying figure shows, samples that consisted of 90% forsterite and 10% periclase (MgO) could withstand strains of more than 500%. Moreover, two electronic diagnostics, electron back-scattered diffraction and transmission electron microscopy, revealed that grains in the mantle analogues grew like grains in materials whose superplasticity is established. Having measured the temperatures and strain rates under which mantle analogues become superplastic, the team estimated that superplasticity could help Earth's mantle accommodate a 200-km slab that takes 60 million years to penetrate 3000 km. (T. Hiraga, T. Miyazaki, M. Tasaka, H. Yoshida, Nature 468, 1091, 2010.)—Charles Day

Global news coverage of the blown-out BP-operated Macondo well documented several efforts to stem the roughly 8 million liters per day of crude oil that gushed into the Gulf of Mexico from April to July 2010. One notable failure was the “top-kill” method to plug the well by pumping a dense slurry of drilling mud into it—reportedly, much of the mud was swept out by the spewing oil. A recent study by scientists at Lawrence Livermore National Laboratory and Washington University in St. Louis points to fluid shearing as the chief culprit in top-kill’s failure. Turbulent eddies, caused by the velocity differences between the counterstreaming mud and the oil, likely sheared the mud, breaking it up into packets of fluid whose settling velocities were an order of magnitude smaller than the upward velocity. In laboratory experiments, the researchers confirmed their theory and demonstrated a possible solution, adding a viscoelastic polymer to an aqueous cornstarch mixture to represent the drilling mud. As the images show, the control fluid (left, in green) suffered turbulent breakup, but the polymer-laced fluid (center) descended as a coherent slug or, at lower flow rates (right), as stringy, connected globules. The researchers calculate that a polymer-doped slug of drilling mud at the Macondo well would have descended with a terminal velocity of roughly 7 m/s, nearly double the estimated 3.7 m/s ascent of escaping oil. (P. Beiersdorfer et al., Phys. Rev. Lett., in press.)—Jermey N. A. Matthews

Force a fluid gently and its response is an orderly, laminar flow. Disturb it vigorously and that well-organized procession gives way to turbulent whirls and eddies such as those shown in the image. Now, new insights into turbulent flow arrive by way of two independent experiments—one by Detlef Lohse and colleagues at the University of Twente in the Netherlands, the other by Daniel Lathrop and Matthew Paoletti at the University of Maryland, College Park. Both teams studied Taylor–Couette flows, in which fluid is sheared in the gap between concentric, rotating cylinders, and both achieved Reynolds numbers on the order of 10⁶—records for such a device. Lohse and company were intrigued by the parallels between Taylor–Couette and Rayleigh–Bénard cells, the latter comprising a fluid confined between two horizontal plates and heated from below. They found that angular momentum transport in turbulent Taylor–Couette flows obeyed the same scaling laws that had been predicted, but never conclusively observed, for heat transfer in Rayleigh–Bénard flows. Lathrop and Paoletti explored a regime of Taylor–Couette flow in which the innermost fluid rotates faster than the outermost, but with less angular momentum—conditions that approximate the Keplerian trajectories ubiquitous in astrophysics. Their data suggest that such flows, long presumed laminar, might actually become turbulent at large Reynolds numbers. If so, that would help to explain the behavior of accretion disks and other astrophysical flows. (D. P. M. van Gils et al., Phys. Rev. Lett., in press; M. S. Paoletti, D. P. Lathrop, Phys. Rev. Lett., in press; image courtesy of Dennis van Gils.)—Ashley G. Smart

