Cavitation pressure in liquid helium

Recent experiments have suggested that, at low enough temperature, the homogeneous nucleation of bubbles occurs in liquid helium near the calculated spinodal limit. This was done in pure superfluid helium 4 and in pure normal liquid helium 3. However, in such experiments, where the negative pressure is produced by focusing an acoustic wave in the bulk liquid, the local amplitude of the instantaneous pressure or density is not directly measurable. In this article, we present a series of measurements as a function of the static pressure in the experimental cell. They allowed us to obtain an upper bound for the cavitation pressure P_cav (at low temperature, P_cav < -2.4 bar in helium 3, P_cav < -8.0 bar in helium 4). From a more precise study of the acoustic transducer characteristics, we also obtained a lower bound (at low temperature, P_cav > -3.0 bar in helium 3, P_cav > - 10.4 bar in helium 4). In this article we thus present quantitative evidence that cavitation occurs at low temperature near the calculated spinodal limit (-3.1 bar in helium 3 and -9.5 bar in helium 4). Further information is also obtained on the comparison between the two helium isotopes. We finally discuss the magnitude of nonlinear effects in the focusing of a sound wave in liquid helium, where the pressure dependence of the compressibility is large.Comment: 11 pages, 9 figure

Development of an innovative approach for complex, causally determined failure chains

A high number of examples in today’s global industry prove that it is difficult to ensure a certain level of quality. One possible reason is the increasing complexity of modern mechatronic product systems. Interface problems due to several involved science disciplines and the lack of understanding subsystems, which are delivered by different suppliers, make it difficult to identify critical components and specifications of elements. Furthermore, it is even harder to estimate the effects of elements because of their unknown dependencies and relations. Therefore it becomes more and more improbable to develop a safe, sustainable and high quality product right from the beginning. In order to understand failures in complex system structures today’s engineering industries need a generic approach that increases the understanding of systems, identifies critical components and considers the different levels of a system. It shall examine failures that cause system errors but also failures that limit system functions. Thus, it is necessary to model and simulate the causally failure chains and analyse over several system levels and elements with their dependencies. This leads to a better understanding of the whole picture of cause and effect. This research paper points out which requirements this new model-based approach has to fulfill and how it is possible to link quality and reliability methods to achieve an overall understanding and analysis of a complex system to assure high quality and sustainability

Thermovoltages in vacuum tunneling investigated by scanning tunneling microscopy

By heating the tunneling tip of a scanning tunneling microscope the thermoelectric properties of a variable vacuum barrier have been investigated. The lateral variation of the observed thermovoltage will be discussed for polycrystalline gold, stepped surfaces of silver, as well as for copper islands on silver

Predicting the adhesion strength of thermoplastic/glass interfaces from wetting measurements

© 2018 Elsevier B.V. To evaluate compatibility between a substrate and a thermoplastic polymer, the established methodology is to estimate their surface composition in terms of surface energy components, utilizing the results of contact angle measurements of probe liquids onto substrate and polymer surfaces at room temperature. Using this methodology, polymer surfaces are studied in solid state, however, during spreading of polymers on a substrate, polymers are in molten state and at high temperature, having different surface energies and more complex polymer/substrate interactions due to polymer chain mobility. This paper presents a model study with practical relevance to predict polymer/substrate compatibility including contact angle measurements at high temperature directly performed between molten thermoplastics; polypropylene (PP), polyvinylidene fluoride (PVDF) and maleic anhydride-grafted polypropylene (MAPP), on smooth glass fibres and plates. The values of total surface energy of thermoplastics at high temperature (260 °C) are down to 57% of that measured at room temperature, which has a strong influence on the wetting prediction. Surface energies of both the polymer and the substrate were found not to be the only factor controlling the wetting behaviour of molten polymers and the level of adhesion with the substrate, but also some intrinsic characteristics of the polymer melt play a role. We also observed that the wetting behaviour of molten MAPP is affected by the maleic anhydride (MA) content, demonstrating dramatically different results to room temperature measurements, which is suspected to be due to the formation of covalent bonds of MA groups with the glass surface enhancing the interface strength beyond the shear strength of MAPP.status: publishe