Stress and strain fields in rotating elastic/plastic annular disks of pressure-dependent material

The Drucker–Prager yield criterion is used in conjunction with its associated flow rule to find the elastic/plastic stress and strain distributions within the rotating annular disks under plane stress conditions. The main distinguished feature of the model, as compared to typical models used for analysis of disks, is that the material is plastically compressible. Using an approach proposed elsewhere, the solution for strain rates is reduced to one nonlinear ordinary differential equation and two linear ordinary differential equations. These equations can be solved one by one, which significantly simplifies the numerical treatment and increases the accuracy of solution.     

Exploring the undrained cyclic behavior of methane-hydrate-bearing sediments using CFD–DEM

Based on the mechanical experimental results of methane hydrate (MH), a bond contact model considering the rate-dependency of MH is proposed. A CFD–DEM scheme considering fluid compressibility is used to simulate a series of undrained cyclic shear tests of numerical methane-hydrate-bearing sediment (MHBS) samples. The dynamic behavior, including stress–strain relationship, dynamic shear modulus, and damping ratio, is investigated. In addition, the force chains, contact fabric and averaged pure rotation rate (APR) are examined to investigate the relationships between micromechanical variables and macromechanical responses in the DEM MH samples. The effects of temperature, confining pressure and MH saturation are also analyzed. Due to the micro-structural strengthening by the MH bonds, no obvious change in microscopic quantities is observed, and the samples remain at the elastic stage under the applied low-shear stress level. When confining pressure and MH saturation increase, the dynamic elastic modulus increases, while the damping ratio decreases. An increasing temperature (leading to weakening of MH bonds) can lower the dynamic elastic modulus, but has almost no impact on the damping ratio. On the contrary, an increasing cyclic shear stress level lowers the damping ratio, but has almost no effect on the dynamic elastic modulus.     

Enhancing Immiscible Fluid Displacement in Porous Media by Capillary Pressure Discontinuities

Fluid displacement in porous media plays an important role in many industrial applications, including biological filtration, carbon capture and storage, enhanced oil recovery, and fluid transport in fuel cells. The displacement front is unstable, which evolves from smooth into ramified patterns, when the mobility (ratio of permeability to viscosity) of the displacing fluid is larger than that of the displaced one; this phenomenon is called viscous fingering. Viscous fingering increases the residual saturation of the displaced fluid, considerably impairing the efficacy of fluid displacement. It is of practical importance to develop suitable methods to improve fluid displacement. This paper presents an experimental study on applying the discontinuity of capillary pressure to improve immiscible fluid displacement in drainage for which the displacing fluid (air) wets the porous media less preferentially than does the displaced fluid (silicone oil). The concept involves using a heterogeneous packing system, where the upstream region features large pores and small capillary pressure, and the downstream region features small pores and large capillary pressure. The increase in capillary pressure prevents fingering from directly crossing the media interface, thus enhancing the displacement. The experimental apparatus was a linear cell comprising porous media between two parallel plates, and glass beads of 0.6 and 0.125 mm diameter were packed to compose the heterogeneous porous media. The time history of the finger flow was recorded using a video camera. Pressure drops over the model from the inlet to the outlet were measured to compare viscous pressure drops with capillary pressures. The results show that the fluid displacement was increased by the capillary discontinuities. The optimal displacement was determined through linear regression by adjusting the relative length of the large- and small-pore region. The results may assist in the understanding of fingering flow across the boundaries of different grain-sized bands for the gas and oil reservoir management, such as setting the relative location of the injection and production wells. The findings may also serve as a reference for industrial applications such as placing the grain bands in an adequate series to improve the displacement efficacy in biological filtration.     

Self-assembly of Ag nanopowder on OTS-patterned glass

Ag ink was spontaneously patterned on glass substrate by using the surface energy difference of a pre-patterned octadecyltrichlorosilane (OTS) layer. Ag ink was confined into the hydrophilic area, where OTS layer was not formed. OTS layer was selectively transferred by micro-contact printing (μCP) method and significantly decreased surface energy. As a result, surface of glass substrate was separated as hydrophobic and hydrophilic with and without OTS layer, respectively. Ag line could be successfully patterned with the width of below 10 μm on the glass. The patterned Ag line was dense and abrupt on the edge and the thickness was about 0.25 μm. Ag film showed good adhesion on a glass substrate after anneal above 200 °C. The minimum resistivity was about 4 μΩ cm.     

Improved performances on surface‐enhanced Raman scattering based on electrochemically roughened gold substrates modified with SiO2 nanoparticles

In this work, we use electrochemical oxidation–reduction cycles (ORC) methods to prepare surface‐enhanced Raman scattering (SERS)‐active gold substrates modified with SiO₂ nanoparticles to improve the corresponding SERS performances. Based on the modified substrates, the SERS of Rhodamine 6G (R6G) exhibits a higher intensity by 3‐fold of magnitude, as compared with that of R6G adsorbed on a SERS‐active Au substrate without the modification of SiO₂ nanoparticles. Moreover, the SERS enhancement capabilities of the modified and the unmodified Au substrates are seriously destroyed at temperatures higher than 250 and 200 °C, respectively. These results indicate that the modification of SiO₂ nanoparticles can improve the thermal stability of SERS‐active substrates. The aging in SERS intensity is also depressed on this modified Au substrate due to the contribution of SiO₂ nanoparticles to SERS effects. Copyright © 2009 John Wiley & Sons, Ltd.     

