September 2010 Archives

Since its introduction in the 1970s, optical trapping has become a mainstay in science research. It has been used to tug at strands of DNA, to levitate aerosols, and to cool atoms to microkelvin temperatures. It has not, however, been very effective at manipulating light-absorbing particles, particularly in air or other gases. That's because conventional traps rely on radiation pressure, the momentum imparted by refracting light, to pull a particle into an optical beam. But photophoretic forces, which result from the imbalanced momentum exchange between an anisotropically heated particle and its surrounding gas, tend to push an absorbing particle away from intense light. Andrei Rode and colleagues at the Australian National University in Canberra have now developed a technique that exploits photophoretic forces to confine and move light-absorbing particles in air. And unlike conventional techniques that manipulate particles over just millimeter distances, theirs could conceivably send particles gliding across an entire room. The key was to use a Laguerre–Gaussian beam, in which a donut-like ring of light surrounds a dark core. A particle that finds its way into the core, such as the 50-µm carbon-coated glass sphere shown here, gets trapped in the transverse (x–y) plane and propelled in the direction of light propagation (z) by photophoretic pushes. Two beams aimed from opposite directions can hold a particle in place, a potentially useful technique to isolate particles such as carbon soot—a suspected global warming contributor—for sensitive measurements. (V. G. Shvedov et al., Phys. Rev. Lett. 105, 118103, 2010.)—Ashley G. Smart

Oxygen in the air and dissolved oxygen in the ocean are crucial for sustaining animal life on Earth. But that oxygen wasn’t always there. Researchers can gain some information about the history of ocean oxygenation from the compositions of sedimentary rocks, because dissolved O₂ affects the solubility and precipitation of certain elements. Now, an international team of researchers led by Tais W. Dahl (University of Southern Denmark) has studied the problem using a new technique based on molybdenum isotopes. In oxygenated water, Mo forms the soluble molybdate ion, MoO₄²⁻, and precipitates in small quantities by a mechanism that favors lighter isotopes. But when hydrogen sulfide is present instead of O₂, all Mo isotopes precipitate quickly. By analyzing the Mo in 180 rock samples with ages spanning almost 2 billion years, the researchers concluded that O₂ levels increased twice: 550 million years ago and 400 million years ago, as shown in the figure. The first increase coincides with the emergence of the first motile animals, and the second coincides with the evolution of much larger animals including large predatory fish. The method does not provide a quantitative measure of O₂ levels over time. Still, based on the physiological requirements of fish, the researchers suggest that prior to 400 million years ago the ocean and atmosphere contained O₂ at just 15–50% of their present levels, which means that the earliest animals survived with much less oxygen than we breathe today. (T. W. Dahl et al., Proc. Natl. Acad. Sci. USA, in press, doi:10.1073/pnas.1011287107.)—Johanna Miller

Messenger RNA (mRNA) is the shuttle that carries genetic information across a cell’s nuclear membrane and into the cytoplasm, where the information is translated into a protein sequence. However, the movement of mRNA, which is about 25 nm in diameter, through the nuclear pore complex, roughly 120 nm in diameter, has been difficult to resolve visually since both mRNA and the NPC are well below the 200-nm diffraction limit for optical microscopes. Now, Robert Singer at Yeshiva University’s Albert Einstein College of Medicine in New York and David Grünwald at the Delft University of Technology in the Netherlands have developed a new nanometer-resolution imaging technique that they used to track mRNA’s passage. Emission signals from mRNA and the NPC—labeled with spectrally different fluorescent probes and shown in the image as green and red, respectively—were chromatically separated and tracked by two synchronized high-speed CCD cameras. The researchers achieved 26-nm spatial resolution by resolving the misalignment between the mRNA and the NPC in the images collected before and during tracking by both cameras—a technique they’ve dubbed super-registration microscopy. In their experiments, they also observed that mRNA spent about 5-20 ms crossing the NPC, a fraction of the time it spends at the pore’s entrance and exit—as if, say the researchers, the mRNA was being double screened for quality. That information may support research into understanding how defective mRNA is prevented from escaping the nucleus. (D. Grünwald, R. H. Singer, Nature, in press, doi:10.1038/nature09438.)—Jermey N. A. Matthews

Related link:

