Physics Update

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

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Loud, fast, rhythmic music is exciting. It mirrors aspects of human behavior—we are loud and active when excited. Likewise slow, soft, static music mimics how we behave when subdued. That congruence might explain why music affects us as it does. Also, melodies in a major key generally come across as happy; minor melodies are sad. But why those tonalities have their particular emotive effects is not clear. Now, Duke University neuroscientist Dale Purves, his graduate student Daniel Bowling, and colleagues report that qualities of major- or minor-key melodies also mirror human behavior—specifically, speech—according to the mood of the speaker. One of their analyses focuses on the intervals (tone pairs) implied by melodies. Major-key melodies emphasize the major-third interval, whose two notes have a frequency ratio of about 5:4. Minor-key melodies feature an interval, the minor third, with a 6:5 frequency ratio. How is that interval dichotomy mirrored in speech? The Duke team asked 20 subjects to read a word or short passage in an excited or subdued manner (click the figure below to enlarge). They then analyzed the ratios of the two lowest (and strongest) frequencies of vocal-tract resonance associated with each vowel sound. The prevalence of major-third intervals (5:4 ratios) as compared with minor thirds (6:5) was much greater in excited than in subdued speech. Musically speaking, at least, our thrills are major; our disappointments, minor. (D. L. Bowling et al., J. Acoust. Soc. Am., in press.) —Steven K. Blau

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

Our historical record of seismic activity is very short, by geological time scales. So extrapolating that record to predict future earthquakes can lead to nasty surprises, such as 2008's devastating earthquake in Sichuan, China, which occurred on a fault that had seen little recent activity. Large earthquakes are typically followed by aftershocks whose frequency decays to some background level of seismicity, following an empirical relation known as Omori's law. But determining the time scale of the decay and the baseline activity can be difficult. A new model by Seth Stein of Northwestern University and Mian Liu of the University of Missouri–Columbia posits an inverse relationship between the aftershock-sequence durations and the slip rates along faults. Large earthquakes are most common along the boundaries of tectonic plates, and the occurrences of aftershocks tend to decay quickly—within a decade or so—to a relatively high background. The relative plate motion at such boundaries can be rapid, faster than 10 mm/yr. Continental interiors, far away from plate boundaries, deform much more slowly, typically less than 1 mm/yr. And thanks to that slower rate of fault loading, aftershocks can last hundreds of years or longer, as shown in the figure. Thus, warn the researchers, interpreting continental earthquakes as steady-state seismicity can overestimate the hazard in presently active areas and underestimate it elsewhere. (S. Stein, M. Liu, Nature 462, 87, 2009.)—Richard J. Fitzgerald

In the absence of an applied voltage, an induced electrical current rapidly decays thanks to the scattering of electrons from defects, phonons, and each other. But in a cold metal ring smaller than the electron’s coherence length, it’s possible to induce a dissipationless current, even if the metal is not superconducting. The trick, theorists predicted in the early 1980s, is to thread the ring with a magnetic field, which breaks time-reversal symmetry. The current is revealed only by its magnetic moment μ. And although researchers confirmed the effect early on, mostly using superconducting quantum interference devices (SQUIDs), complete agreement between theory and experiment, and even among experiments, has remained elusive. Jack Harris and colleagues from Yale University and the Free University of Berlin have now developed an elegantly simple alternative measurement scheme. The team deposited aluminum rings on a cantilever whose vibration frequency can be precisely monitored. In a magnetic field B, each ring’s current produces a torque τ = μ × B, recorded as a shift in the cantilever’s resonance frequency of vibration. From that frequency shift, the researchers deduce the current with a precision two orders of magnitude greater than is possible using SQUIDs. For a magnetic flux threading the ring, the current exhibits an Aharonov–Bohm effect, measurable as oscillations, shown here, whose period corresponds to the addition of one flux quantum h/e through the ring. In experiments taken over a broad range of fields, temperatures, and ring sizes, Harris and coworkers find perfect agreement with a noninteracting electron model. (A. C. Bleszynski-Jayich et al., Science 326, 272, 2009.)—R. Mark Wilson

Stretched out completely, a human chromosome would be several centimeters long. It is packed, along with its 45 companions, into a few-microns-wide cell nucleus in such a way that all the necessary genes are accessible to RNA transcription. Figuring out how that packing is done is no easy task. Microscopy helps, but provides nowhere near a complete picture. Now a research team led by Eric Lander of MIT and Job Dekker of the University of Massachusetts has developed a method for probing chromosomes’ folded structures. The researchers chemically join segments of a folded chromosome that are close in space, cut away and sequence the DNA around the crosslink, and compare those sequences to genome libraries to determine which parts of the chromosome are in contact. A matrix of the observed contacts, as shown in the figure, reveals large-scale organization. Analyzing the plaid pattern, the researchers found that most of the cell’s actively transcribed DNA was spatially segregated from most of the inactive DNA. On a smaller scale, chromosome segments a millimeter or so in extended length appeared to form so-called fractal globules with self-similar structures very different from that of a tangled polymer in equilibrium. So far, the researchers have studied only cultured cell lines: one derived from a tumor and another modified by a virus. They hope to apply their method to healthy cells and to look for differences in chromosome structure among cells of different types. (E. Lieberman-Aiden et al., Science 326, 289, 2009.) —Johanna Miller

