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T Speck

Publications and source records attributed to T Speck.

12 recordsLinked to original sources
PubMed2006

Measurement of stochastic entropy production.

Using fluorescence spectroscopy we directly measure entropy production of a single two-level system realized experimentally as an optically driven defect center in diamond. We exploit a recent suggestion to define entropy on the level of a single stochastic trajectory [Seifert, Phys. Rev. Lett. 95, 040602 (2005)10.1103/PhysRevLett.95.040602]. Entropy production can then be split into one of the system itself and one of the surrounding medium. We demonstrate that the total entropy production obeys various exact relations for finite time trajectories.

Journal Article↗
PubMed2006

Thermodynamics of a colloidal particle in a time-dependent nonharmonic potential.

We study the motion of an overdamped colloidal particle in a time-dependent nonharmonic potential. We demonstrate the first lawlike balance between applied work, exchanged heat, and internal energy on the level of a single trajectory. The observed distribution of applied work is distinctly non-Gaussian in good agreement with numerical calculations. Both the Jarzynski relation and a detailed fluctuation theorem are verified with good accuracy.

Journal Article↗
PubMed2005

Experimental and modeling study of the ion-molecule association reaction H3O+ + H2O (+M) --> H5O2(+) (+M).

Experimental results for the rate of the association reaction H3O+ + H2O (+M) --> H5O2(+) (+M) obtained with the Cinetique de Reactions en Ecoulement Supersonique Uniforme flow technique are reported. The reaction was studied in the bath gases M=He and N2, over the temperature range of 23-170 K, and at pressures between 0.16 and 3.1 mbar. At the highest temperatures, the reaction was found to be close to the limiting low-pressure termolecular range, whereas the limiting high-pressure bimolecular range was approached at the lowest temperatures. Whereas the low-pressure rate coefficients can satisfactorily be reproduced by standard unimolecular rate theory, the derived high-pressure rate coefficients in the bath gas He at the lowest temperatures are found to be markedly smaller than given by simple ion-dipole capture theory. This result differs from previous observations on the related reaction NH4(+) + NH3 (+M) --> N2H7(+) (+M). This observation is tentatively attributed to more pronounced contributions of the valence part of the potential-energy surface to the reaction in H5O2(+) than in N2H7(+). Falloff curves of the reaction H3O+ + H2O (+M) --> H5O2(+) (+M) are constructed over wide ranges of conditions and represented in compact analytical form.

Journal Article↗
PubMed2005

Experimental test of the fluctuation theorem for a driven two-level system with time-dependent rates.

A single defect center in diamond periodically excited by a laser is shown to provide a simple realization for a system obeying a fluctuation theorem for nonthermal noise. The distribution of these fluctuations is distinctly non-Gaussian, which has also been verified by numerical calculation. For driving protocols symmetric under time reversal a more restricted form of the theorem holds, which is also known from entropy fluctuations caused by thermal noise.

Journal Article↗
PubMed2004

Two-dimensional tension tests in plant biomechanics--sweet cherry fruit skin as a model system.

Splitting of fruits is a function of two-dimensional tension caused by different growth rates of tissues and turgor, especially water uptake shortly before harvest. In order to analyse the mechanical properties of spheroid plant material close to stress-strain conditions in vivo, a new hydraulic two-dimensional testing device was set up. Sweet cherry (Prunus avium L.) fruit skin was chosen as a model system. The recorded pressure-deflection curves were non-linear, with a considerable initial "lag phase" and a distinct increasing end part. Taking into account the special geometry, these curves could be modelled with a newly developed analytical approach based on linear elastic material behaviour. The results demonstrated good correlation if a modulus of elasticity ranging from 160 to 250 MPa for the cherry fruit skin was chosen. In addition, a mean strength value of 47 MPa was calculated based on the theory of thin shells and spheres. The results are compared with mechanical data found for fruits and other plant material. In order to test the theoretical approach, two- and one-dimensional tension tests were performed on packaging PE foil, revealing a mean modulus of 171 MPa in bi-axial tension, and 193 and 242 MPa in uni-axial tension, depending on the test speed. The results demonstrate that it seems to be feasible to use this method to analyse the two-dimensional stress-strain conditions of spheroid plant materials such as cherry fruit skins. It may be applied as a tool for crop testing to elucidate the mechanical basis of cracking susceptibility of fruits.

Biomechanical Phenomena↗
PubMed2001

Evolutionary trends in safety factors against wind-induced stem failure.

