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Characteristic Size (characteristic + size)
Selected AbstractsBulk Nanoporous Metal for ActuationADVANCED ENGINEERING MATERIALS, Issue 8 2010Hai-Jun Jin Abstract Nanoporous metals prepared by controlled chemical or electrochemical corrosion of alloys can provide prototypical manifestations of bulk nanostructured material. Samples are readily prepared with dimensions at the millimeter or centimeter scale, while at the same time the microstructure is a homogeneous array of interpenetrating solid skeleton phase and pore channels with a characteristic size that can reach down to below 5,nm. The interest in nanoporous metals as functional materials derives from recent observations of unique materials behavior resulting from their extremely small structure size and their open porosity with large volume-specific surface area. As an example, this article discusses the possible use of nanoporous metal for actuation. [source] Swelling-Induced Surface Patterns in Hydrogels with Gradient Crosslinking DensityADVANCED FUNCTIONAL MATERIALS, Issue 19 2009Murat Guvendiren Abstract Hydrogels with controlled surface patterns are useful for a range of applications, including in microdevices, sensors, coatings, and adhesives. In this work, a simple and robust method to generate a wide range of osmotically driven surface patterns, including random, lamellar, peanut, and hexagonal structures is developed. This method does not require the use of organic solvents for swelling, pre-patterning of the film surface, or coating of a second layer on the gel. The patterns are fabricated by exposing a photocurable formulation to light while open to air and then swelling, using oxygen inhibition of the radical polymerization at the surface to create a gradient of crosslinking with depth, which was confirmed by measuring the double bond conversion at the surface, surface mechanics, and molecule diffusion into the network. The modulus gradient, and hence osmotic pressure, is controlled by the crosslinker concentration, and the characteristic size of the patterns is determined by the initial film thickness. The patterns are stable in both swollen and dry states, creating a versatile approach that is useful for diverse polymers to create complex patterns with long-range order. [source] A fractal comminution approach to evaluate the drilling energy dissipationINTERNATIONAL JOURNAL FOR NUMERICAL AND ANALYTICAL METHODS IN GEOMECHANICS, Issue 5 2002Alberto Carpinteri The drilling comminution is theoretically and experimentally analysed by a fractal approach. An extension of the Third Comminution Theory is developed to evaluate the energy dissipation in the process: it occurs in a fractal domain intermediate between a surface and a volume. The theoretical assumption of a material ,quantum' is experimentally observed. The experimental fragment analysis evidences the characteristic size of separation between primary cutting and secondary milling. A global power balance for the drilling process is also presented and permits the prediction of drilling velocity. It shows also how the dissipation energy density (drilling strength) is not a constant parameter, but decreases considerably with the size scale. Copyright © 2002 John Wiley & Sons, Ltd. [source] Homogenizing the acoustic properties of a porous matrix containing an incompressible inviscid fluidMATHEMATICAL METHODS IN THE APPLIED SCIENCES, Issue 10 2003J. L. Ferrin We undertake a rigorous derivation of the Biot's law for a porous elastic solid containing an inviscid fluid. We consider small displacements of a linear elastic solid being itself a connected periodic skeleton containing a pore structure of the characteristic size ,. It is completely saturated by an incompressible inviscid fluid. The model is described by the equations of the linear elasticity coupled with the linearized incompressible Euler system. We study the homogenization limit when the pore size ,tends to zero. The main difficulty is obtaining an a priori estimate for the gradient of the fluid velocity in the pore structure. Under the assumption that the solid part is connected and using results on the first order elliptic systems, we obtain the required estimate. It allows us to apply appropriate results from the 2-scale convergence. Then it is proved that the microscopic displacements and the fluid pressure converge in 2-scales towards a linear hyperbolic system for an effective displacement and an effective pressure field. Using correctors, we also give a strong convergence result. The obtained system is then compared with the Biot's law. It is found that there is a constitutive relation linking the effective pressure with the divergences of the effective fluid and solid displacements. Then we prove that the homogenized model coincides with the Biot's equations but with the added mass ,a being a matrix, which is calculated through an auxiliary problem in the periodic cell for the tortuosity. Furthermore, we get formulas for the matricial coefficients in the Biot's effective stress,strain relations. Finally, we consider the degenerate case when the fluid part is not connected and obtain Biot's model with the relative fluid displacement equal to zero. Copyright © 2003 John Wiley & Sons, Ltd. [source] The basis of asymmetry in IS2 transpositionMOLECULAR MICROBIOLOGY, Issue 4 2001Leslie A. Lewis In the first step of IS2 transposition, the