It’s easy to think of water as a typical liquid, but many aspects of its behavior are highly atypical. For example, its freezing and boiling points are high compared to those of similarly sized molecules. To better understand water’s anomalous nature, and to test models that connect its microscopic and macroscopic properties, researchers look at the liquid’s behavior under tension, or negative pressure. When liquid water, with no air or vapor present, is forced to occupy a larger and larger volume, it enters a metastable state: The water could lower its potential energy by forming a vapor bubble that allows the liquid to shrink to a smaller volume, but an energy barrier bars the bubble’s formation. However, negative-pressure experiments present a mystery of their own: Different methods of stretching water achieve vastly different maximum tensions before vapor bubbles form. To try to resolve the discrepancy, researchers at the École Normale Supérieure in Paris measured the equation of state (the relationship between temperature, pressure, and density) of stretched water. Working at constant temperature, they launched intense ultrasound waves from a hemispherical piezoelectric transducer (the red element in the photograph), thereby creating a region of strongly oscillating positive and negative pressure at the sphere’s center. There, they performed two independent optical measurements (using the horizontal green beam and the vertical optical fiber) from which they deduced the pressure and density. But the results pose even more questions: The ultrasound method can’t generate tensions beyond −30 MPa before vapor bubbles form, but the measured equation of state agrees with models predicting much greater tensions should be possible. (K. Davitt et al., J. Chem. Phys. 133, 174507, 2010.)—Johanna Miller

Shuttled around a microfluidic circuit, water droplets can serve as test tubes in which controlled chemical reactions occur. That microenvironment has proven useful in applications such as genetic sequencing and the screening of huge libraries of molecular compounds in the search for new drugs. The central challenge in reactor applications is controllably adding reagents to the drops, which is particularly problematic in those cases where drops are coated with an oily surfactant layer to ensure that the small chemical factories remain isolated from each other. A Harvard University team led by David Weitz has now designed a pressurized microfluidic system that injects, with subpicoliter precision, an aqueous reagent into individual drops in the presence of an electric field. The figure illustrates the technique. As tightly packed drops pass through a narrow channel in single file, carrier oil is added to space them apart. Downstream, a reagent-filled injector channel tapers to a small opening and connects to the main channel. The watery reagent and oily carrier fluid are immiscible, and interfacial tension keeps the reagent, except for a small bulge, out of the main channel. As a drop flows under the bulge and voltage is applied to the electrodes, the electric field destabilizes the drop’s oil–water interface and reagent flows into it. By adjusting the drop’s velocity, the injection pressure, and the rate at which the field is switched on and off, the researchers can control how much reagent is added—between 0.1 and 3 pL in their device—and selectively target particular drops. (A. R. Abate et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1006888107.)—R. Mark Wilson

As tiny creatures swim, the flows they create in their liquid environment can affect the motion of their neighbors and the viscosity of the ambient suspension. Those flows have been mathematically modeled, but, until recently, they had not been imaged near a swimming microorganism. The feat has now been accomplished by two research teams working independently, one at the University of Cambridge in the UK and one at Haverford College in the US. Both obtained flow fields by taking sequential photographs of microscopic tracer particles displaced by the alga Chlamydomonas reinhardtii, which swims by executing a breaststroke-like motion with its two approximately 10-µm-long flagella. The image, from the Cambridge group, shows the streamlines created by the swimming alga, averaged over a stroke cycle (left). That fluid flow, the researchers note, is well described by a simple model (right) in which C. reinhardtii exerts two point forces to balance the drag force on its body. (The Cambridge team also studied the larger alga Volvox carteri.) The Haverford collaboration focused on the changes in the alga’s flow field during the course of a swimming cycle (see the accompanying video). The group discovered, for example, that as it carries out its forth-and-back motion, C. reinhardtii expends about four times the energy it would if it could swim steadily along its trajectory. (K. Drescher et al., Phys. Rev. Lett. 105, 168101, 2010; J. S. Guasto, K. A. Johnson, J. P. Gollub, Phys. Rev. Lett. 105, 168102, 2010.)—Steven K. Blau

The complex mechanical properties of colloidal coatings are hard to measure because they vary spatially and temporally. Paint, for instance, starts as a fluid that, as the solvent evaporates, dries into a brittle solid that can crack and peel away from a substrate. To better understand the stresses that drive the fracture process, researchers led by Yale University’s Eric Dufresne have now adapted a technique from cell biology known as traction force microscopy. In the technique’s biological application, researchers observe a cell crawling across a rubber substrate and monitor the deformations within the rubber. Knowing the rubber’s mechanical properties, the researchers convert the displacement field into a stress field and deduce which parts of the cell exert force on the substrate. Dufresne and colleagues replaced the cell with a drying film of a silica-particle suspension, which they applied to a soft layer of silicone rubber that would deform as the film dried and cracked. To map those deformations and convert them to a three-dimensional stress field, the team monitored the motion of tiny, fluorescent tracers mixed into the rubber. In this plot of stress as a function of distance from the crack front, the normal stress (solid dots) shoots up rapidly just ahead of the crack front—with much greater magnitude than does the in-plane stress (open dots) and, reassuringly, with a scaling that roughly agrees with that predicted by classic fracture theory (red). (Y. Xu et al., Proc. Natl. Acad. Sci. USA 107, 14964, 2010.)—R. Mark Wilson

