A new instrument for time-resolved measurement of HO2 radicals

OH and HO2 radicals are closely coupled in the atmospheric oxidation and combustion of volatile organic compounds (VOCs). Simultaneous measurement of HO2 yields and OH kinetics can provide the ability to assign site-specific rate coefficients that are important for understanding the oxidation mechanisms of VOCs. By coupling a fluorescence assay by gaseous expansion (FAGE) laser-induced fluorescence (LIF) detection system for OH and HO2 with a high-pressure laser flash photolysis system, it is possible to accurately measure OH pseudo-1st-order loss processes up to ∼100 000 s−1 and to determine HO2 yields via time-resolved measurements. This time resolution allows discrimination between primary HO2 from the target reaction and secondary production from side reactions. The apparatus was characterized by measuring yields from the reactions of OH with H2O2 (1:1 link between OH and HO2), with C2H4∕O2 (where secondary chemistry can generate HO2), with C2H6∕O2 (where there should be zero HO2 yield), and with CH3OH∕O2 (where there is a well-defined HO2 yield). As an application of the new instrument, the reaction of OH with n-butanol has been studied at 293 and 616 K. The bimolecular rate coefficient at 293 K, (9.24±0.21)×10−12 cm3 molec.−1 s−1, is in good agreement with recent literature, verifying that this instrument can measure accurate OH kinetics. At 616 K the regeneration of OH in the absence of O2, from the decomposition of the β-hydroxy radical, was observed, which allowed the determination of the fraction of OH reacting at the β site (0.23±0.04). Direct observation of the HO2 product in the presence of oxygen has allowed the assignment of the α-branching fractions (0.57±0.06) at 293 K and (0.54±0.04) at 616 K, again in good agreement with recent literature; branching ratios are key to modelling the ignition delay times of this potential “drop-in” biofuel

On the effect of hydrogen on the elastic moduli and acoustic loss behaviour of Ti-6Al-4V

The elastic moduli and acoustic loss behaviour of Ti-6Al-4V (wt.%) in the temperature range 5–298 K have been studied using Resonant Ultrasound Spectroscopy. A peak in the acoustic dissipation was observed at 160 K within the frequency range 250–1000 kHz. Analysis of the data acquired in this study, coupled with complementary data from the literature, showed that this was consistent with a Snoek-like relaxation process with an associated activation energy of 23 3 kJ mol$^{−1}$. However, the loss peak was broader than would be expected for a Snoek-like relaxation, and the underlying process was shown to have a spread of relaxation times. It is suggested that this effect arises as a result of variations in the strain experienced by the β phase due to different local microstructural constraint by the bounding secondary α phase.The authors would like to acknowledge Dr M Thomas of TIMET UK for providing compositional analysis, and the EPSRC / Rolls-Royce Strategic Partnership for funding (SLD under EP/H022309/1, NGJ and HJS under EP/H500375/1 and EP/M005607/1). RUS facilities were established in Cambridge through grants from the Natural Environment Research Council of Great Britain (NE/B505738/1 and NE/F017081/1).This is the final version of the article. It first appeared from Taylor & Francis via https://doi.org/10.1080/14786435.2016.119805

Physiological effects of diet mixing on consumer fitness: a meta-analysis

The degree of dietary generalism among consumers has important consequences for population, community, and ecosystem processes, yet the effects on consumer fitness of mixing food types have not been examined comprehensively. We conducted a meta-analysis of 161 peer-reviewed studies reporting 493 experimental manipulations of prey diversity to test whether diet mixing enhances consumer fitness based on the intrinsic nutritional quality of foods and consumer physiology. Averaged across studies, mixed diets conferred significantly higher fitness than the average of single-species diets, but not the best single prey species. More than half of individual experiments, however, showed maximal growth and reproduction on mixed diets, consistent with the predicted benefits of a balanced diet. Mixed diets including chemically defended prey were no better than the average prey type, opposing the prediction that a diverse diet dilutes toxins. Finally, mixed-model analysis showed that the effect of diet mixing was stronger for herbivores than for higher trophic levels. The generally weak evidence for the nutritional benefits of diet mixing in these primarily laboratory experiments suggests that diet generalism is not strongly favored by the inherent physiological benefits of mixing food types, but is more likely driven by ecological and environmental influences on consumer foraging

Temperature and Pressure Dependent Kinetics of QOOH Decomposition and Reaction with O2: Experimental and Theoretical Investigations of QOOH Radicals Derived from Cl + (CH3)3COOH

QOOH radicals are key species in autoignition, produced by internal isomerisations of RO2 radicals, and are central to chain branching reactions in low temperature combustion. The kinetics of QOOH radical decomposition and reaction with O2 have been determined as a function of temperature and pressure, using observations of OH radical production and decay following H-atom abstraction from tertiary-butyl hydroperoxide ((CH3)3COOH) by Cl atoms to produce QOOH (.CH2(CH3)2COOH) radicals. The kinetics of QOOH decomposition have been investigated as a function of temperature (251 to 298 K), and pressure (10 to 350 Torr), in helium and nitrogen bath gases, and those of the reaction between QOOH and O2 have been investigated as a function of temperature (251 to 304 K), and pressure (10 to 100 Torr) in He and N2. Decomposition of the QOOH radicals was observed to display temperature and pressure dependence, with a barrier height for decomposition of (44.7 ± 4.0) kJ mol-1 determined by master equation fitting to the experimental data. The rate coefficient for the reaction between QOOH and O2 was determined to be (5.6 ± 1.7) × 10-13 cm3 s-1 at 298 K, with no significant dependence on pressure, and can be described by the Arrhenius parameters A = (7.3 ± 6.8) × 10-14 cm3 s-1 and Ea = -(5.4 ± 2.1) kJ mol-1 in the temperature range 251 to 304 K. This work represents the first measurements of any QOOH radical kinetics as a function of temperature and pressure