The intensity gradients of inhomogeneous laser-light fields impose ponderomotive forces on charged particles. Such forces have been used to trap and manipulate ions, diffract electrons, and generate charge waves in plasmas. But they were thought to act only very weakly on neutral atoms—having to rely on the polarizability of an atom’s charge distribution. Now, however, a group at the Max Born Institute in Berlin has reported the use of intense ultrashort laser pulses to accelerate neutral helium atoms for about 100 femtoseconds at 10¹⁵ m/s². That’s eight orders of magnitude greater than the acceleration (or deceleration) one can get with the continuous-wave techniques used in laser cooling of neutral atoms. The ponderomotive force of an inhomogeneous light field pushes a charged particle toward lower light intensity with a force proportional to the square of its charge—irrespective of sign—and inversely proportional to its mass. The Berlin group argues that the strong laser pulse excites an electron to the outer reaches of the helium atom where it “quivers” in the oscillating light field and experiences the ponderomotive force almost as a free electron would. But still bound to the atom’s ionic core, it tugs the much heavier core with it away from the laser beam’s focus. The figure shows how the maximum velocity thus acquired by neutral atoms in the Berlin experiment increases with pulse duration. The dashed curves show the theoretical expectation for the group’s model of electron excitation and the consequent ponderomotive force. Such “ultrastrong” acceleration of neutral atoms, they suggest, could be exploited for atomic-beam optics, atom deposition, and controlled chemical reactions. (U. Eichmann et al., Nature 461, 1261, 2009.) —Bertram Schwarzschild

Photonic devices that can detect and control the polarization of light across a range of wavelengths are rare. More common are materials such as quartz that can be made into monochromatic optical retarders, which through their intrinsic birefringence convert a specific wavelength of linearly polarized light into circularly polarized light, or vice versa. Some multilayered thin films exhibit achromatic retardation through fabricated periodic nanoscale structures that effectively combine the dispersive properties of each layer to achieve wavelength-independent birefringence. But engineering nanoscale structures is tricky, and even the best synthetic achromatic retarders perform poorly across the full visible range, varying by as much as 9.1°. But Nature has already solved the puzzle in animals that have evolved biophotonic structures for signaling, vision, and coloration (see Physics Today, January 2004, page 18). Now, an international team of researchers from the UK, Australia, and the US has discovered a near-ideal achromatic retarder in the eyes of the colorful peacock mantis shrimp, Odontodactylus scyllarus, shown in the image. This mantis shrimp’s biophotonic retarder is the R8 photoreceptor cell—a UV-photopigment-filled lipid bundle with critical radii of 26 nm and 40 nm, which are subwavelength for visible light. When subjected to linearly polarized light, the R8 cell acted as a quarter-wave retarder, converting the incident light to circularly polarized light, as confirmed by close experimental agreement with theoretically determined Stokes parameter values. Moreover, the extent of retardation varied by only 2.7° across the visible spectrum. (N. W. Roberts et al., Nat. Photonics, in press, doi:10.1038/nphoton.2009.189. Image courtesy of Roy Caldwell, University of California Berkeley.)—Jermey N. A. Matthews

With political sentiment growing in favor of greenhouse gas (GHG) restrictions, biofuels from plant cellulose are being considered among the alternatives to fossil fuels: Plants are renewable and biodegradable, and they sequester carbon. Yet a new report validates concerns that a global biofuels program could put intense pressure on land supply and distribution. To predict the impact of a biofuels-based economy on climate change, an international team of researchers from the US, Brazil, and China linked an economic model of land use with a terrestrial biogeochemical model of global GHG levels. The team considered two cases for cellulosic biofuel crop growth: The primary focus of case1 is on converting unfarmed areas such as forests, as shown in the image; of case 2, on exploiting existing farmland to the extent possible. In both cases, biofuel feedstock becomes a dominant global crop by year 2100, but in the process, total forest area is cut—by 56% in case 1 and by 24% in case 2. The loss of carbon-sequestering trees in case 1 results in a net release of carbon. In case 2, the gains from biofuel production ultimately lead to increased carbon sequestration in the farmed soil from the addition of nitrogen fertilizer, which paradoxically releases N₂O, another potent GHG. The research suggests that stabilizing GHG levels will require a limited and more efficient use of forests and fertilizers for biofuel crop production. (J. M. Melillo et al., Science Express, 22 October 2009, doi:10.1126/science.1180251. Image courtesy of Chris Neill, Marine Biological Laboratory, Woods Hole, MA.)—Jermey N. A. Matthews

A common step in industrial cooling processes is the liquefaction of a vapor on a condenser. If, however, a liquid film forms on the condenser, the cooling may be compromised. The problem can be addressed by coating the condenser with a hydrophobic material conducive to drop formation and then letting the drops slide off due to gravity. Now Chuan-Hua Chen of Duke University and his student Jonathan Boreyko report a different approach. By depositing carbon nanotubes on silicon micropillars and coating both with hexadecanethiol (C₁₆H₃₄S), they engineered a rough “superhydrophobic” surface. The water drops that condensed on it were about a hundred times smaller than those on a conventional hydrophobic surface that the Duke team considered as a standard; the surface roughening offers the promise for more efficient cooling. Furthermore, as the figure and video show, when two sufficiently large drops coalesce into a single drop, that drop literally springs off the condenser—no external prompting needed.

The post-combination drop has less surface energy than do the two drops from which it forms. Most of the released surface energy is dissipated, but Chen and Boreyko observed that the vertical component of the drop’s velocity can be as much as one-sixth of the theoretical maximum. Nature has her own version of the jumping trick. Coalescence of a wet portion of a spore with a dew drop provides the energy for spore ejection in certain mushrooms. (J. B. Boreyko, C.-H. Chen, Phys. Rev. Lett., in press.) —Steven K. Blau