Cavitation Bubbles Generated by Vibrating Quartz Tuning Fork in Liquid $$^$$He Close to the $$\lambda $$λ-Transition

We report direct optical observation of cavitation bubbles in liquid helium, both in classical viscous He I and in superfluid He II, close to the \(\lambda \)-transition. Heterogenous cavitation due to the fast-flowing liquid over the rough surface of prongs of a quartz tuning fork oscillating at its fundamental resonant frequency of \(4\,\mathrm {kHz}\) occurs in the form of a cluster of small bubbles rapidly changing its size and position. In accord with previous investigators, we find the cavitation threshold lower in He I than in He II. In He I, the detached bubbles last longer than one camera frame (10 ms), while in He II the cavitation bubbles do not tear off from the surface of the fork up to the highest attainable drive.     

Assessing the effects of hydrological and chemical stressors on macroinvertebrate community in an Alpine river: The Adige River as a case study

In this study, the combined effects of hydrological and chemical stressors on benthic macroinvertebrates were evaluated in order to explore the response of the biological community to multiple stressors. The Adige River, located in the south‐eastern Alps, was selected as a case study because representative of the situation of a large river in which the variety of stressors present in the Alpine region act simultaneously. As expected, streamflow showed a seasonal pattern, with high flows in the spring–summer period; however, locally, the natural hydrological regime was altered by the presence of hydropower systems, which chiefly affected low flows. Multivariate analysis showed seasonal and spatial patterns in both chemical and hydrological parameters with a clear gradient in the concentration of nitrate, personal care, and pharmaceutical products moving from headwaters to the main stem of the river. The macroinvertebrate community composition was significantly different in summer and winter and between up and downstream sites. Streamflow alteration chiefly due to water use by hydropower affected community composition but not richness or diversity. Gammarus sp., Hirudinea, and Psychomyia sp., were positively correlated with flow variability, increasing their densities in the sites with higher streamflow variability because of hydropeaking. The results obtained in this study show that the composition of the macroinvertebrate community responded to seasonality and to changes in the main stressors along the river and highlights the importance of the spatial and temporal variability of stressors in this Alpine river. Taking into account, this variability will help the decision‐making process for improving basin management.     

XPS study of carbon in electrochemical reduction products of poly(tetrafluoroethylene)

XPS spectra of reaction products of poly(tetrafluoroethylene) with lithium amalgam were studied. The primary reaction product is a mixture of LiF and elementary carbon in the sp- state in a molar ratio of 2:1. This carbon is very reactive, among others also with respect to air oxidation at room temperature leading to the formation of surface oxides with a well-defined chemical shift of the C 1s photoemission band, which can be attributed to COOH groups. The binding energy of 1s electrons in C atoms of the basic skeleton hidden in LiF is markedly higher than with other known modifications of carbon. The carbonaceous materials formed by the leaching out of LiF with water, or by the removal of LiF by melting, contain, after air oxidation, various types of surface oxides. The binding energy of C 1s photoelectrons in the resulting skeletons is comparable with that of other carbonaceous materials.     

In situ EPR spectroelectrochemistry of single-walled carbon nanotubes and C60 fullerene peapods

The first in situ electron paramagnetic resonance (EPR) spectroelectrochemical study of C₆₀ fullerene peapods (C₆₀@SWCNT) as well as that of single-walled carbon nanotubes (SWCNTs) in different electrolyte solutions describes the formation of spin states by charge transfer reactions. Electrochemical reduction of peapods at high negative potentials causes the production of spins at the SWCNT site, while the intratubular fullerene is unchanged.Slightly anisotropic EPR signals were detected during electrochemical reduction of single-walled carbon nanotubes and fullerene peapods in the potential region from −1.75 to −2.15 V vs. decamethylferrocene/decamethylferrocinium couple. They are centered at g = 2.0038 and exhibit a hyperfine structure indicating the presence of functional groups containing N, O, H atoms in neighborhood. They differ from the EPR signals of chemically (potassium) doped SWCNT and C₆₀@SWCNT. As the EPR signal is influenced by the electrolyte counter ions a reaction with electrolysis products of tetraalkylammonium cations is taken into consideration. No EPR lines of fullerene anions were found in electrochemically treated peapods, but these anions are detectable, if a free C₆₀ in solution is cathodically reduced on a SWCNT electrode.     

