Multiaxial Mechanical Characterization of Interpenetrating Polymer Network Reinforced Acrylic Elastomer

The acrylic elastomer membrane VHB 4910 is a material widely used for applications as Dielectric Elastomer Actuators DEA. For suitable actuation performance however, it is necessary to pre-strain the very compliant membrane. This reduces the lifetime of DEA due to early failure of the tensioned membrane. Interpenetrating Polymer Network Reinforced Acrylic Elastomers (IPN) are produced by introducing a curable additive into the pre-strained acrylic elastomer membrane. While curing at elevated temperature, the additive forms a second polymeric network that supports part of the pre-strain in the acrylic membrane. This leads to a free standing material that combines the actuation performance of pre-strained VHB 4910 with an excellent long-term reliability. This work presents a detailed mechanical characterization of acrylic IPN membranes. To reduce the experimental effort required to characterize the nonlinear elastic behavior, we developed a unique specimen design that enables the assessment of uni- and biaxial stress states within one experiment. Slight changes in the material composition of IPN-membranes lead to substantial variations in their mechanical properties. The extraction of material behavior in different kinematic states within a single sample thus reduces the uncertainty on the determination of constitutive models. An extensive experimental campaign was carried out involving uniaxial and equibiaxial tension and relaxation. Image based local deformation measurements and iterative finite element calculations were applied to derive constitutive model parameters that describe the mechanical response in a wide range of planar strain and strain rate.     

Simultaneous analysis of serotonin, melatonin, piceid and resveratrol in fruits using liquid chromatography tandem mass spectrometry

An analytical method was developed for the simultaneous quantification of serotonin, melatonin, trans- and cis-piceid, and trans- and cis-resveratrol using reversed-phase high performance liquid chromatography coupled to mass spectrometry (HPLC-MS) with electrospray ionization (ESI) in both positive and negative ionization modes. HPLC optimal analytical separation was achieved using a mixture of acetonitrile and water with 0.1% formic acid as the mobile phase in linear gradient elution. The mass spectrometry parameters were optimized for reliable quantification and the enhanced selectivity and sensitivity selected reaction monitoring mode (SRM) was applied. For extraction, the direct analysis of initial methanol extracts was compared with further ethyl acetate extraction. In order to demonstrate the applicability of this analytical method, serotonin, melatonin, trans- and cis-piceid, and trans- and cis-resveratrol from 24 kinds of commonly consumed fruits were quantified. The highest serotonin content was found in plantain, while orange bell peppers had the highest melatonin content. Grape samples possessed higher trans- and cis-piceid, and trans- and cis-resveratrol contents than the other fruits. The results indicate that the combination of HPLC-MS detection and simple sample preparation allows the rapid and accurate quantification of serotonin, melatonin, trans- and cis-piceid, and trans- and cis-resveratrol in fruits.     

Single-site- and single-atom-resolved measurement of correlation functions

Correlation functions play an important role for the theoretical and experimental characterization of many-body systems. In solid-state systems, they are usually determined through scattering experiments, whereas in cold gases systems, time-of-flight, and in situ absorption imaging are the standard observation techniques. However, none of these methods allow the in situ detection of spatially resolved correlation functions at the single-particle level. Here, we give a more detailed account of recent advances in the detection of correlation functions using in situ fluorescence imaging of ultracold bosonic atoms in an optical lattice. This method yields single-site- and single-atom-resolved images of the lattice gas in a single experimental run, thus gaining direct access to fluctuations in the many-body system. As a consequence, the detection of correlation functions between an arbitrary set of lattice sites is possible. This enables not only the detection of two-site correlation functions but also the evaluation of non-local correlations, which originate from an extended region of the system and are used for the characterization of quantum phases that do not possess (quasi-)long-range order in the traditional sense.     

