Recently in Microstructures and nanostructures Category

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

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

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

Late Stone Age metal smiths added a little tin to copper to usher in the eponymous Bronze Age; over the ensuing five millennia, many new combinations and applications of the two metals have appeared. Today, for example, a thin tin coating on a copper substrate often serves to interconnect electronic components of various kinds, such as are found in medical devices and satellite equipment. Unfortunately, micron-sized tin whiskers (see figure) sometimes arise spontaneously and can short out the equipment, with great technological and economic repercussions. After decades of widespread effort, the actual mechanism underlying such whisker growth has only now been elucidated. Led by Eric Mittemeijer, a group from the Max Planck Institute for Metals Research in Stuttgart, Germany, working with the Robert Bosch company, examined growing whiskers and their crystallographic environment. Using Laue diffraction measurements made at the Advanced Photon Source at Argonne National Laboratory in Illinois, the researchers noted that at the Cu–Sn interface, Cu₆Sn₅ develops along the tin grain boundaries and is most pronounced directly beneath a whisker's root. That observation, coupled with residual strain measurements, led the team to propose the following mechanism: Deep penetration of Cu₆Sn₅ into the 3-μm-thick tin layer induces in-plane compressive strains near the Cu–Sn interface and in-plane tensile strains nearer the surface. Out-of-plane and in-plane strain gradients—not the strains themselves—then provide the driving force that leads to whisker growth by transporting Sn atoms to the whisker nucleation site as a strain-relief mechanism. (M. Sobiech et al., Appl. Phys. Lett., in press.) —Stephen G. Benka

Before they form snowflakes and other hexagonal crystals, water molecules nucleate in smaller configurations. Determining the structure of those precursors—even in the outwardly simple case of water on a clean metal surface—is an area of ongoing interest and controversy. For example, at submonolayer coverage on a copper (110) surface, water molecules form chains that can grow to many tens of nanometers in length but are just 1 nm wide. The chains’ structure has been a mystery, since no arrangement of water molecules into hexagonal units entirely explains the experimental data. Now, Andrew Hodgson and colleagues of the University of Liverpool in the UK have collaborated with Angelos Michaelides’ group at University College London to find the structure. Michaelides and postdoc Javier Carrasco ran calculations on some 50 possible chain structures; they found that the most energetically stable one also gave the best fit to the Liverpool group’s high-resolution scanning tunneling microscopy images (as shown in the top panel) and vibrational spectra. That structure (bottom panel) is an arrangement of pentagons, not hexagons. The water molecules shown in red and yellow are perpendicular to the Cu surface—the hydrogen atoms pointing up are responsible for the bright spots in the STM images, and the ones pointing down (not visible in the figure) interact with the Cu atoms. The researchers suggest that nonhexagon arrangements might be involved at other water–metal interfaces where the structure of water is unknown. (J. Carrasco et al., Nat. Mater., doi:10.1038/nmat2403.) — Johanna Miller

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

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

Inorganic crystal aggregates known as biomorphs earn their name by virtue of a remarkable resemblance to the fossils of primitive organisms. But although the structures can be varied and complex—leaflike sheets, wormy ropes, and helical filaments, among others—biomorphs are exceedingly simple to make. They self assemble when an alkaline earth halide such as barium chloride is mixed with a silica-rich solution under high pH conditions at ambient pressure and temperature. As carbon dioxide from the air dissolves into solution, barium carbonate and silica precipitate out and produce the complex structures. A long-standing question is how. Juan Manuel García-Ruiz, his postdoc Emilio Melero-García (both at the University of Granada), and Stephen Hyde (Australian National University) now propose a mechanism for the morphogenesis. As the carbonate crystallizes, it lowers the pH of the local environment and triggers the precipitation of silica. The silica precipitation, in turn, raises the local pH, which prompts another round of carbonate formation. The sensitivity of silica and carbonate species to opposite trends in pH fluctuation creates a chemical feedback that eventually produces rodlike, carbonate particles, each coated with silica. Freed from the hexagonal symmetry restrictions imposed by carbonate growth, the silica-coated nanoparticles form clusters that can adopt various shapes. For reasons unexplained by their mechanism, the clusters align themselves on the micron scale and grow as two-dimensional sheets. The edges of those sheets can then curl like a scroll to create the sort of curved and twisted filaments captured here by optical microscopy. (J. M. García-Ruiz, E. Melero-García, S. T. Hyde, Science 323, 362, 2009.) — R. Mark Wilson

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

In typical loudspeakers, a coil surrounds the apex of a flexible cone; when a varying current flows through the coil, the cone moves toward and away from a fixed permanent magnet and produces pressure waves we hear as sound. But researchers from Tsinghua University and Beijing Normal University have demonstrated a radically simpler loudspeaker design based on nanotubes: They showed that a thin film of nanotubes can reproduce sounds over a wide frequency range--including the full human audible range--with high sound pressure level, low total harmonic distortion, and no magnets. The team created the film by drawing nanotubes from a so-called superaligned array grown on a wafer, a technique the group introduced six years ago (see also PHYSICS TODAY, October 2005, page 23). The resulting film, only tens of nanometers thick but up to 10 cm wide, is transparent and has a nearly purely resistive impedance. When electrodes are placed along its ends and an alternating current is applied, the film produces clear tones that can be as loud as a conventional speaker. Moreover, since the film is flexible, the nanotube speaker can be configured into arbitrary shapes or mounted onto curved substrates; the figure shows an omnidirectional cylindrical loudspeaker 9 cm in diameter and 8.5 cm high. The film can even be stretched with essentially no degradation of the sound reproduction. The researchers attribute the sound generation not to vibration but to a thermoacoustic effect first proposed nearly a century ago: Thanks to the nanotube film's extremely low heat capacity per unit area, changes in the current flowing through the film are reflected in the film's temperature, and those temperature changes excite pressure waves in the surrounding air. The mechanism is independent of the sign of the current, which leads to a frequency doubling of the input signal, but that drawback can be overcome by applying a constant current bias. The movie shows a nanotube loudspeaker being periodically stretched with almost no noticeable effect on the sound intensity. (L. Xiao et al., Nano Lett., in press, doi:10.1021/nl802750z.) -- Richard J. Fitzgerald