Theory of the acoustic radiation force exerted on a sphere by standing and quasistanding zero-order Bessel beam tweezers of variable half-cone angles

A rigorous theory is developed to predict the radiation force (RF) exerted on a sphere immersed in an ideal fluid by a standing or quasistanding zero-order Bessel beam of different half-cone angles. A standing or a quasistanding acoustic field is the result of counter propagating 2 equal or unequal amplitude zero-order Bessel beams, respectively, along the same axis. Each Bessel beam is characterized by its halfcone angle β_l;l = 1, 2 of its plane wave components, such that β_l = 0 represents a plane wave. Analytical expressions of RF are derived for a homogeneous viscoelastic sphere chosen as an example. RF calculations for a polyethylene sphere immersed in water are performed. Particularly, the half-cone angle dependency on the RF is analyzed for standing and quasistanding waves. Changing the half-cone angle is equivalent to changing the beamwidth. Potential applications include particle manipulation in microfluidic lab-on-chips as well as in reduced gravity environments.     

Ignition and Charring of PVC-Based Electric Cables

Fire Technology - The ignition of four different PVC-based electric cables was studied using cone calorimeter and the influence of the charring phenomenon on ignition was investigated. The...     

Measuring permeability of porous materials at low frequency range via acoustic transmitted waves

An acoustical transmission method is proposed for measuring permeability of porous materials having rigid frame. Permeability is one of the several parameters required by acoustical theory to characterize porous materials such as plastic foams and fibrous or granular materials. The proposed method is based on a temporal model of the direct and inverse scattering problem for the diffusion of transient low frequency waves in a homogeneous isotropic slab of porous material having a rigid frame. This time domain model of wave propagation was initially introduced by the authors [Z.E.A Fellah and C. Depollier, J. Acoust. Soc. Am. 107, 683 (2000)]. The viscous losses of the medium are described by the model devised by Johnson et al. [J. Fluid. Mech. 176, 379 (1987)]. Reflection and transmission scattering operators for a slab of porous material are derived from the responses of the medium to an incident acoustic pulse. The permeability is determined from the expressions of these operators. Experimental and numerical validation results of this method are presented. This method has the advantage of being simple, rapid, and efficient.     

Graciela Iturbide’s Cholos/as Series: Images of Cross-Border Identities

This article looks at Mexican photographer Graciela Iturbide’s Cholos/as series taken on both sides of the border: in Los Angeles in 1986 and in Tijuana in 1989. The cholos/as, or gang members, have a complex relationship to Mexico; this response grows in part from the constructed nature of the border in general – a symbolic and relatively recent division between nations that does not represent lived reality or divisions between identities and cultures. As an outsider, Iturbide inserts visual cues throughout the series that tie the cholos/as’ lifestyle to its Mexican roots and historical legacies, thus recuperating the mexicanidad, or Mexican identity, that exists in the borderlands. The narrative thrust of this series is strong, foregrounding female agency and matrilineal family structures over traditional, heteropatriarchal conceptions of family; complex negotiations of personal identity; and generally emphasising the social ties of border communities and subcultural groups through a distinct aesthetic style that intervenes in traditional photographic genres like studio portraiture. Iturbide’s images form an origin story for the topicality of the US–Mexico border at the present moment and demonstrate that the issues of the border have been a critical point of inquiry for artists since the 1980s.     

Automatic detection of Moroccan coastal upwelling zones using sea surface temperature images

An efficient unsupervised method is developed for automatic segmentation of the area covered by upwelling waters in the coastal ocean of Morocco using the Sea Surface Temperature (SST) satellite images. The proposed approach first uses the two popular unsupervised clustering techniques, k-means and fuzzy c-means (FCM), to provide different possible classifications to each SST image. Then several cluster validity indices are combined in order to determine the optimal number of clusters, followed by a cluster fusion scheme, which merges consecutive clusters to produce a first segmentation of upwelling area. The region-growing algorithm is then used to filter noisy residuals and to extract the final upwelling region. The performance of our algorithm is compared to a popular algorithm used to detect upwelling regions and is validated by an oceanographer over a database of 92 SST images covering each week of the years 2006 and 2007. The results show that our proposed method outperforms the latter algorithm, in terms of segmentation accuracy and computational efficiency.     

A DFT Study of Ethylene Hydrogenation Reaction Mechanisms on Ni13 Nanocluster

Metal nanoclusters can potentially exhibit high catalytic activity and selectivity due to not only a high number of exposed surface atoms but also active sites with different coordination numbers as compared to bulk catalyst surface. In this study ethylene hydrogenation was used as a model reaction in an effort to elaborate reactivity of small Ni nanoclusters and the ethylene hydrogenation reaction mechanism. Two mechanistic pathways; representing Eley–Rideal and Horiuti–Polanyi type of mechanism for ethylene hydrogenation reaction are studied. It has been concluded that Horiuti–Polanyi type is more favorable when compared to Eley–Rideal type.     

Epoxidation of Ethylene by Silver Oxide (Ag<Subscript>2</Subscript>O) Cluster: A Density Functional Theory Study

Abstract  

Density functional theory (DFT) calculations were employed to study epoxidation of ethylene on a [Ag₁₄O₉] cluster model representing silver oxide (001) surface. Theoretical results obtained in this study showed that formation paths of acetaldehyde and vinyl alcohol have higher activation barriers than that of ethylene oxide formation path on silver oxide (35 and 35 vs. 20 kcal/mol). Formation of the ethylene oxometallocycle intermediate is found to have a low probability on Ag₂O(001) surface. The essential reason for this may be lower basicity of surface oxygen atom on silver oxide surface and the absence of a surface vacancy position to activate ethylene. Adsorbed EO is formed on Ag₂O surface cluster without an activation barrier. The activation barriers of the rate-limiting steps for the production of EO mechanisms (via ethyleneoxy and non-activated paths, 20 vs. 14 kcal/mol) are in relatively good agreement with the experimental activation energy values (14, 17 and 22 kcal/mol) reported for EO formation on silver catalyst.     

Microstructure and mechanical properties of ultrafine-grained fcc/hcp cobalt processed by a bottom-up approach

Bulk Co samples having a mean grain size of ～300 nm were processed by hot isostatic pressing of a high purity Co nanopowder synthesized by chimie douce. The grain interior exhibited a highly faulted nanoscale lamellar microstructure comprising an intricate mixture of face-centered cubic, hexagonal close-packed phases and nanotwins. Room temperature compression tests carried out at a strain rate of ～2 × 10^?4 s^?1 revealed a yield stress of ～1 GPa, a strain to rupture of ～5%. During straining it was found that the hexagonal close-packed phase content increased from 55% to 65% suggesting a deformation mechanism based on stress-assisted face-centered cubic to hexagonal close-packed phase transformation. In addition, an apparent activation volume of ～3b³ was computed which indicates that the deformation mechanism was controlled by dislocation nucleation from the numerous boundaries. Nonetheless, in such an intricate microstructure, the overall mechanical properties are discussed in term of a complex interplay between lattice dislocation plasticity, transformation-induced plasticity and possibly twin-induced plasticity.