Robert Singer explains super-registration microscopy

Thermodynamics teaches that the efficiency of a heat engine operating between a hot reservoir at temperature T_h and a cold one at T_c can be no greater than the Carnot value η_C = 1 - T_c/T_h. To achieve its theoretical maximum, the engine must run infinitely slowly and generate zero power—surely an unsatisfactory state of affairs in the real world. Now Massimiliano Esposito (Free University of Brussels) and colleagues have derived efficiency bounds for engines operating at maximum power. They assume that the engine operates in a Carnot cycle and interacts with the hot reservoir for a finite time τ_h, presumed much greater than the duration of the adiabatic steps. They then express entropy as a sum of the standard term of heat over temperature and a term of the form a_h/τ_h (the cold reservoir is treated analogously); placing the interaction time in the denominator ensures that the reversible-process result is obtained in the infinite-interaction-time limit. After deriving the maximum power as a function of the interaction times, Esposito and company can readily calculate the efficiency, which depends in particular on a_c/a_h. The figure shows the allowed range of efficiencies at maximum power. The upper bound corresponds to a_c/a_h = 0; the lower bound to an infinite ratio. The points in the figure give observed efficiencies for several heat engines worldwide. Those engines may not satisfy the assumptions of the Esposito model or even run in Carnot cycles; still, their efficiencies lie within or near the idealized bounds. (M. Esposito et al., Phys. Rev. Lett., in press; available at http://arxiv.org/abs/1008.2464.)—Steven K. Blau

An axicon lens is flat on one side, conical on the other. After passing through the flat surface, a beam of light with a Gaussian profile emerges from the conical surface with a profile that looks approximately like a zeroth-order Bessel function. Bessel beams interest physicists because of a remarkable property: Thanks to the way the beams' energy and phase components are spread in space, the beams barely diffract—even around certain obstacles. Now, Florian Fahrbach, Philipp Simon, and Alexander Rohrbach of the University of Freiburg in Germany have demonstrated that Bessel beams can propagate without significant diffraction through inhomogeneous media. The accompanying figure shows the intensity of fluorescence emitted by a transparent fluorescent gel when three kinds of beam passed through from left to right: a sheet-shaped beam from a cylindrical lens, a Gaussian beam scanned from top to bottom, and a Bessel beam scanned from top to bottom. The different patterns arose from the beams' interactions with hundreds of 2-μm-diameter glass beads that the Freiburg researchers had dispersed throughout the gel. As the light sheet and Gaussian beam encountered the beads, their scattered components interfered constructively, leading to bright streaks and therefore to what would amount to image artifacts. The Bessel beam had far less streaking. The researchers obtained similar results with a sample of fresh human skin, whose densely packed cells, like those of other biological tissues, are micron sized. Bessel beams could therefore form the basis of a new kind of biological microscopy. (F. O. Fahrbach, P. Simon, A. Rohrbach, Nat. Photon., in press.)—Charles Day

The recently demonstrated ability to image single atoms in an ultracold quantum gas is welcome news to those who hope to use the relatively clean and more highly controlled atomic systems to simulate complex electronic interactions, such as superconductivity, that take place in solids. The simulations would be based on optical lattices, an array of potential wells that hold atoms in much the same way as the potential wells of a solid’s atomic structure hold electrons. Last year, Markus Greiner and his colleagues at Harvard University built a fluorescent microscope that was a step ahead because it not only could see individual atoms in an optical lattice but could distinguish very closely spaced atoms, which are necessary to simulate the strong electronic interactions that researchers most want to explore. Now, two groups have used such high-resolution microscopes to study a quantum gas of atoms as it transitioned from a superfluid to a Mott insulator (MI) state in which particle interactions hinder hopping between sites. Images such as the accompanying pictures of the superfluid (left) and two types of MI (center, right) give insight not gleaned from less direct measurements. The experiments were done by Greiner and his group and by Stefan Kuhr, Immanuel Bloch, and colleagues at the Max Planck Institute of Quantum Physics in Garching, Germany. (W. S. Bakr et al., Science 329, 547, 2010; J. F. Sherson et al., Nature 467, 68, 2010.)—Barbara Goss Levi

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

Like a batch of cookie dough, Earth is constantly being stirred: Volcanism, tectonic-plate movements, and thermal convection can all transport material between and within the mantle and crust. Even the lower mantle is not immune to mixing: Studies of seismic waves’ speeds (see Physics Today, August 1997, page 17), which measure the temperature profile across Earth’s interior, show slabs of cool, dense oceanic crust sinking nearly to the core–mantle boundary. But now, Boston University’s Matthew Jackson and collaborators have found that rock samples from Baffin Island in northeastern Canada, the products of an enormous volcanic eruption some 60 million years ago, show all the signs of having come from an ancient mantle reservoir, unstirred since not long after Earth’s formation 4.5 billion years earlier. The researchers based their conclusion on isotopic measurements of several chemical elements, each with some isotopes produced by radioactivity and others that are purely primordial. If any mixing had occurred, it would have changed the relative amounts of a radioactive parent element and its daughter—and thus the daughter’s present-day isotopic composition, which is easier to measure than the parent–daughter ratio. If the Baffin samples did indeed come from a primitive reservoir, geophysical modelers will be challenged to explain how any part of the mantle could have remained so isolated for so long; one possible solution involves convective eddies that trap and preserve the pristine mantle material. (M. G. Jackson et al., Nature 466, 853, 2010.)—Johanna Miller