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The phenomenon of dynamic stabilization can be demonstrated with an inverted pendulum: If the pivot point vibrates fast enough and strongly enough, the pendulum aligns with the vibration direction and can stably stand upside down, even at an angle, seeming to defy gravity. Physicists Greg Swift and Scott Backhaus (Los Alamos National Laboratory) looked at an analogous situation with gas in a so-called pulse tube that has one end much hotter than the other. Colder gas is denser and therefore sinks below the hotter gas; a vertical tube with the cold end down is like an undisturbed pendulum with the heavy bob at the bottom. However, raise the cold end above the hot end and convection sets in—the cold gas falls due to gravity and the hot gas rises in a natural convective flow. Such orientation-dependent effects are undesirable for cryogenic thermoacoustic pulse-tube refrigerators, like the commercial one shown here, in which the gas is used to transmit acoustic power but not heat. (For more on thermoacoustics, see Physics Today, July 1005, page 22.) Swift and Backhaus found that suppression of convection when these refrigerators run at high enough frequency and amplitude is related to the well-understood stabilization of the inverted pendulum. Although their experiments and theoretical analysis are beginning to unravel the essentially nonlinear physics at the core of the system, many mysteries remain, including the actual role of the oscillating pressure. (G. W. Swift, S. Backhaus, J. Acoust. Soc. Am. 126, 2273, 2009.) —Stephen G. Benka

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

Familiar software-based “random number generators” rely on deterministic algorithms, so their outputs are not actually random. For some applications, such as Monte Carlo simulations or randomized music playlists, that’s not a problem. But for others, such as secure communications or online gaming, it’s important to use numbers that are truly unguessable, such as can be generated from measurements of stochastic physical processes. One candidate is the chaotic output of a semiconductor laser: When a certain fraction of laser light is fed back into the laser, small intensity fluctuations (thought to be quantum in origin) are amplified into large, irregular, subnanosecond oscillations, as shown in the figure. Last year, Atsushi Uchida (Takushoku University, Tokyo) and colleagues used digitized measurements of two semiconductor lasers to generate random binary sequences at a rate of 1.7 billion bits per second—much faster than any previous method based on a physical system (A. Uchida et al., Nat. Photonics 2, 728, 2008). Small drifts in the lasers’ average intensities can throw off the balance of ones and zeros, but with the lasers suitably tuned, the sequences meet statistical criteria for randomness. Now, Michael Rosenbluh, Ido Kanter, and their students at Bar-Ilan University in Ramat-Gan, Israel, have developed a new method, using just one laser, that avoids the problem of intensity drift. From the differences between successive intensity measurements, they can generate 12.5 billion random bits per second. (I. Reidler et al., Phys. Rev. Lett., in press.) —Johanna Miller

The unusual stiffness or sponginess of dead and decaying biological tissue is readily apparent to the human touch. However, early detection of such mechanical property changes in a tissue's extracellular matrix could signal the onset of disease. To measure the elasticity of tissue in living patients, needle-based indentation methods are more direct and less expensive alternatives to MRI, ultrasound, and electrical impedance. Such a probe has recently been developed by University of California, Santa Barbara, physicist Paul Hansma, an atomic force microscopy expert, and his collaborators. The handheld tissue diagnostic instrument (TDI) consists of a stainless steel probe, 175 µm to 1 mm in diameter depending on the tissue sample, which longitudinally oscillates at 4 Hz in a needle-thin stationary sheath. The force from the magnetically controlled oscillation of the probe produces a corresponding displacement in the tissue. The tissue's elastic modulus, or stiffness, is proportional to the slope of the force-displacement curve, and energy dissipation in the tissue is proportional to the area under that curve. The researchers measured, with millimeter spatial resolution, healthy and diseased tissue samples ranging in elastic moduli from around 1 kPa to 12 GPa. Among them were mouse breast tissue, which hardens when it becomes tumorous, and human tooth dentin (see schematic), which softens and decays when infection sets in. The researchers say the instrument could be used in the future to simultaneously test and biopsy a tumor or, if the probe is coated with antibodies, to measure single-molecule interaction forces. (P. Hansma et al., Rev. Sci. Instrum., in press.) — Jermey N. A. Matthews

Lightness, strength, and moldability are among the most desired material properties for aircraft, sporting equipment, and many structural applications. Those sometimes opposing properties converge in bulk metallic glasses—supercooled amorphous metal alloys that can be cast into complex shapes and are resilient under large elastic strains. However, their toughness is suspect: Under repeated stress, BMGs fatigue and develop fatal cracks much more quickly than crystalline metal alloys do. To control crack propagation, Caltech's William Johnson, Lawrence Berkeley National Laboratory's Robert Ritchie, and their collaborators focused on controlling the microstructure of a particularly tough BMG composite made of zirconium, titanium, and other metals. Its fingerlike crystalline dendrites (67% by volume) are surrounded by an amorphous matrix, as seen in this optical micrograph. By heating the precursor alloys between their melting points then rapidly quenching the solution, the researchers were able to control dendrite size and the spacing between the glassy and crystalline phases. The width of the glassy region between the much tougher dendrite fingers was tailored to be short enough to serve as a "microstructural arrest barrier" for just-formed cracks. Compared with existing dendrite-containing BMGs, the new material holds up under three times more stress cycles and is comparable in toughness to high-strength steel or aluminum. (M. E. Launey et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.0900740106.) — Jermey N. A. Matthews