During the collapse of a cavitation bubble, the gas inside it can reach temperatures of more than 15 000 K—as hot as the surface of a star—and the energy can be released in the form of shock waves, heat, light, turbulent vortices, and high-speed jets of fluid. For decades, medical researchers have worked to harness that energy for therapeutic applications, such as the disintegration of cancerous tumors using focused ultrasound (see the article by Gail ter Haar in Physics Today, December 2001, page 29) and the delivery of drugs or genes into living cells (see the article by Detlef Lohse in Physics Today, February 2003, page 36). Although studies have demonstrated that microbubbles can rupture nearby cells, control over the bubble–cell interaction has remained difficult. Duke University researchers led by Pei Zhong have now demonstrated an approach to puncturing a cell’s membrane that entails carefully manipulating the fluid dynamics around it. The high-speed sequence of photographs captures the process: Two laser pulses, offset in space by 40 μm and time by 4 μs, create two bubbles (B₁ and B₂) that act in concert. The rapid expansion of the second bubble causes the collapse of the first bubble by pressing against it; the interaction deforms the shape of both. The bubbles’ asymmetric collapse gives rise to two localized microjets—one toward the cell between 6 and 7 μs, one away from it 2 μs later. The researchers can control the microjets’ impact by adjusting the bubbles’ position, spacing, and orientation relative to the cell. (G. N. Sankin, F. Yuan, P. Zhong, Phys. Rev. Lett., in press.)—R. Mark Wilson

Newtonian fluids, such as water, are described by the Navier-Stokes equations. But many everyday fluids lack a similar complete description, and researchers still seek better observations and models of their flow. Yield stress fluids (YSFs), a subset of non-Newtonian fluids that includes toothpaste and mayonnaise, hold their shape under low stress but flow under high stress. Some YSFs are also thixotropic, meaning their viscosities decrease with time during continued flow. Thixotropy in a YSF can result in heterogeneous flow—confinement of the fluidlike behavior to part of the material, which flows more and more easily, while the rest remains solid—an important phenomenon to understand and control when handling YSFs industrially. Now, Sébastien Manneville, of the École Normale Supérieure de Lyon, and colleagues have unexpectedly observed similar localized flow in a nonthixotropic YSF subjected to a shear stress. The observed behavior was transient, but it lasted a surprisingly long time: more than a day in one of their trials, several hours in others. Many of the researchers’ observations, such as the power-law dependence of the transient duration on the shear rate, remain unexplained. Even so, the data indicate that nonthixotropic YSFs are more complicated than was previously assumed, and they exemplify the importance of distinguishing between transient and steady-state behavior in YSF experiments. (T. Divoux et al., Phys. Rev. Lett. 104, 208301, 2010.)—Johanna Miller

Several methods exist for growing nanowires, whether attached to a substrate or dispersed in a liquid. Using those wires to make designated electrical connections in a circuit, however, has been difficult. Yves Galerne and his colleagues at the University of Strasbourg, France, now demonstrate a procedure that produces conducting wires across a gap between two electrodes. The chemical physicists first paint the electrodes with a polymer so as to create "anchors" in predetermined locations; when the gap is filled with nematic liquid crystals, an isolated defect line—a disclination—connects the anchors and therefore the electrodes. Next, silica particles coated with a conducting polymer are introduced and gather along the disclination like beads on a necklace. In the third step, a voltage across the electrodes welds the necklace beads together into a robust wire. Although ragged with extra polymer aggregates, the central region of a 150-micron-long wire, shown in the photo, demonstrates the team’s initial result. The researchers note that the wire’s size, smoothness, and conductivity can be improved—for example, by decreasing the silica particles’ size and concentration and by electroplating them. (J.-B. Fleury, D. Pires, Y. Galerne, Phys. Rev. Lett., in press.) —Stephen G. Benka

