244 research outputs found
Effect of Hydrodynamic Force on Microcantilever Vibrations: Applications to Liquid-Phase Chemical Sensing
At the microscale, cantilever vibrations depend not only on the microstructureâs properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantileverâs vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, and improve sensitivity in sensing applications, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include in-plane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. However, such systems require sampling (no in-situ measurements) and a relatively large sample volume (a few milliliters). Moreover, the frequency range is limited to low frequencies (less than 200Hz). To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range
Influence of Fluid-Structure Interaction on Microcantilever Vibrations: Applications to Rheological Fluid Measurement and Chemical Detection
At the microscale, cantilever vibrations depend not only on the microstructureâs properties and geometry but also on the properties of the surrounding medium. In fact, when a microcantilever vibrates in a fluid, the fluid offers resistance to the motion of the beam. The study of the influence of the hydrodynamic force on the microcantileverâs vibrational spectrum can be used to either (1) optimize the use of microcantilevers for chemical detection in liquid media or (2) extract the mechanical properties of the fluid. The classical method for application (1) in gas is to operate the microcantilever in the dynamic transverse bending mode for chemical detection. However, the performance of microcantilevers excited in this standard out-of-plane dynamic mode drastically decreases in viscous liquid media. When immersed in liquids, in order to limit the decrease of both the resonant frequency and the quality factor, alternative vibration modes that primarily shear the fluid (rather than involving motion normal to the fluid/beam interface) have been studied and tested: these include inplane vibration modes (lateral bending mode and elongation mode). For application (2), the classical method to measure the rheological properties of fluids is to use a rheometer. To overcome the limitations of this classical method, an alternative method based on the use of silicon microcantilevers is presented. The method, which is based on the use of analytical equations for the hydrodynamic force, permits the measurement of the complex shear modulus of viscoelastic fluids over a wide frequency range
Loss of strength in Ni3Al at elevated temperatures
Stress decrease above the stress peak temperature (750 K) is studied in h123i single crystals of Ni3(Al, 3 at.% Hf ). Two thermally activated deformation mechanisms are evidenced on the basis of stress relaxation and strain rate change experiments. From 500 to 1070 K, the continuity of the activation volume/temperature curves reveals a single mechanism of activation enthalpy 3.8 eV/atom and volume 90 b3 at 810K with an athermal stress of 330 MPa. Over the very same temperature interval, impurity or solute diffusion towards dislocation cores is evidenced
through serrated yielding, peculiar shapes of stressâstrain curves while changing the rate of straining and stress relaxation experiments. This complicates the
identification of the deformation mechanism, which is likely connected with cube glide. From 1070 to 1270 K, the high-temperature mechanism has an activation
enthalpy and volume of 4.8 eV/atom and 20 b3, respectively, at 1250 K
Long-Term Impact of Cyclosporin Reduction with MMF Treatment in Chronic Allograft Dysfunction: REFERENECE Study 3-Year Follow Up
Calcineurin inhibitor (CNI) toxicity contributes to chronic allograft nephropathy (CAN). In the 2-year, randomized, study, we showed that 50% cyclosporin (CsA) reduction in combination with mycophenolate mofetil (MMF) treatment improves kidney function without increasing the risk for graft rejection/loss. To investigate the long-term effect of this regimen, we conducted a follow up study in 70 kidney transplant patients until 5 years after REFERENCE initiation. The improvement of kidney function was confirmed in the MMF group but not in the control group (CsA group). Four graft losses occurred, 2 in each group (graft survival in the MMF group 95.8% and 90.9% in control group). One death occurred in the control group. There was no statistically significant difference in the occurrence of serious adverse events or acute graft rejections. A limitation is the weak proportion of patient still remaining within the control group. On the other hand, REFERENCE focuses on the CsA regimen while opinions about the tacrolimus ones are still debated. In conclusion, CsA reduction in the presence of MMF treatment seems to maintain kidney function and is well tolerated in the long term
Sinteza i antihipoksiÄno djelovanje alifatskih i arilalifatskih amida kofein-8-tioglikolne kiseline
The synthesis of some aliphatic and arylaliphatic amides of caffeine-8-thioglycolic acid was studied. The structures of synthesized compounds were proved by microanalyses, IR- and 1H NMR data. Values of acute p.o. and i.p. toxicity in mice show lower toxicity compared to caffeine. Declines in spontaneous locomotor activity support the idea of depressive CNS activity of the compounds. Two compounds exhibited brain antihypoxic activity (5a and 5b against haemic and circulatory hypoxia, respectively).U radu je opisana sinteza alifatskih i arilalifatskih amida kofein-8-tioglikolne kiseline i njihova karakterizacija elementarnom analizom, IR- i 1H NMR spektroskopijom. Testiranja na miĆĄevima pokazuju da su sintetizirani spojevi primijenjeni p.o. i i.p. manje toksiÄni od kofeina. Smanjenje lokomotoriÄke aktivnosti podupire ideju o njihovom depresivnom djelovanju na SĆœS. Spojevi 5a i 5b djeluju antihipoksiÄki u uvjetima krvne i cirkulacijske hipoksije u mozgu
Kink pair production and dislocation motion