Non-affine strain measures for continuum models of network materials

Based on stretch averaging, two non-affine strain measures for materials with random network microstructure are suggested. The first concept provides a scaling between affine and non-affine behaviour that is predefined by the network functionality. The second approach applies to fibres that experience strong reorientation upon macroscopic deformation of the network. (© 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Ultrasonic surface crack characterization on complex geometries using surface waves

The characterization of surface cracks on complex geometries using surface waves is investigated numerically and experimentally. The specimen geometry is implemented in a finite difference code by approximation of the contour using a Cartesian grid. In the experiments the out-of-plane surface displacement is measured by means of a heterodyne laser interferometer. Good agreement is shown by comparison of the calculated out-of-plane displacement with experimental results for both cracked and non-cracked specimens. The crack depth is measured down to a size of 0.7 times the surface wavelength using a time delay approach. The many Rayleigh pulses propagating after the crack can be separated from the other modes by a filtering procedure based on the surface wave propagation velocity. Only a detailed analysis of the scattering phenomenon using the simulation allows an identification of the transmitted pulse required for crack depth measurement. Application of the method to a specimen with a real fatigue crack shows a systematical error possibly due to the inclined crack profile.     

Some remarks on Betti numbers of random polygon spaces

Polygon spaces such as , or the three‐dimensional analogs N_? play an important rle in geometry and topology, and are also of interest in robotics where the l_i model the lengths of robot arms. When n is large, one can assume that each l_i is a positive real valued random variable, leading to a random manifold. The complexity of such manifolds can be approached by computing Betti numbers, the Euler characteristics, or the related Poincaré polynomial. We study the average values of Betti numbers of dimension p_n when p_n → ∞ as n → ∞. We also focus on the limiting mean Poincaré polynomial, in two and three dimensions. We show that in two dimensions, the mean total Betti number behaves as the total Betti number associated with the equilateral manifold where . In three dimensions, these two quantities are not any more asymptotically equivalent. We also provide asymptotics for the Poincaré polynomials. © 2009 Wiley Periodicals, Inc. Random Struct. Alg., 2010     

Intraoperative mechanical characterization of human liver

The tissue aspiration experiment, the instrumentation for intraoperative measurements and the procedure for quantitative evaluation of the measurement results are presented. This experiment is currently used in a clinical study on the correlation between the mechanical properties of liver tissue and its histopathologic state. To date the liver of 25 patients has been tested. The results indicate a difference of 40% in the initial stiffness between peritumoral normal liver tissue and liver tumor. (© 2009 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim)     

Differences in preparative loadability between the charged and uncharged forms of ionizable compounds

Ionizable compounds experience a drastic difference in preparative loadability as a function of pH. It can be shown that the preparative loadability of a compound in the ionic form is by a factor of 20 or more inferior to the loadability of the same compound in the unionized form. In this paper, we demonstrate the reason for this behavior, and show practical applications of the principle.     

Limiting Dynamics for Spherical Models of Spin Glasses at High Temperature

We analyze the coupled non-linear integro-differential equations whose solution is the thermodynamical limit of the empirical correlation and response functions in the Langevin dynamics for spherical p−spin disordered mean-field models. We provide a mathematically rigorous derivation of their FDT solution (for the high temperature regime) and of certain key properties of this solution, which are in agreement with earlier derivations based on physical grounds. AMS (2000) Subject Classification: Primary: 82C44, Secondery: 82C31, 60H10, 60F15, 60K35     

Photoemission investigations on nanostructured TiO2 grown by cluster assembling

Nanostructured titanium dioxide (ns-TiO₂) films were grown by supersonic cluster beam deposition method. Transmission electron microscopy demonstrated that films are mainly composed by TiO₂ nanocrystals embedded in an amorphous TiO₂ phase while their electronic structure was studied by photoemission spectroscopy. The cluster assembled ns-TiO₂ films are expected to exhibit several structural and chemical defects owing to the large surface to volume ratio of the deposited clusters. Ultraviolet photoemission spectra (hv = 50 eV) from the valence band unveil the presence of a restrained amount of surface Ti 3d defect states in the band gap, whereas Ti 2p core level X-ray photoelectron (hv = 630 eV) spectra do not manifestly disclose these defects.