Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session V4: Biological Nanostructures for Photonics and Adhesion |
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Sponsoring Units: DPOLY Chair: Mohan Srinivasrao, Center for Advnaced Research on Optical Microscopy Room: Oregon Ballroom 204 |
Thursday, March 18, 2010 8:00AM - 8:36AM |
V4.00001: Butterfly wing coloration studied with a novel imaging scatterometer Invited Speaker: Animal coloration functions for display or camouflage. Notably insects provide numerous examples of a rich variety of the applied optical mechanisms. For instance, many butterflies feature a distinct dichromatism, that is, the wing coloration of the male and the female differ substantially. The male Brimstone, \textit{Gonepteryx rhamni}, has yellow wings that are strongly UV iridescent, but the female has white wings with low reflectance in the UV and a high reflectance in the visible wavelength range. In the Small White cabbage butterfly, \textit{Pieris rapae crucivora}, the wing reflectance of the male is low in the UV and high at visible wavelengths, whereas the wing reflectance of the female is higher in the UV and lower in the visible. Pierid butterflies apply nanosized, strongly scattering beads to achieve their bright coloration. The male Pipevine Swallowtail butterfly, \textit{Battus philenor}, has dorsal wings with scales functioning as thin film gratings that exhibit polarized iridescence; the dorsal wings of the female are matte black. The polarized iridescence probably functions in intraspecific, sexual signaling, as has been demonstrated in \textit{Heliconius} butterflies. An example of camouflage is the Green Hairstreak butterfly, \textit{Callophrys rubi}, where photonic crystal domains exist in the ventral wing scales, resulting in a matte green color that well matches the color of plant leaves. The spectral reflection and polarization characteristics of biological tissues can be rapidly and with unprecedented detail assessed with a novel imaging scatterometer-spectrophotometer, built around an elliptical mirror [1]. Examples of butterfly and damselfly wings, bird feathers, and beetle cuticle will be presented. \\[4pt] [1] D.G. Stavenga, H.L. Leertouwer, P. Pirih, M.F. Wehling, Optics Express 17, 193-202 (2009) [Preview Abstract] |
Thursday, March 18, 2010 8:36AM - 9:12AM |
V4.00002: Beetle-inspired Capillarity-based Switchable Adhesion Invited Speaker: In a striking display from Nature, the palm beetle defends itself by adhering to the palm leaf with extraordinary strengths. Its survival depends on surface tension through its ability to manipulate an array of 10$^{5}$ micron-sized liquid bridges. Inspired by this example, we seek to make a wet reversible super-adhesion device and, more generally, to actively manipulate arrays of coupled droplets/bridges to make useful devices/materials. We first review the chosen activation strategy: electroosmosis (eo) is observed to pump effectively against capillary pressures at small scales. We illustrate using an eo pump to actively toggle between states of the droplet-droplet switch, a two-component bi-stable system. Next, we focus on stability of larger systems in the absence of active elements. Coarsening by capillarity occurs by volume-scavenging amongst many droplets in an array. That is, coupled, communicating droplets naturally reconfigure owing to surface-area minimization. This coarsening behavior evolves to concentrate volume as neighbors scavenge from one other until a single `winner' emerges. Predicting the identity of the winner and the dynamics of coarsening evolution, which depend on the coupling network size and topology, is the focus. Our solution to this dynamical systems problem will be presented. In closing, there will be a summary of progress toward the super-adhesive device goal. [Preview Abstract] |
Thursday, March 18, 2010 9:12AM - 9:48AM |
V4.00003: Bioinspired Structures and Devices for Nanophotonics Invited Speaker: The catalog of novel material structures and device concepts being discovered in biological systems continues to grow with astonishing speed. Nature's ``innovations'' include various strategies for structural coloration,\footnote{ M. Srinivasarao, Chemical Reviews 99, 1935 (1999)} broadband nanostructured low- and high-reflective surfaces, photonic crystal light collection schemes and unique multi-colored polarization based vision systems. Nature achieves these effects using very low-index structures and hierarchal fabrication schemes. In this presentation we review some of these key discoveries and present physical based fabrication strategies that emulate nature. For example, the green wing color of the \textit{Papilio palinurus} butterfly results from a micro-bowl array formed from multilayers of air and chitin and is a consequence of the mixing of yellow light reflected from the bottom of the bowls and blue light reflected from the sides of the bowls.