We learn in introductory physics classes that the friction force is the product of a friction coefficient and the force normal to the interface. That relationship, embodied in the first of Guillaume Amontons's two laws of friction, has been superseded over the past 50 years by the recognition that the lateral friction or retention force is, in fact, proportional to the true contact area (see Physics Today, September 1998, page 22). Amontons's law turns out to be a special but common case in which the contact area scales linearly with the normal force. In new measurements of liquid drops on surfaces, Rafael Tadmor and colleagues at Lamar University in Beaumont, Texas, observe the opposite behavior: a lowered lateral force despite a larger normal force and an increased contact area. Key to the observations was the ability to decouple the normal and lateral forces while monitoring the drop. To achieve that separation, the researchers mounted the sample at an adjustable angle in a horizontal centrifuge arm, shown here, that could be rotated about the vertical axis at a variable speed. A comounted camera wirelessly transmitted video to a nearby computer. Comparing the situation in which the drop of liquid was on top of a horizontal substrate to that in which the drop was hanging below a horizontal substrate, the team found that the hanging drop had the larger lateral retention force, despite a smaller contact area and a smaller normal force. That counterintuitive result agrees with theories that incorporate the effects of surface deformation and molecular reorientation. (R. Tadmor et al., Phys. Rev. Lett., in press.)—Richard J. Fitzgerald

The tunable elasticity and porosity of colloidal gels lead to some interesting applications, among them tissue scaffolding and drug delivery. Conventionally, colloidal particles interact and assemble under entropic and electrostatic forces to form predictable structures. But greater control can be achieved from an approach developed by Paul Clegg, Michael Cates, and their collaborators at the University of Edinburgh in the UK. The researchers disperse silica particles in the single-phase region of two partially miscible solvents—water and the organic base 2,6-lutidine. When the solution is heated above a critical temperature, the solvents separate and the particles become trapped at the liquid–liquid interfaces. The bulky particle domains then jam together and arrest the solvent separation, forming a two-phase network the researchers call a bijel. But cool the solution and remix the solvents too soon and the distinct structure disappears, as shown in movie 1 and the two left images in which the colloids appear green, the water black, and the lutidine red. Now the researchers have discovered an approach to stabilize the bijel structure. When the phase-separated solution is allowed to sit for at least 24 hours before it is cooled, the bijel surprisingly keeps its shape, as shown in the two right images and movie 2. From Monte Carlo simulations, the researchers deduce how the resulting network of colloidal monolayers, or monogel, stays intact: the particles become compressed by capillary forces, remain attracted by van der Waals forces, and are kept from collapsing into each other by repulsive electrostatic forces. (E. Sanz et al., Phys. Rev. Lett., in press.) —Jermey N. A. Matthews

Movie 1

Movie 2

Turbulent flows of a liquid along a surface experience frictional drag, a macroscopic phenomenon that affects the speed and efficiency of marine vessels, the cost of pumping oil through a pipeline, and countless other engineering parameters. The drag arises from shear stress, the rate per unit area of momentum transfer from the flow to the surface. To reduce the flux, engineers could add polymers to the flow, inject bubbles against the surface, or combine the two methods, but those approaches bear their own cost. Jonathan Rothstein and colleagues at the University of Massachusetts Amherst now offer a proof-of-principle demonstration of a new, passive option for reducing drag in turbulent flow. They tailored the microscale structure of a hydrophobic material—polydimethylsiloxane, similar to the rubbery polymer used to caulk bathtubs—to create air pockets at the surface, as shown in the figure, that allow the flow to “slip” (shear free) at the liquid–air interface. The greater the area covered by air pockets, the greater the reduction in shear stress—up to 50%, they estimate, judging from particle-image velocimetry and pressure-drop experiments over a wide range of Reynolds numbers. The researchers found that the critical Reynolds number at which the onset of drag reduction occurs is related to the ratio of two length scales—one associated with the geometry of the hydrophobic surface corrugations, the other with the thickness of the viscous boundary layer there. (R. J. Daniello, N. E. Waterhouse, J. P. Rothstein, Phys. Fluids 21, 085103, 2009.) —R. Mark Wilson

