A laser flash photolysis-resonance fluorescence technique has been employed to investigate the kinetics of reactions of the important stratospheric species bromine nitrate (BrONO 2 ) with ground-state atomic bromine (k 1 ), chlorine (k 2 ), and oxygen (k 3 ) as a function of temperature (224-352 K) and pressure (16-250 Torr of N 2 ). The rate coefficients for all three reactions are found to be independent of pressure and to increase with decreasing temperature. The following Arrhenius expressions adequately describe the observed temperature dependencies (units are 10 -11 cm 3 molecule -1 s -1 ): k 1 ) 1.78 exp(365/T), k 2 ) 6.28 exp(215/T), and k 3 ) 1.91 exp(215/T). The accuracy of reported rate coefficients is estimated to be 15-25% depending on the magnitude of the rate coefficient and on the temperature. Reaction with atomic oxygen is an important stratospheric loss process for bromine nitrate at altitudes above ∼25 km; this reaction should be included in models of stratospheric chemistry if bromine partitioning is to be correctly simulated in the 25-35 km altitude regime.
We have performed systematic experiments on vane intruders of different sizes and aspect ratios that are immersed and slowly rotated in beds of monodisperse glass beads of different diameters. We find that the torque and lift force on the vane increase with bead size. The measured torque on the rotating vanes follows a scaling behavior that depends on the effective immersion depth and the effective vane diameter. The torque increases with the square of the effective immersion depth and the square of the effective vane diameter, and closely resembles the scaling behavior previously reported for the torque on rotating cylinders. We also find that the vertical lift forces have a supralinear dependence on the effective immersion depth, and qualitatively resemble the plunging forces produced when an intruder is slowly immersed into beds of glass beads.
The thermal decomposition of Si(OCH3)4 (TMOS) has been studied by FTIR at temperatures between 858 and 968 K. The experiment was carried out in a static cell at a constant pressure of 700 Torr under highly diluted conditions. Additional experiments were performed by using toluene as a radical scavenger. The species monitored included TMOS, CHzO, C&. and CO. According to these measurements, the first-order global rate constants for the disappearance of TMOS without and with toluene can be given by k, = 1.4 x 10l6 exp(-81 200/RT) s-l and kg = 2.0 x 1014 exp(-74 500/RT) s-l, respectively. The noticeable difference between the two sets of Arrhenius parameters suggests that, in the absence of the inhibitor, the reactant was consumed to a significant extent by radical attacks at higher temperatures. The experimental data were kinetically modeled with the aid of a quantum-chemical calculation using the BAC-MP4 method. The results of the kinetic modeling, using the mechanism constructed on the basis of the quantum-chemical data and the known C/H/O chemistry, identified two rate-controlling reactions: TMOS -CH30H + (CH30)2SiOCHz (reaction 2 ) and CHzOSi(OCH3)3 -CHzO + Si(OCH3)3 (reaction 3), which have the following respective first-order rate constants, given in the units of s-l: kZ = 1.6 x 1014 exp(-74 OOO/RT) and k3 = 3.8 x 1014 exp(-60 OOO/RT). In addition to these new kinetic data, the heats of formation of many relevant SiO,C,H, species computed with the B AC-MP4 method are presented herein.
We developed the foam drainage rheology technique in order to perform rheological measurements of aqueous foams at a set liquid fraction epsilon and fixed bubble radius R without the usual difficulties associated with fluid drainage and bubble coarsening. The shear stress exhibits a power-law dependence on strain-rate, tau approximately gamma[over]n where n approximately 0.2. The stress exhibits an inverse dependence on liquid content, tau approximately (1+h'epsilon)(-1), where h'=theta(10) exhibits a diminishing logarithmic trend with gamma[over]. We propose a model based upon film shearing as the dominant source of viscous dissipation.
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