The impact of natural and artificial weathering on the visual,colour and structural changes of seven tropical woods

This study investigates the effects of wood weathering on changes in its macroscopic and colour characteristics in connection with changes in its molecular and anatomical structure. Seven hardwoods suitable for outdoor architecture—bangkirai, cumaru, cumaru rosa, ipé, jatobá, kusia, and massaranduba—were exposed to the exterior out of ground contact for 1–36 months according to EN 927-3, and for 1–12 weeks in Xenotest with water spraying according to partly modified EN 927-6. With prolonged weathering, the following changes occurred in the top surfaces of all tropical woods: (1) visual—creation of longitudinal macro-cracks, (2) spectrophotometry and CIE-L ^? a ^? b ^? —darkening in exterior exposure mainly due to pollutants, except for ipé, and vice versa lightening in Xenotest, as well as greening and blueing in both modes of exposure, (3) FTIR—faster decrease of guaiacyl than syringyl lignin, absolute decrease of conjugated and unconjugated carbonyl groups in the newly formed lignin-polysaccharide-extractive substrate in the photo-oxidized and washed-out cell walls, and decrease of cellulose crystallinity, (4) SEM—damaging of cell-walls by micro-cracks, and their degradation by thinning. Connections between changes of the individual characteristics of weathered woods, for example, between the colour (ΔE*, etc.) and the molecular structure (carbonyls, etc.), were also determined.     

In situ Raman spectroelectrochemistry as a tool for the differentiation of inner tubes of double-wall carbon nanotubes and thin single-wall carbon nanotubes

In situ Raman spectroelectrochemistry has been used to distinguish between thin single-wall carbon nanotubes (SWCNT) and the inner tubes of double-wall carbon nanotubes (DWCNT). The spectroelectrochemical method is based on the different change of the electronic structure of the inner tube in DWCNT and that of SWCNT during electrochemical charging, which is reflected in the Raman spectra. During electrochemical charging the inner tubes of DWCNT exhibit a delayed attenuation of the intensities of their Raman modes as referred to the behavior of SWCNT of similar diameter. The changes are pronounced for the radial breathing mode (RBM), and thus, these modes are diagnostic for the distinction of inner tubes of DWCNT from the thin SWCNT. The different sensitivities of inner and outer tubes to the applied electrochemical charging is a simple analytical tool for differentiation of SWCNT and DWCNT in a mixture. The significance of the proposed method is demonstrated on a commercial DWCNT sample.     

A Virtual Puncture Surgery System Based on Multi-Layer Soft Tissue and Force Mesh

Puncture is a common operation in surgery, which involves all kinds of tissue materials with different geometry and mechanical properties. As a new cross-disciplinary research area, Virtual Surgery (VS) makes simulation of soft tissue in puncture operation possible in virtual environment. In this paper, we introduce a VS-based puncture system composed by three-layer soft tissue, simulated with spherical harmonic function (SHF), which is covered with a force mesh, constructed by mass spring model (MSM). The two models are combined together with a parameter of SHF named surface radius, which provides MSM with real-time deformation data needed in force calculation. Meanwhile, force calculation, divided into the surface spring force and the puncture damping force, makes the force presentation better accord to the corresponding tissue characteristics. Moreover, a deformation resumption algorithm is leveraged to simulate the resumption phenomenon of the broken tissue surface. In evaluation experiment, several residents are invited to grades our model along with other four mainstream soft tissue models in terms of 7 different indicators. After the evaluation, the scores are analyzed by a comprehensive weighted grading method. Experiment results show that the proposed model has better performance during puncture operation than other models, and can well simulate surface resumption phenomenon when tissue surface is broken.     