In principle, setting a droplet in motion inside a microfluidic channel is straightforward: Apply pressure and the liquid flows. In practice, however, precise control of droplet flow simultaneously along multiple channels is technically challenging; conventional pressure pumps are not feasible for microfluidic systems. Inspired by the potential of finely tunable acoustic-pressure generators, a group of engineers at the University of Michigan set out to control droplet motion with music. First, they composed a computer-synthesized sequence of single notes and chords. That signal was then sent to four resonance cavities that were tuned according to their lengths to extract and amplify narrow, non-overlapping frequency bands from the input tones. As shown in the figure and the movie, unidirectional droplet flow was generated from the difference between positive air pressure in the oscillating cavity and relative negative pressure at vent ports near the cavity's outlet. Although the relatively high frequencies of the selected tones produced steady flow, the researchers adjusted the relative amplitudes of the input tones as needed to compensate for variations in average flow velocity. Maybe someday, conducting complex lab-on-chip microfluidic operations will be as simple as stringing together a few musical notes. (S. M. Langelier et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900043106.) — Jermey N. A. Matthews

Traveling smoothly through a turbulent medium is no mean feat, as anyone who regularly flies in an airplane can attest. Scientists have investigated how fish navigate through turbulent currents, but until recently they had not addressed the analogous issue of animal flight through turbulent air. Now biologist Stacey Combes has filmed male orchid bees (genus Euglossa) flying in turbulent airstreams and, with colleague Robert Dudley, has described the effects of the turbulent air on the bee’s flight stability and maximum speed. Combes induced the bees to fly in a turbulent airstream by luring them with an attractive scent. As the airspeed increased, the bees found it increasingly difficult to avoid the rolling illustrated in the left image. When the airspeed was high enough and maintaining stable flight difficult enough, the bees extended their hind legs, as depicted in the right photograph.

That move increased the moment of inertia about the roll axis by roughly 50% and improved stability, but it also increased body drag and energy expenditure by about 30%. In a second experiment, Combes altered the turbulence of the stream by inserting different geometric grids. Bees flying in the lower-turbulence environment were able to reach higher speeds before instabilities caused them to be ejected from the air stream. (S. A. Combes, R. Dudley, Proc. Natl. Acad. Sci. USA, doi:10.1073/pnas.0902186106.) —Steven K. Blau

Related link: Dragonfly Flight, Z. Jane Wang, Physics Today October 2008, page 74.

In his "Milkdrop Coronet," strobe-photography pioneer Harold Edgerton famously captured the splash produced by a milk droplet falling into a saucer. But our understanding of the underlying physics remains poor. It's known that before a liquid droplet splashes upward from a surface, a thin sheet of liquid spreads out from the impact point. Four years ago experiments by Sidney Nagel and colleagues at the University of Chicago showed, surprisingly, that splashing on a dry surface can be suppressed by reducing the ambient air pressure. The researchers concluded that compressible effects in the air are responsible for the splashing (L. Xu, W. W. Zhang, S. R. Nagel, Phys. Rev. Lett. 94, 184505, 2005). Now Michael Brenner and coworkers at Harvard University have further looked into the air's role in how droplets splash on a dry surface. Taking into account the compressibility and viscosity of the gas and the surface tension of the liquid, they modeled the behavior of the approaching droplet as it reaches the surface. They find that instead of spreading out over the surface, the liquid spreads over a very thin film of air. When the droplet nears the surface, pressure builds beneath it and the bottom of the droplet deforms by flattening and then becoming dimpled. The droplet's bottom perimeter develops a kink that, still over a layer of air, moves out and creates capillary waves. The calculations don't, however, show any indications of splashing; the researchers suggest that other parameters, such as the droplet viscosity and thermal transfer, must become important after the initial spreading phase. (S. Mandre, M. Mani, M. P. Brenner, Phys. Rev. Lett., in press.) — Richard J. Fitzgerald

