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Surface Stress (surface + stress)
Selected AbstractsEutectic Gallium-Indium (EGaIn): A Liquid Metal Alloy for the Formation of Stable Structures in Microchannels at Room Temperature,ADVANCED FUNCTIONAL MATERIALS, Issue 7 2008Michael D. Dickey Abstract This paper describes the rheological behavior of the liquid metal eutectic gallium-indium (EGaIn) as it is injected into microfluidic channels to form stable microstructures of liquid metal. EGaIn is well- ;suited for this application because of its rheological properties at room temperature: it behaves like an elastic material until it experiences a critical surface stress, at which point it yields and flows readily. These properties allow EGaIn to fill microchannels rapidly when sufficient pressure is applied to the inlet of the channels, yet maintain structural stability within the channels once ambient pressure is restored. Experiments conducted in microfluidic channels, and in a parallel-plate rheometer, suggest that EGaIn's behavior is dictated by the properties of its surface (predominantly gallium oxide, as determined by Auger measurement s); these two experiments both yield approximately the same number for the critical surface stress required to induce EGaIn to flow (,0 .5,N/m). This analysis,which shows that the pressure that must be exceeded for EGaIn to flow through a microchannel is inversely proportional to the critical (i.e., smallest) dimension of the channel,is useful to guide future fabrication of microfluidic channels to mold EGaIn into functional microstructures. [source] Stress-adapted numerical form finding of pre-stressed surfaces by the updated reference strategyINTERNATIONAL JOURNAL FOR NUMERICAL METHODS IN ENGINEERING, Issue 2 2005R. Wüchner Abstract In this paper we present the updated reference strategy for numerical form finding of pre-stressed membranes, which is based on standard finite element discretization. The singularities of the inverse problem are regularized by a homotopy mapping. A projection scheme is proposed where anisotropic pre-stress is defined with respect to an additional reference plane, which reflects the initially developable surface of membrane strips in the production process. Physically problematic combinations of edge geometry and surface stress are solved by a self-adaptive stress correction scheme. The algorithm is based on a local criterion derived from differential geometry. Several examples illustrate the success of each idea and implementation. Copyright © 2005 John Wiley & Sons, Ltd. [source] Small non-coding RNAs, co-ordinators of adaptation processes in Escherichia coli: the RpoS paradigmMOLECULAR MICROBIOLOGY, Issue 4 2003F. Repoila Summary Adaptation to the changing environment requires both the integration of external signals and the co-ordination of internal responses. Around 50 non-coding small RNAs (sRNAs) have been described in Escherichia coli; the levels of many of these vary with changing environmental conditions. This suggests that they play a role in cell adaptation. In this review, we use the regulation of RpoS (,38) translation as a paradigm of sRNA-mediated response to environmental conditions; rpoS is currently the only known gene regulated post-transcriptionally by at least three sRNAs. DsrA and RprA stimulate RpoS translation in response to low temperature and cell surface stress, respectively, whereas OxyS represses RpoS translation in response to oxidative shock. However, in addition to regulating RpoS translation, DsrA represses the translation of HNS (a global regulator of gene expression), whereas OxyS represses the translation of FhlA (a transcriptional activator), allowing the cell to co-ordinate different pathways involved in cell adaptation. Environmental cues affect the synthesis and stability of specific sRNAs, resulting in specific sRNA-dependent translational control. [source] Bulk formulation of surface fluxes extended to weak-wind stable conditionsTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 630 2008L. Mahrt Abstract The bulk formulation for surface fluxes based on surface-layer similarity theory is found to be a poor approximation for weak-wind stably-stratified conditions. These conditions are often non-stationary, with very weak turbulence, and have often been excluded from analysis. As a consequence of such exclusion and the failure to account for self-correlation in the bulk aerodynamic formula for the surface stress, the validity of the similarity theory has been overestimated for weak-wind stable conditions. Four months of turbulence data from FLOSSII are analysed, in order to generalize the bulk formula to include the influence of the non-stationary mesoscale motions on the generation of turbulence. The generalization of the bulk formula leads to a more systematic variation of the drag coefficient. Excluding cases of significant non-stationarity of the wind field also leads to a more systematic behaviour of the drag coefficient, although it removes most of the very stable cases, thus introducing bias. Additional data are required in order to establish confidence in the generalized bulk formula. Copyright © 2008 Royal Meteorological Society [source] Boundary layer mechanisms in extratropical cyclonesTHE QUARTERLY JOURNAL OF THE ROYAL METEOROLOGICAL SOCIETY, Issue 623 2007Robert J. Beare Abstract This paper revisits the mechanism for the interaction of the boundary layer with extratropical cyclones. Two diagnostic approaches are compared: Ekman pumping and potential vorticity. Ekman pumping derives from the boundary layer stress which induces convergence and ascent. boundary layer potential vorticity contains in a single quantity both the vorticity and stratification. These quantities are compared for an idealized extratropical cyclone life cycle simulated with the Met Office Unified Model. A significant component of the boundary layer stress and thus Ekman pumping at occlusion is forced by the cold conveyor-belt jet in the unstable boundary layer. In contrast, much of the boundary layer depth-averaged potential vorticity is contained within the stable warm-sector region. Inversion of the warm-sector PV indicates a small local deepening of about 2.5 hPa. Moreover, switching off the boundary layer mixing in the unstable cold sector has much more impact than in the stable warm sector. The sensitivity of the cyclone and its boundary layer to basic-state jet strength is then investigated. The maximum friction velocity scales closely with the initial maximum jet strength. This demonstrates the important role of the large-scale flow in organizing the boundary layer structure. Changes in the minimum pressure produced by altering the boundary layer parametrization correspond closely to changes in the surface stress averaged over the cyclone. Different operational changes to the boundary layer scheme produce small and compensating changes to the cyclone minimum pressure over three days. © Crown Copyright 2007. Reproduced with the permission of the Controller of HMSO. Published by John Wiley & Sons, Ltd [source] Stresses in ion-exchange layers of soda-lime-silicate glassFATIGUE & FRACTURE OF ENGINEERING MATERIALS AND STRUCTURES, Issue 6 2005T. FETT ABSTRACT This paper presents a new method to determine both the magnitude and the sign of the surface stresses that develop as a consequence of sodium/hydrogen ion exchange in soda-lime-silicate glass immersed in water. At 90 °C, very thin layers that develop at the surfaces of polished glass specimens are found to have extremely high compressive stresses, ,2.4 GPa. The negative sign of the stress is consistent with earlier findings that the ion-exchange process involves hydronium ions (H3O+) and not bare protons (H+). [source] | ||||||||||