A possible role of amorphous regions at grain-boundary vertex points as linear defects on flux pinning in MgB2 films with columnar grain structure

We measured the transport properties of MgB₂ films having columnar grain structure with their axis normal to the substrate. When an external magnetic field was applied parallel to the grain axis, an enhanced critical current density has been observed, and this result has been ascribed to flux pinning induced by grain boundaries. The shape of the angular dependence of critical current density and its magnetic field dependence showed a quite similar resemblance to those of YBa₂Cu₃O_x films containing columnar defects, implying a possible existence of linear defects in MgB₂ films of columnar structure. We propose that the amorphous regions at the vertex points of three or more grain boundaries observed in microstructural studies correspond to the linear defects and these linear defects anchor the end points of the flux line dislocations of Frank-Read sources, by which the shear in the flux line lattice is actuated. This assumed mechanism is found to reasonably explain the magnetic field dependence of the flux pinning force density of MgB₂ films with columnar grain structure.     

Femtosecond laser ablation of neurons in <Emphasis Type="Italic">C</Emphasis>. <Emphasis Type="Italic">elegans</Emphasis> for behavioral studies

In this paper, the interaction between the inhomogeneous and anisotropic cloak covered infinitely long PEC circular cylinder and the parallel electric-line-source is investigated in detail. First, the permittivity and permeability tensors of the cloak are defined according to Pendry’s model, then the wave equation in the metamaterial cloak is derived; second, the far/near properties of the model are investigated, and using the boundary conditions the electromagnetic fields of the whole region are calculated analytically; finally, based on the analytical expressions, the electric fields of the whole region are demonstrated through numerical results which show the effects of invisibility. Finally, the effects of loss on the performance of the invisible cloak are investigated.     

An improvement of tilted AlN for shear and longitudinal acoustic wave

This study investigates c-axis tilted aluminum nitride (AlN) piezoelectric films for the improvement of both shear and longitudinal acoustic wave resonances. Solidly-mounted resonator (SMR) structure is adopted for the applications of high frequency wireless communications and high sensitivity sensors. As to the piezoelectric layer, c-axis tilted AlN has the capability to excite the dual-mode resonances, namely, the longitudinal and shear mode resonances. In this study, SMR devices made with a seven-layer molybdenum/silicon dioxide (Mo/SiO₂) Bragg reflector and the c-axis tilted AlN are carried out. A conventional off-axis sputtering technique is applied to grow the tilted AlN. The outcome frequency responses show dual resonant characteristics. However, the longitudinal resonance fades away with the AlN c-axis tilted angle, and the quality factor of the longitudinal resonance decreases. Consequently, we make an improvement by tilting the off-center substrates toward the sputtering source and successfully enhance the longitudinal resonance while preserving the shear resonance at the same time. Not only the shear resonance for the liquid-based sensing application, but also an outstanding longitudinal resonance could be obtained. The practicability of the dual-mode resonator is extended.     

Theoretical studies on the electronic and optical properties of arene‐ versus fluoroarene‐thiophene co‐oligomer

A series of regiochemically varied and core size extension‐modulated arene‐ and fluoroarene‐thiophene co‐oligomers and the unsubstituted sexithiophene α6T were investigated theoretically to explore their electronic and optical properties. These phenylene‐thiophene oligomers show great potential for application in organic light‐emitting diodes (OLEDs), organic diode lasers, and organic thin‐film transistors (OTFTs) because of their feasible tuning of optical and electronic properties by the various structural tunings. Density functional theory (DFT) and the ab initio HF were employed to investigate the geometric and electronic structures of the oligomers in the ground state, and the singles configuration interaction (CIS) methods were used to study the lowest singlet excited state. The lowest excitation energies (E_gs), the radiative lifetime τ, and the maximal absorption/emission wavelength of the oligomers were studied within time‐dependent DFT (TDDFT). All calculations were performed using the 6‐31G(d) basis set. The results show that the HOMOs, LUMOs, energy gaps, ionization potentials (IPs), electron affinities (EAs), and reorganization energies are significantly affected by the various structural tunings in these co‐oligomers, which is important for the improvement of the hole and electron injection into OLEDs. Interestingly, the LUMO energy of 1b , 2b , and 3b is lower than that of α6T and 1a , 2a , 3a by about 0.12 ～ 0.47 eV, indicating that the fluorophenyl‐substitution has significantly improved the electron injection properties of the oligomers. Copyright © 2008 John Wiley & Sons, Ltd.     

Discrimination of strain and temperature based on a polarization-maintaining photonic crystal fiber incorporating an erbium-doped fiber

A simple sensing method for simultaneous measurement of temperature and strain is investigated by using a Sagnac fiber loop mirror composed of a polarization-maintaining photonic crystal fiber (PM-PCF) incorporating an erbium-doped fiber (EDF). Amplified spontaneous emission created by a pumped EDF is transmitted to a Sagnac fiber loop mirror. The interference between two counter-propagating signals in a Sagnac fiber loop mirror generates a periodic transmission spectrum with respect to wavelength. When external temperature is increased, the transmission peak power reduces because the amplified spontaneous emission of the EDF is decreased by the applied temperature change (0.04 dB/°C). The peak wavelength is shifted into the shorter wavelength because of the negative temperature dependence of the birefringence of the PM-PCF (0.3 pm/°C). As the applied strain increases, the peak wavelength of the transmission spectrum of the Sagnac loop mirror incorporating the EDF shifts into a longer wavelength (1.3 pm/με) because the phase change of the proposed sensing probe is directly proportional to the applied strain. The transmission peak power, however, is not changed by the applied strain. Since the source and the sensing probe are integrated, the overall system configuration is significantly simplified without requiring any additional broadband light source. Therefore, it is possible to simultaneously measure temperature and strain by monitoring the variation of transmission peak power and peak wavelength, respectively.