THERMAL-WAVE MEASUREMENTS OF MULTI-LAYER SUPERINSULATION FOILS

Abstract Thermal wave measurements rely on modulated laser heating and IR detection of the thermal response, using a MCT detector with IR optics and lock-in amplifier. Both, the amplitude and the phase retardation of the thermal wave response with respect to the heating modu-lation, provide information on the effective thermal transport properties of the measured samples. Here we apply this method to determine the shielding properties of multilayer superinsulation foils, used for the thermal insulation of superconducting magnetic coils in particle accelerators, e.g. in LHC at CERN. The measurements, performed at ambient temperature and ambient and reduced pressure, have been interpreted using a theoreti-cal model, including both conductive and radiative heat transport. The results show that the radiative heat transport can be well identified, although the conductive heat transport is dominant across multi-layer samples. At reduced pressures, the conductive heat transport decrea-ses considerably and, depending on the number of spacer layers, the radiative heat transport can become dominant. Applying this new photothermal technique, the shielding efficiencies of multi-layer superinsulation foils have been compared in this work for the first time

Cambridge Monographs in Experimental Biology

and the magnitude of FCT. Because active torque is proportional to n 2 and passive torque to n, the ratio of active to passive torque increases as n increases (Eq. 5), even while both quantities increase individuallẏ The increase in the ratio indicates an enhanced capability for active maneuvers and active stabilization, whereas the increase in FCT adds to passive stability. Thus, increasing wingbeat frequency enhances both maneuverability and stability. Hummingbirds provide an interesting example; males typically have greater wingbeat frequencies (21) and smaller body sizes as compared to females of the same species, potentially conferring a benefit in maneuverability and therefore an advantage in display flights (22) as well as greater stability when experiencing an external perturbation. These benefits are not without cost, because increasing wingbeat frequency increases the inertial and profile power requirements of flapping flight. Finally, the success of our FCT model in predicting yaw deceleration dynamics implies that passive damping may be important to flight control in flying animals across a wide range of body sizes. For example, if a steadily flapping animal experiences a brief perturbation in midstroke, by the time it is prepared to execute a corrective wingbeat, FCT will have eroded much of the effect of the perturbation, regardless of the wingbeat frequency employed by the animal. Thus, FCT provides open loop stability for some aspects of animal flight, reducing its neuromuscular and neurosensory requirements. These are not eliminated, because FCT results in asymmetric forces from symmetric flapping, implying that the animal's muscles must generate asymmetric forces and suggesting neural regulation to enforce symmetry. Furthermore, FCT does not address all the stability problems faced by flying animals. This study is limited to yaw dynamics in hovering or slow-speed flight; FCT is likely to be influential in fast forward flight, but no data are available to test such predictions. More important, a full description of body dynamics involves many factors beyond FCT and includes modes such as pitching and longitudinal dynamics known to be inherently unstable in open loop conditions (23, 24) and subject to active control (25, 26). Finally, yaw damping due to FCT is a feature of flapping flight that is not found in human-made fixed-wing or rotary-wing flyers and may lead to improvements in the stability and maneuverability of biomimetic micro-air vehicles. 11. S. P. Sane, J. Exp. Biol. 206, 4191 (2003). 12. J. R. Usherwood, C. P. Ellington, J. Exp. Biol. 205, 1565 Synonymous mutations do not alter the encoded protein, but they can influence gene expression. To investigate how, we engineered a synthetic library of 154 genes that varied randomly at synonymous sites, but all encoded the same green fluorescent protein (GFP). When expressed in Escherichia coli, GFP protein levels varied 250-fold across the library. GFP messenger RNA (mRNA) levels, mRNA degradation patterns, and bacterial growth rates also varied, but codon bias did not correlate with gene expression. Rather, the stability of mRNA folding near the ribosomal binding site explained more than half the variation in protein levels. In our analysis, mRNA folding and associated rates of translation initiation play a predominant role in shaping expression levels of individual genes, whereas codon bias influences global translation efficiency and cellular fitness. T he theory of codon bias posits that preferred codons correlate with the abundances of iso-accepting tRNAs (1, 2) and thereby increase translational efficiency (3) and accuracy (4). Recent experiments have revealed other effects of silent mutations (5-7). We synthesized a library of green fluorescent protein (GFP) genes that varied randomly in their codon usage, but encoded the same amino acid sequence (8). By placing these constructs in identical regulatory contexts and measuring their expression, we isolated the effects of synonymous variation on gene expression. The GFP gene consists of 240 codons. For 226 of these codons, we introduced random silent mutations in the third base position, while keeping the first and second positions constant We expressed the GFP genes in E. coli using a T7-promoter vector, and we quantified expression by spectrofluorometry. Fluorescence levels varied 250-fold across the library, and they were highly reproducible for each GFP construct (Spearman r = 0.98 between biological replicates)