Today's laptop computers, cell phones, hybrid vehicles, and other technologies rely on rechargeable batteries. As discussed in Physics Today, December 2008, page 43, batteries— in particular, the popular lithium-ion batteries— typically have a high energy density but a low power density: They can't deliver their stored energy particularly quickly. Often the limiting step in Li⁺ batteries is not getting the ions through the electrolyte and electrode structure but getting them into the active electrode material itself. Using nanoscale materials in the electrodes and doping the materials are among present techniques to improve battery rates. Now Byoungwoo Kang and Gerbrand Ceder of MIT have shown that using particles of a common electrode material, lithium iron phosphate (LiFePO₄), covered with a glassy coating of iron-doped lithium phosphate can significantly increase the charging and discharging rates. Moreover, the particles and coating can be formed together in a single step. In test experiments, the researchers obtained discharge rates 100 times as fast as today's commercial Li⁺ batteries. The researchers suggest that the amorphous coating may improve Li⁺ transport across the surface of the electrode particles; uncoated LiFePO₄, in contrast, conducts ions poorly except in a narrow range of directions. Additionally, they say that the coating may modify the surface potential and provide adsorption sites for a range of ion energies. (B. Kang, G. Ceder, Nature 458, 190, 2009.) — Richard J. Fitzgerald

MRI excels at revealing subtle features in soft tissue. Hydrogen nuclei are detected through the electromotive force induced in a nearby coil when their spins flip from an RF pulse. Typically, one coil transmits the pulse and another detects the induced signal. That configuration has been used in clinical settings for decades with imagers built from 1.5 T magnets. In recent years, imagers have been developed with greater field strength to boost sensitivity. But as the field increases, so does the resonance frequency required to excite nuclei. The corresponding wavelength in tissue in a 7-T magnet is about 12 cm, on par with the size of resonator coils that encircle a human head. The result: interference and standing-wave RF patterns. Those inhomogeneities in the RF field are disastrous because they perturb the image contrast between different types of tissue. A group led by Klaas Pruessmann at ETH Zürich has now solved the problem by removing the RF coils entirely and using the conductive lining in a 7-T MRI cavity as a waveguide with an antenna placed at one end. A patient inside the waveguide is exposed to a homogeneous traveling RF wave launched from the same antenna that subsequently detects the spin signals. The in vivo images of a human leg demonstrate that traveling-wave MRI (left) can excite spins more uniformly than can inductive MRI (right). The researchers speculate that, with the tight-fitting induction coils gone, patients may suffer less from claustrophobia and engineers may enjoy more design freedom. (D. O. Brunner, N. De Zanche, J. Frölich, J. Paska, K. P. Pruessmann, Nature 457, 994, 2009.) — R. Mark Wilson

The several approaches being pursued for fuel cells vary in their chemical reactions, materials, and optimal operating conditions, but they share a basic configuration (see Physics Today, November 1994, page 54, and October 2006, page 38). A fuel, often hydrogen, is oxidized at the anode, where it liberates electrons. The electrons travel through and power an external circuit and eventually reach the cathode, where oxygen is reduced. Meanwhile, to complete the redox reaction, ions travel through an electrolyte that separates the electrodes. In alkaline fuel cells, first developed for the Apollo space missions in the 1960s, oxygen combines with water and electrons at the cathode to form hydroxyl ions (OH^-) that travel through an aqueous alkaline electrolyte to produce water by combining with protons from the anode. Commercialization of those fuel cells, however, has been limited by the high cost of the platinum used for the cathode. New work by Liming Dai of the University of Dayton and colleagues at the Air Force Research Laboratory and the University of Akron has shown that vertically aligned carbon nanotubes doped with nitrogen provide an efficient, lower-cost alternative for the cathode. Nitrogen-doped nanotubes have better long-term stability and, unlike Pt, are not harmed by the presence of carbon monoxide or any fuel molecules that cross the electrolyte from the anode to the cathode. The researchers attribute the catalytic performance to the relatively high positive charge density on the carbon atoms adjacent to the nitrogen atoms. (K. Gong et al., Science 323, 760, 2009.) — Richard J. Fitzgerald

To map molecules in cells and tissue, researchers prefer biomedical imaging techniques that rely solely on the intrinsic responses of chemical bonds to optical stimulation. Although fluorescence microscopy and other chemical tagging methods yield high-resolution images, they also introduce foreign species or synthetic derivatives that can alter the dynamics of intracellular processes. Spontaneous Raman scattering, which uses a single laser beam to excite the vibrational and rotational modes in chemical bonds, requires no chemical labels but generates a weak signal that gets muddled by Rayleigh scattering. A more sensitive technique known as coherent anti-Stokes Raman scattering uses multiple laser beams to generate coherent optical signals that enhance resonant frequencies in the sample; that method, however, also produces nonresonant background noise. Recently a team led by Harvard University chemist Sunney Xie demonstrated a new technique based on stimulated Raman scattering that tunes the difference between the frequencies of two laser beams to match a desired molecule's resonant frequency, thus amplifying the Raman signal. The measurable intensities of the transmitted beams change only when a match occurs; nonresonant signals are not picked up. The images show the top view (a) and the depth profile (b) of an acne medication (blue) that penetrated a mouse's skin, thus demonstrating the potential of the new technique to monitor drug delivery. (C. W. Freudiger et al., Science 322, 1857, 2008.)
— Jermey N. A. Matthews

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