With their ability to manipulate microliter to nanoliter volumes of liquids, microfluidic devices have found increasing application in a variety of fields, from ink-jet technology to proteomics and DNA analysis. Most current microfluidic devices are made from glass or polymers, and advances in design and fabrication have opened the realm of three-dimensional, complex flow paths. George Whitesides and colleagues at Harvard University have recently demonstrated 3D devices made from stacked layers of ordinary paper and tape. Thanks to paper's wicking ability, the devices don't require external pumps to drive the liquids through. Indeed, the wicking property of paper is routinely exploited in medical tests such as those for blood glucose, pregnancy, and HIV. To define the microfluidic pathways in the paper-based microfluidic device, the team impregnated each paper layer with a common photoresist, a hydrophobic polymer that could be patterned with UV light. With their channels thus established, the layers of paper were alternated with layers of double-sided tape; holes cut in the tape connected channels in adjacent paper layers. The figure illustrates the complex routing that can be achieved: Four differently colored liquids deposited on the top of a 5 cm × 5 cm, nine-layer stack (left) are, within 5 minutes, wicked through horizontally and vertically to the array of 1024 detection zones on the bottom (right). With reagents or antibodies placed in detection zones prior to assembly, such devices would provide highly parallel, independent assays. The Harvard team sees particular potential for their paper-based devices in medical diagnostics in developing countries. (A. W. Martinez, S. T. Phillips, G. M. Whitesides, Proc. Natl. Acad. Sci. USA, in press.) — Richard J. Fitzgerald

At the very moment a droplet of water breaks away from a dripping faucet, a singularity is formed. The dynamics leading up to the singularity are governed by the competition between the water’s inertia and its surface tension. (Water’s viscosity is low enough that it does not play a role.) In the reverse setup—an air bubble breaking away from an underwater nozzle—the pinch-off process is driven instead by the difference in pressure between the air and the water. As a result, the bubble and droplet systems differ both in the shapes formed and in the dependence on time. Now, Justin Burton and Peter Taborek of the University of California, Irvine, have observed both bubble-like and droplet-like behavior in a single continuously variable system: xenon bubbles in water over a range of pressures (and hence xenon densities). At low pressures, xenon bubbles behave like air bubbles, as shown in the top row of the figure. At 68 atmospheres, the highest practical pressure for the system, the xenon bubbles are 70% as dense as water and look like upside-down water droplets, as shown in the bottom row. To quantify the behavior of the Xe–water system, the researchers measured the width of the pinch-off region’s neck as a function of time before pinch off. For water droplets, the neck width is proportional to time to the 2/3 power; for air bubbles, it is proportional to time to the 0.57 power. By that standard, the researchers observed a sharp boundary between the bubble-like and droplet-like regimes at a xenon density that is 25% of the density of water. (J. C. Burton, P. Taborek, Phys. Rev. Lett. 101, 214502, 2008.) — Johanna Miller

During an effusive volcanic eruption—one that produces flowing lava, as shown here, as opposed to projectile material or clouds of ash—civil authorities need to know which way and how far the lava will flow so that they can decide whether and when to order evacuations. But lava is a difficult fluid to model, because as it cools, it crystallizes and eventually stops flowing. Robert Wright and colleagues at the University of Hawaii in Honolulu have developed a new model for forecasting lava flows. Their model combines two previously published ones: FLOWGO, which simulates lava's heat loss to predict how far it will flow before solidifying, and DOWNFLOW, which takes a stochastic approach to predict the lava's direction. Since DOWNFLOW’s stochastic method is computationally simple—but still accurate—Wright and colleagues’ model yields results much more quickly than other forecasting techniques. Moreover, FLOWGO accounts for the effusion rate—the rate of lava coming out of the ground—which strongly affects the flow length and can change substantially over the course of an eruption. When combined with satellite monitoring of the effusion rate, Wright and colleagues’ model can potentially provide updated forecasts in near real time. (R. Wright, H. Garbeil, A. J. L. Harris, Geophys. Res. Lett., in press.) —Johanna L. Miller