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Characteristic Wavelength (characteristic + wavelength)
Selected AbstractsHarnessing Surface Wrinkle Patterns in Soft MatterADVANCED FUNCTIONAL MATERIALS, Issue 16 2010Shu Yang Abstract Mechanical instabilities in soft materials, specifically wrinkling, have led to the formation of unique surface patterns for a wide range of applications that are related to surface topography and its dynamic tuning. In this progress report, two distinct approaches for wrinkle formation, including mechanical stretching/releasing of oxide/PDMS bilayers and swelling of hydrogel films confined on a rigid substrate with a depth-wise modulus gradient, are discussed. The wrinkling mechanisms and transitions between different wrinkle patterns are studied. Strategies to control the wrinkle pattern order and characteristic wavelength are suggested, and some efforts in harnessing topographic tunability in elastomeric PDMS bilayer wrinkled films for various applications, including tunable adhesion, wetting, microfluidics, and microlens arrays, are highlighted. The report concludes with perspectives on the future directions in manipulation of pattern formation for complex structures, and potential new technological applications. [source] Wavefront healing: a banana,doughnut perspectiveGEOPHYSICAL JOURNAL INTERNATIONAL, Issue 2 2001S.-H. Hung SUMMARY Wavefront healing is a ubiquitous diffraction phenomenon that affects cross-correlation traveltime measurements, whenever the scale of the 3-D variations in wave speed is comparable to the characteristic wavelength of the waves. We conduct a theoretical and numerical analysis of this finite-frequency phenomenon, using a 3-D pseudospectral code to compute and measure synthetic pressure-response waveforms and ,ground truth' cross-correlation traveltimes at various distances behind a smooth, spherical anomaly in an otherwise homogeneous acoustic medium. Wavefront healing is ignored in traveltime tomographic inversions based upon linearized geometrical ray theory, in as much as it is strictly an infinite-frequency approximation. In contrast, a 3-D banana,doughnut Fréchet kernel does account for wavefront healing because it is cored by a tubular region of negligible traveltime sensitivity along the source,receiver geometrical ray. The cross-path width of the 3-D kernel varies as the square root of the wavelength , times the source,receiver distance L, so that as a wave propagates, an anomaly at a fixed location finds itself increasingly able to ,hide' within the growing doughnut ,hole'. The results of our numerical investigations indicate that banana,doughnut traveltime predictions are generally in excellent agreement with measured ground truth traveltimes over a wide range of propagation distances and anomaly dimensions and magnitudes. Linearized ray theory is, on the other hand, only valid for large 3-D anomalies that are smooth on the kernel width scale ,(, L). In detail, there is an asymmetry in the wavefront healing behaviour behind a fast and slow anomaly that cannot be adequately modelled by any theory that posits a linear relationship between the measured traveltime shift and the wave-speed perturbation. [source] Origins and Applications of London Dispersion Forces and Hamaker Constants in CeramicsJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 9 2000Roger H. French The London dispersion forces, along with the Debye and Keesom forces, constitute the long-range van der Waals forces. London's and Hamaker's work on the point-to-point dispersion interaction and Lifshitz's development of the continuum theory of dispersion are the foundations of our understanding of dispersion forces. Dispersion forces are present for all materials and are intrinsically related to the optical properties and the underlying interband electronic structures of materials. The force law scaling constant of the dispersion force, known as the Hamaker constant, can be determined from spectral or parametric optical properties of materials, combined with knowledge of the configuration of the materials. With recent access to new experimental and ab initio tools for determination of optical properties of materials, dispersion force research has new opportunities for detailed studies. Opportunities include development of improved index approximations and parametric representations of the optical properties for estimation of Hamaker constants. Expanded databases of London dispersion spectra of materials will permit accurate estimation of both nonretarded and retarded dispersion forces in complex configurations. Development of solutions for generalized multilayer configurations of materials are needed for the treatment of more-complex problems, such as graded interfaces. Dispersion forces can play a critical role in materials applications. Typically, they are a component with other forces in a force balance, and it is this balance that dictates the resulting behavior. The ubiquitous nature of the London dispersion forces makes them a factor in a wide spectrum of problems; they have been in evidence since the pioneering work of Young and Laplace on wetting, contact angles, and surface energies. Additional applications include the interparticle forces that can be measured by direct techniques, such as atomic force microscopy. London dispersion forces are important in both adhesion and in sintering, where the detailed shape at the crack tip and at the sintering neck can be controlled by the dispersion forces. Dispersion forces have an important role in the properties of numerous ceramics that contain intergranular films, and here the opportunity exists for the development of an integrated understanding of intergranular films that encompasses dispersion forces, segregation, multilayer adsorption, and structure. The intrinsic length scale at which there is a transition from the continuum perspective (dispersion forces) to the atomistic perspective (encompassing interatomic bonds) is critical in many materials problems, and the relationship of dispersion forces and intergranular films may represent an important opportunity to probe this topic. The London dispersion force is retarded at large separations, where the transit time of the electromagnetic interaction must be considered explicitly. Novel phenomena, such as equilibrium surficial films and bimodal wetting/dewetting, can result in materials systems when the characteristic wavelengths of the interatomic bonds and the physical interlayer thicknesses lead to a change in the sign of the dispersion force. Use of these novel phenomena in future materials applications provides interesting opportunities in materials design. [source] Light propagation in a coupled waveguide-triple quantum well structurePHYSICA STATUS SOLIDI (C) - CURRENT TOPICS IN SOLID STATE PHYSICS, Issue 9 2008M. Yamaguchi Abstract The light propagation in a waveguide which was weakly coupled to a quantum well (QW) structure was studied to demonstrate the detectability of the electronic states in the small area of QW. The small area of QW was made with a GaAs/AlGaAs asymmetric triple QW. Since the electronic states in the triple QW were easily controlled by an electric field which was induced by an optical injection, we observed the optical spectra obtained from the waveguide under the various intensities of optical injection. The spectral change was observed around the characteristic wavelengths of the QW. In order to verify the observed spectral change, we measured the optical spectra of the traveling wave in a sample whose QW surface has been etched off. By comparing the results, we interpreted that the spectral change is due to the reconstruction of the electronic states in the small area of QW due to the optically excited carriers. This proves the detection of small electronic state changes in the small area of QW with the traveling light in the waveguide. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source] | ||||||||||