RESEARCH/RESEARCHERS

solution (2–3 mol%) for about 5 min, fol-lowed by immersions in a diluted solution(0.5–1 mol%) for 10 min. In this way, theresearchers achieved a 16-layer woodpilePC with an in-plane spacing of 1 μm and alayer spacing of 1.2 μm, built up bysmooth rods, 150 nm in thickness, andexcellent mechanical strength. The resolu-tion limit of the direct-write fabricationprocess and multi-step etching process isbeyond the diffraction limit and compara-ble to the fabrication limit achievable inphotosensitive polymers. The researchersattributed this resolution limit to thethreshold of the fabrication technique,combined with the multi-step etchingprocess. Due to the ellipsoidal shape ofeach rod and the geometry of the wood-pile structure, no complete bandgap couldbe achieved, and the researchers onlyobserved partial bandgaps along thestacking direction.

According to calculations produced bythe researchers, the fabrication processshould produce two pronounced dips inthe transmission spectrum, a first-orderstop gap at a normalized frequency of0.28–0.29 and 74% suppression ratio intransmission, and a higher order gap at afrequency of 0.40–0.41 and 25% suppres-sion ratio. The measured optical responseof the woodpile PC is in good agreementwith the results of the calculations.Further calculations performed by the

researchers revealed that these bandgapscould be engineered and tuned by eitherchanging the filling ratio or the lattice con-stant of the PCs, which they confirmedexperimentally.

The researchers consider that the use ofsuch higher order bandgaps “provide aneffective alternative to achieve photonicbandgaps in the telecommunication wave-length region, circumventing the need forstructural miniaturization of the PCs.”

JOAN J. CARVAJAL

Materials Design Principles fromAncient Armored Fish Give Cluesfor Improved EngineeredBiomimetic Structural Materials

Dermal armor in fish first appeared ~500million years ago in the Paleozoic period.As ancient fish became more predatory,armor design evolved incorporating small-er plates instead of larger plates and fewer,thinner layers replacing multiple, thick lay-ers resulting in a lighter weight, balancingprotection with mobility for maximumsurvival. B. Bruet, J. Song, M. Boyce, andC. Ortiz of the Massachu setts Institute ofTechnology have used nanoindentationand finite element analysis simulations touncover structure–property–function rela-tionships in individual scales of a modelspecies of armored fish, Polypterussenegalus. The materials design principlesof P. senegalus fish armor in the context of

their primary environmental threat, pene-trating bite attacks, and mechanically pro-tective function, could lead to better bio-inspired engineered designs for humanbody armor, according to the researchers.

In the September 2008 issue of NatureMaterials (DOI: 10.1038/nmat2231 p. 748),the researchers reported on a multiscaleexperimental and computational approachto examine multilayering and gradationwithin individual scales from P. senegalus.These freshwater fish, also known as theGray bichir or Senegal bichir, appearedabout 96 million years ago but still retaincharacteristics of their ancient predeces-sors. The researchers reported that P. sene-galus scales consist of four organic–inorganic nanocomposite layers: ganoine(~10 μm thick), dentine (~50 μm thick),isopedine (~40 μm thick), and a bone basalplate (~300 μm thick), from the outer toinner surface, respectively. Penetration-resistance and elastic and plastic mechani-cal properties of the layers were examinedspatially on individual scales by nano -indentation.

One overlying mechanical design themerevealed by these experiments was the jux -ta position of multiple distinct reinforcinglayers each with its own unique deforma-tion and energy dissipation mechanisms.Multiple cross-sectional indents revealed adecrease in Oliver–Pharr indentationmodulus and hardness from outer to inner

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scale surfaces. Observed mechanical prop-erties correlated with the known composi-tion of each layer. The researchers con-cluded that layer thickness, grading ofmechanical properties within and betweenlayers, as well as material layer sequencewithin individual scales of P. senegalus arephysiologically important for abating pen-etration and facilitating protection whileyielding biomechanical flexibility andmaneuverability of the entire fish.Materials design principles learned fromthese “living fossils” may hold promise forimproved design of engineered biomimet-ic structural materials, such as protectivearmor for humans or military vehicleapplications, said the researchers.

SAMESHA R. BARNES

Spatial Light Scattering RevealsNew Information on AtmosphericIce Crystals

Uncertainty about the future of Earth’sclimate has opened avenues of researchinto the elements shaping the climate.Meteorological evidence suggests thatthere are correlations between cloud prop-erties and climate change. Clouds, com-prised mainly of water vapor, droplets,and ice crystals, both absorb heat andreflect radiation from the sun. However,the magnitude of their ability to alter cli-mate is unknown. What is known is thatthe size, shape, and number of particles

present in the cloud determines its radia-tive properties. Currently, analysis of thesefeatures is frequently performed by instru-ments such as the Cloud Particle Imager(CPI), which uses a pulsing laser to cap-ture real images of ice crystals on a CCDcamera. However, the CPI is limited toimaging particles greater than ~25 μm.The use of the device described in thisstudy will allow data on particle size,shape, and quantity to be acquired fromparticles approaching 1 μm.

P.H. Kaye and colleagues at the Uni ver -sity of Hertfordshire, P.J. DeMott ofColorado State University, and C. Saundersand colleagues at the Uni ver sity of Man -chester have used a light scatteringapproach to analyze atmospheric ice crys-tals. In the July 1 issue of Optics Letters(DOI:10.1364/OL.33.001545; p. 1545), theresearchers described the design and test-ing of a novel instrument that can capturehigh-resolution light scattering patternsfor sizes well below the resolution of con-ventional cloud imaging probes. Theinstrument is composed of a temperature-controlled light scattering chamber with atapered aerosol inlet tube and a vent, anintensified CCD camera, a particle detec-tion trigger photomultiplier detector, apellicle beam splitter, and a frequency-doubled NdYAG laser. Air flows throughthe inlet tube at 80 m/s and exits into thescattering chamber with a flow diameter

of 2.5 mm, where it crosses the laser beam.The pattern of light scattered by individ-ual particles carried in the flow is thencaptured by the optical system, with asmall percentage of light being directed bythe pellicle beam splitter onto the detectorand the remainder directed to the ICCDcamera. Particle size is determined by themagnitude of the signal produced by thephotomultiplier detector, and the samesignal is used to trigger the ICCD camerato record a high-resolution image of theparticle’s scattering pattern. This wasdemonstrated in preliminary experiments,using the University of Manchester IcingCloud Chamber where ice crystals rangingfrom 3.5 μm to 22 μm in diameter wereobserved for a range of temperatures andhumidity. The ice crystals displayed scat-tering typical of the hexagonal symmetryfound in ice. Interpretation of the scatter-ing patterns is required to precisely deter-mine the shape of the crystals. Currently,for less complex crystal shapes, under-standing of crystal morphology isachieved by inversion of scattering pat-terns using theoretical scattering modelsor by comparison with scattering patternsfrom known crystal shapes. Future pro -gress in modeling will allow for increasedaccuracy of the interpretation of scatteringpatterns for complex crystal shapes. Theresearch in this study was able to uncoverinformation on the morphology and size

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