The motion of extended defects called dislocations controls the mechanical properties of crystalline materials such as strength and ductility. Under moderate applied loads, this motion proceeds via the thermal nucleation of kink pairs. The nucleation rate is known to be a highly nonlinear function of the applied load, and its calculation has long been a theoretical challenge. In this article, a stochastic path integral approach is used to derive a simple, general, and exact formula for the rate. The predictions are in excellent agreement with experimental and computational investigations, and unambiguously explain the origin of the observed extreme nonlinearity. The results can also be applied to other systems modelled by an elastic string interacting with a periodic potential, such as Josephson junctions in superconductors
Special electronic structures and quantum conduction of B/P co-doping carbon nanotubes under electric field using the first principle
Boron (B)/phosphorus (P) doped single wall carbon nanotubes (B-PSWNTs) are
studied by using the First- Principle method based on density function theory
(DFT). Mayer bond order, band structure, electrons density and density of
states are calculated. It concludes that the B-PSWNTs have special band
structure which is quite different from BN nanotubes, and that metallic carbon
nanotubes will be converted to semiconductor due to boron/phosphorus co-doping
which breaks the symmetrical structure. The bonding forms in B-PSWNTs are
investigated in detail. Besides, Mulliken charge population and the quantum
conductance are also calculated to study the quantum transport characteristics
of B-PSWNT hetero-junction. It is found that the position of p-n junction in
this hetero-junction will be changed as the applied electric field increase and
it performs the characteristics of diode.Comment: 11 pages, 6 fiugres, 2 table
Density Functional Theory investigations of titanium gamma-surfaces and stacking faults
Properties of hcp-Ti such as elastic constants, stacking faults and
gamma-surfaces are computed using Density Functional Theory (DFT) and two
central force Embedded Atom interaction Models (EAM). The results are compared
to previously published calculations and to predicting models. Their
implications on the plastic properties of hcp-Ti are discussed.Comment: 12 pages, 3 figures, 3 table
Dislocation Creep of Olivine: Backstress Evolution Controls Transient Creep at High Temperatures
Transient creep occurs during geodynamic processes that impose stress changes on rocks at high temperatures. The transient is manifested as evolution in the viscosity of the rocks until steady-state flow is achieved. Although several phenomenological models of transient creep in rocks have been proposed, the dominant microphysical processes that control such behavior remain poorly constrained. To identify the intragranular processes that contribute to transient creep of olivine, we performed stress-reduction tests on single crystals of olivine at temperatures of 1250â1300°C. In these experiments, samples undergo timeâdependent reverse strain after the stress reduction. The magnitude of reverse strain is ~10-3 and increases with increasing magnitude of the stress reduction. High-angular resolution electron backscatter diffraction analyses of deformed material reveal lattice curvature and heterogeneous stresses associated with the dominant slip system. The mechanical and microstructural data are consistent with transient creep of the single crystals arising from accumulation and release of backstresses among dislocations. These results allow the dislocationâglide component of creep at high temperatures to be isolated, and we use these data to calibrate a flow law for olivine to describe the glide component of creep over a wide temperature range. We argue that this flow law can be used to estimate both transient creep and steadyâstate viscosities of olivine, with the transient evolution controlled by the evolution of the backstress. This model is able to predict variability in the style of transient (normal versus inverse) and the load-relaxation response observed in previous work.LH and DW acknowledge support from the Natural Environment Research Council, grant NE/M000966/1, LH and CT acknowledge support from the Natural Environment Research Council, grant 1710DG008/JC4, and DW acknowledges support from the Netherlands Organisation for Scientific Research, User Support Programme Space Research, grant ALWGO.2018.038, and startup funds from Utrecht University. LH recognizes funds used to develop the uniaxial apparatus from the John Fell Fund at the University of Oxford
Chemical Basis of Prey Recognition in Thamnophiine Snakes: The Unexpected New Roles of Parvalbumins
Detecting and locating prey are key to predatory success within trophic chains. Predators use various signals through specialized visual, olfactory, auditory or tactile sensory systems to pinpoint their prey. Snakes chemically sense their prey through a highly developed auxiliary olfactory sense organ, the vomeronasal organ (VNO). In natricine snakes that are able to feed on land and water, the VNO plays a critical role in predatory behavior by detecting cues, known as vomodors, which are produced by their potential prey. However, the chemical nature of these cues remains unclear. Recently, we demonstrated that specific proteinsâparvalbuminsâpresent in the cutaneous mucus of the common frog (Rana temporaria) may be natural chemoattractive proteins for these snakes. Here, we show that parvalbumins and parvalbumin-like proteins, which are mainly intracellular, are physiologically present in the epidermal mucous cells and mucus of several frog and fish genera from both fresh and salt water. These proteins are located in many tissues and function as Ca2+ buffers. In addition, we clarified the intrinsic role of parvalbumins present in the cutaneous mucus of amphibians and fishes. We demonstrate that these Ca2+-binding proteins participate in innate bacterial defense mechanisms by means of calcium chelation. We show that these parvalbumins are chemoattractive for three different thamnophiine snakes, suggesting that these chemicals play a key role in their prey-recognition mechanism. Therefore, we suggest that recognition of parvalbumin-like proteins or other calcium-binding proteins by the VNO could be a generalized prey-recognition process in snakes. Detecting innate prey defense mechanism compounds may have driven the evolution of this predator-prey interaction
- âŠ