Defects in individual semiconducting single wall carbon nanotubes: Raman spectroscopic and in situ Raman spectroelectrochemical study

Raman spectroscopy and in situ Raman spectroelectrochemistry have been used to study the influence of defects on the Raman spectra of semiconducting individual single-walled carbon nanotubes (SWCNTs). The defects were created intentionally on part of an originally defect-free individual semiconducting nanotube, which allowed us to analyze how defects influence this particular nanotube. The formation of defects was followed by Raman spectroscopy that showed D band intensity coming from the defective part and no D band intensity coming from the original part of the same nanotube. It is shown that the presence of defects also reduces the intensity of the symmetry-allowed Raman features. Furthermore, the changes to the Raman resonance window upon the introduction of defects are analyzed. It is demonstrated that defects lead to both a broadening of the Raman resonance profile and a decrease in the maximum intensity of the resonance profile. The in situ Raman spectroelectrochemical data show a doping dependence of the Raman features taken from the defective part of the tested SWCNT.     

Intracellular disintegration by shockwave pretreatment accelerates “dry fermentation”

Dry fermentation refers to the process of anaerobic digestion that does not take place in a mixable liquid environment but in stacks loaded into airtight chambers. Since it is difficult for anaerobic consortia to decompose biomass without operating fluid, the process of dry fermentation is time consuming and considered technically inefficient. However, the construction costs of such biogas plants are significantly lower in comparison to conventional continuous technologies. Shockwave pretreatment is a modern method of intracellular disintegration that occurs purely on the basis of physical forces. It was confirmed for the first time that shockwaves are capable of accelerating methanogenesis, even in technologies that do not utilise a process liquid. This finding opens up new opportunities with regards to the processing of feedstock such as solid biowaste.     

Analytical Models of Thermal-Stress Induced Phenomena in Isotropic Multi-Particle-Matrix System

The paper presents four topics dealing with phenomena induced by elastic thermal stresses acting in an isotropic multi-particle-matrix system to represent a model system applicable to real multi-phase materials of a precipitation-matrix type. The isotropic multi-particle-matrix system consists of periodically distributed spherical particles in an infinite matrix to be imaginarily divided into cubic cells containing a central spherical particle. Formulae for the thermal stresses to be investigated within the cubic cell represents functions of the particle volume fraction v and the particle radius R. The thermal stresses originate during a cooling process as a consequence of the difference α _m  ? α _p in the thermal expansion coefficients α _m and α _p of the matrix and the particle, respectively. Additionally, such temperature range is considered within which the multi-particle-matrix system exhibits elastic deformations regarding the yield stress and the particle-matrix boundary adhesion strength. Analytical fracture mechanics to represent the first topic of this paper results from the determination of a curve integral of the thermal-stress induced elastic energy density. The curve elastic energy density results in the determination of the critical particle radii R _pc and R _mc as reasons of the crack initiation in the spherical particle and the matrix for α _m  ? α _p  < 0 and α _m  ? α _p  > 0, respectively. Consequently, the crack propagation to follow the crack initiation is a consequence of the particle radius R > R _qc (q = p, m). Finally, a shape of the crack in a plane perpendicular to the direction of the crack propagation in the particle (q = p) and the matrix (q = p) is described by the function f _q related to the ideal-brittle components. With regard to the crack initiation, the analytical determination of the radius R _qc is considered for any multi-phase materials of a precipitation-matrix type. With regard to the crack propagation, the analytical determination of the function f _q along with an analysis concerning the crack dimension and directions of the crack propagation is considered for ceramic multi-phase materials which are generally assumed to be ideal-brittle. The thermal stresses induce resistance against compressive or tensile mechanical loading for α _m  ? α _p  > 0 or α _m  ? α _p  < 0, respectively. The analytical determination of the resistance results from the elastic energy gradient to represent the second topic of this paper. Derived by two equivalent mathematical techniques, the gradient within the cubic cell is defined as a surface integral of the thermal-stress induced elastic energy density. Consequently the ‘surface’ elastic energy density results in the analytical determination of the system strengthening to represent the third topic of this paper. Representing the fourth topic of this paper, an analytical model of stresses originating in isotropic crystalline lattices are derived. The stresses in the lattices are a consequence of the presence of a central substitutive atom. Additionally, elastic energy, induced by the substitute atom and accumulated in the lattices, is also derived. Finally, readers can substitute numerical values of parameters of real multi-phase materials into the presented formulae.