formation of an IS2 minicircle, the roles of the two IS ends differ. Terminal cleavage initiates exclusively at the right inverted repeat (IRR) , the donor end , whereas IRL is always the target. At the resulting minicircle junction, the two abutted ends are separated by a spacer of 1 or 2 basepairs. In this study, we have identified the determinants of donor and target function. The inability of IRL to act as a donor results largely from two sequence differences between IRL and IRR , an extra basepair between the conserved transposase binding sequences and the end of the element, and a change of the terminal dinucleotide from CA-3, to TA-3,. These two changes also impose a characteristic size on the minicircle junction spacer. The only sequences required for the efficient target function of IRL appear to be contained within the segment from position 11,42. Although IRR can function as a target, its shorter length and additional contacts with transposase (positions 1,7) result in minicircles with longer, and inappropriate, spacers. We propose a model for the synaptic complex in which the terminus of IRL makes different contacts with the transposase for the initial and final strand transfer steps. The sequence differences between IRR and IRL, and the behavioural characteristics of IRL that result from them, have probably been selected because they optimize expression of transposase from the minicircle junction promoter, Pjunc. [source] Modification of the 21-cm power spectrum by X-rays during the epoch of reionizationMONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2009L. Warszawski ABSTRACT We incorporate a contribution to reionization from X-rays within analytic and seminumerical simulations of the 21-cm signal arising from neutral hydrogen during the epoch of reionization. The relatively long X-ray mean free path (MFP) means that ionizations due to X-rays are not subject to the same density bias as UV ionizations, resulting in a substantive modification to the statistics of the 21-cm signal. We explore the impact that X-ray ionizations have on the power spectrum (PS) of 21-cm fluctuations by varying both the average X-ray MFP and the fractional contribution of X-rays to reionization. In general, prior to the epoch when the intergalactic medium (IGM) is dominated by ionized regions (H ii regions), X-ray-induced ionization enhances fluctuations on spatial scales smaller than the X-ray MFP, provided that X-ray heating does not strongly suppress galaxy formation. Conversely, at later times when H ii regions dominate, small-scale fluctuations in the 21-cm signal are suppressed by X-ray ionization. Our modelling also shows that the modification of the 21-cm signal due to the presence of X-rays is sensitive to the relative scales of the X-ray MFP and the characteristic size of H ii regions. We therefore find that X-rays imprint an epoch and scale-dependent signature on the 21-cm PS, whose prominence depends on fractional X-ray contribution. The degree of X-ray heating of the IGM also determines the extent to which these features can be discerned. We further show that the presence of X-rays smoothes out the shoulder-like signature of H ii regions in the 21-cm PS. For example, a 10 per cent contribution to reionization from X-rays translates to a 20,30 per cent modulation in the 21-cm PS across the scale of H ii regions. We show that the Murchison Widefield Array will have sufficient sensitivity to detect this modification of the PS, so long as the X-ray photon MFP falls within the range of scales over which the array is most sensitive (,0.1 Mpc,1). In cases in which this MFP takes a much smaller value, an array with larger collecting area would be required. As a result, an X-ray contribution to reionization has the potential to substantially complicate analysis of the 21-cm PS. On the other hand, a combination of precision measurements and modelling of the 21-cm PS promises to provide an avenue for investigating the role and contribution of X-rays during reionization. [source] Hierarchical structures formed by partially crystalline polymers in solution: from fundamentals to applications , a combined conventional, focusing and ultra-small-angle neutron scattering studyJOURNAL OF APPLIED CRYSTALLOGRAPHY, Issue 2007Dietmar Schwahn Multilevel aggregates with characteristic sizes covering four orders of magnitude, from 1,nm to 10,µm, are formed upon cooling decane solutions of poly(ethylene-butene) random copolymers (designated as PEB- n, where n is the number of ethyl side branches per 100 backbone C atoms) and wax-containing mixed solutions. The partially crystalline PEB-7.5 copolymers form two distinct morphologies that evolve on a range of length scales. When these polymers are mixed with wax molecules having a crystallization point lower than the polymer aggregation temperature, a hierarchy of morphologies evolves on decreasing the temperature. The multilevel structures were elucidated by combining conventional small-angle neutron scattering, focusing small-angle neutron scattering and ultra-small-angle neutron scattering investigations with microscopy. Contrast-matching analysis of the wax and copolymer components within the common morphologies revealed the wax-crystal modification capacity of the PEB-7.5 copolymers. Since the copolymers limit the growth of wax crystals, they are potential pour-point depressants for the fuel industry. [source] | ||||||||||