\footnote{ P. Vukusic, J. R. Sambles, C. R. Lawrence, Nature 404, 457 (2000)} We have emulated this strategy by using breath figure templated self-assembly to mimic the microbowl structure and then by atomic layer deposition of TiO$_{2}$/Al$_{2}$O$_{3}$ multilayer films obtained the same coloration as the original butterfly structure. Additionally, other bioinspired schemes, such as those derived from the fluorescence properties displayed by the \textit{Princeps nireus} butterfly which have lead to new concepts for detecting thermal neutrons, are presented. [Preview Abstract] |
Thursday, March 18, 2010 9:48AM - 10:24AM |
V4.00004: Mechanism of the tunable structural color of neon tetra Invited Speaker: Many examples of the structural color can be found in butterfly wings, beetle's elytra and bird feathers. Since the color-producing microstructures of these examples mainly consist of stable materials, for example, dried cuticles in insects and keratin and melanin granules in bird feathers, it is impossible to actively change the microstructure. On the other hand, some fish have the tunability in their structural colors. For example, a small tropical fish, neon tetra, has a longitudinal stripe that looks blue-green in the day time, while it changes into deep violet at night. This fact clearly indicates the variability in the microstructure. It is known that the iridophore of the stripe part of neon tetra contains two stacks of thin light-reflecting platelets that are made of guanine crystal. Since the arrangement of the platelets is observed periodic, the stack is thought to cause the structural color through the multilayer thin-film interference. Consequently, the variability in the color is thought to originate from the variation in the distance between the platelets. Two explanations have been proposed so far for the distance variation. Lythoge and Shand considered that the distance is controlled by osmotic pressure that induces the inflow of the water into the iridophore[1]. On the other hand, Nagaishi et al. proposed a different model, called Venetian blind model, in which the inclination angle of the platelets is varied, resulting in the change in the distance[2]. Recently, we have performed detailed optical measurements on the iridophore of neon tetra. We have paid particular attention to the direction of the reflected light, since the Venetian blind model expects that the direction varies with the color change owing to the tilt of the platelets. We present the experimental results and quantitatively discuss the validity of the Venetian blind model. \\[4pt] [1] J. N. Lythgoe, and J. Shand, J Physiol. 325, 23-34 (1982). \\[0pt] [2] H. Nagaishi, N. Oshima, and R. Fujii, Comp. Biochem. Physiol. 95A, 337-341 (1990). [Preview Abstract] |
Thursday, March 18, 2010 10:24AM - 11:00AM |
V4.00005: Mechanisms Underlying the Emergent Properties of Gecko-like Nanostructures Invited Speaker: Imagine the difficulties a gecko would encounter if it employed a conventional pressure sensitive adhesive (PSA) on its toes. PSAs are soft viscoelastic polymers that degrade, foul, self-adhere, and attach accidentally to inappropriate surfaces. In contrast, gecko toes bear angled arrays of branched, hair-like setae formed from stiff, hydrophobic keratin that act as a bed of angled springs with similar effective stiffness to that of PSAs. We have discovered nine benchmark properties of the gecko adhesive over the past decade: 1) anisotropy, 2) strong attachment with minimal preload, 3) easy and rapid detachment, 4) material independence, 5) self-cleaning 6) anti-self-adhesion, and 7) nonadhesive default state. Most recently, we discovered 8) dynamic adhesion and 9) wear resistance. Rate dependent, wear-free friction and adhesion in a dry hard solid may emerge from uncorrelated stick-slip of the spatulae. We confirmed these predictions in a gecko-like synthetic adhesive (GSA) made from a hard silicone polymer. The GSA slid smoothly while adhering, and its velocity-dependence and stick-slip frequency matched the predictions of the model. There has been rapid progress in understanding the principles underlying these remarkable properties, and in applying the principles of gecko adhesion in the fabrication of GSAs. Properties 1-9 have all been achieved in GSAs (although not yet in a single material). [Preview Abstract] |
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