We explore the hypothesis that the safety factor against wind-induced stem failure remained high during early land plant evolution despite an evolutionary increase in height with concomitant increases in wind-induced drag forces, bending stresses, and moments. This hypothesis was examined for 17 Paleozoic plant species assuming that each (1) existed in a densely packed community of conspecifics with equivalent height, (2) coped with the same wind profile (where ambient wind speed decreased toward ground level), but (3) had different within-canopy wind speeds depending on plant height and general morphology. Drag forces, stresses, and moments were computed, and a safety factor was calculated for each taxon using the quotient of its stem-tissue breaking stress and maximum wind-induced bending stress.The highest factors of safety were calculated among the most ancient rhyniophyte and zosterophyllophyte species examined (e.g., Rhynia and Gosslingia), and, on average, decreased among the taller and geologically younger species. The tallest species examined (e.g., Archaeopteris and Diaphorodendron) had safety factors equal to or higher than those of some of their presumed ancestors (e.g., Psilophyton and Leclercqia). These trends were statistically more robust among rhyniophytes and their presumed descendants.Even though the results comply with the hypothesis, numerous limitations of our protocol exist (e.g., the requirement for reliable whole-plant reconstructions). These are discussed in terms of our theory. Nonetheless, we believe our theory and protocol afford a reasonable opportunity to explore the effects of wind on early plant evolution.

Journal Article↗
PubMed2000

Micromechanics and anatomical changes during early ontogeny of two lianescent Aristolochia species.

The mechanical properties of young stems of Aristolochia macrophylla Lam. and Aristolochia brasiliensis Mart. et Zucc. were studied during elongation growth and primary differentiation. Data for the modulus of elasticity, for the viscoelastic behaviour caused by longitudinal tension and for the shear modulus resulting from torsion around a longitudinal axis were related to the underlying structural changes by quantitative analysis of stem anatomy, tissue distribution, ultrastructure, and cell wall biochemistry. The orientation of cellulose microfibrils was determined by light microscopy and small-angle X-ray diffraction, and the lignin content was determined by thioglycolic acid derivatization and spectroscopic quantification. It was demonstrated that the increase in stability during early development is due to the complementary effects of increase in cell wall material, lignification, and cellulose microfibril alignment. A detailed micromechanical model, considering internal prestresses, is proposed to explain the characteristic biphasic stress-strain behaviour as well as the strain-hardening observed.

Biomechanical Phenomena↗
PubMed2000

Brittleness of twig bases in the genus Salix: fracture mechanics and ecological relevance.

The twig bases within the genus Salix were investigated. Brittleness of twig bases as defined in the literature neither correlates with Young's modulus nor with growth strains, which were measured for S. alba, S. fragilis and S. x rubens. For the species S. alba, S. appendiculata, S. eleagnos, S. fragilis, S. purpurea, S. triandra, S. viminalis, and S. x rubens, fracture surfaces of broken twigs were investigated and semiquantitatively described in terms of 'relative roughness' (ratio of rough area of fracture surface over whole area of fracture surface). The relative roughness clearly corresponds with the classification into brittle and nonbrittle species given in the literature. An attempt was made to quantify brittleness with mechanical tests. The absolute values of stress and strain do not correlate with the brittleness of the twig bases as defined by the relative roughness. However, the 'index stress' (ratio of stress at yield over stress at fracture) or the 'index strain' (ratio of strain at yield over strain at fracture), correlate well with the relative roughness. The graphic analysis of index stress against index strain reveals a straight line on which the eight species are ordered according to their brittleness. Depending on growth form and habitat, brittle twig bases of willows may function ecologically as mechanical safety mechanisms and, additionally, as a propagation mechanism.

Ecology↗
PubMed2000

The functional morphology of the petioles of the banana, Musa textilis.

Bananas are among the largest herbs in the world and their lightweight petioles hold up huge leaves. This study examined how the petioles manage to achieve adequate rigidity to do this, while allowing extensive and reversible reconfiguration in high winds. Morphological and anatomical examination of the petioles and leaves of Musa textilis suggested how these two apparently incompatible abilities are achieved. The hollow U-shaped section of the petiole and the longitudinal strengthening elements in its outer skin give it adequate rigidity, while its ventral curvature help support the leaf without the need for thick lateral veins. These features, however, also allow the petiole to reconfigure by twisting away from the wind, while the leaf can fold away. In addition, two sets of internal structures, longitudinal partitions and transverse stellate parenchyma plates, help prevent dorsoventral flattening, allowing the petiole to flex further away from the wind without buckling. These ideas were tested and verified by a range of mechanical tests. Simple four-point-bending and torsion tests showed that the petioles are indeed far more compliant in torsion than in bending. Axial bending tests and crushing tests showed that petioles could be flexed twice as far and were four times as resistant to dorsoventral flattening when intact than when the internal tissue is removed. The banana petiole, therefore, seems to be an excellent example of natural integrated mechanical design.

Biomechanical Phenomena↗
PubMed1993

The kidney and primary hypertension: contributions from renal transplantation studies in animals and humans.

CLINICAL STUDIES: In clinical renal transplantation studies, recipients of a renal graft from a donor with a genetic predisposition to hypertension had higher blood pressures and required more antihypertensive treatment than recipients of a renal graft from a normotensive donor. In addition, blood pressure normalization in patients suffering from essential hypertension by bilateral nephrectomy and subsequent transplantation of a kidney from a normotensive donor has been reported. The interpretation of these data may be limited by the large number of different factors that can contribute to post-transplantation hypertension in human renal transplant recipients. EXPERIMENTAL STUDIES: In experimental renal transplantation studies the contribution of individual factors to post-transplantation hypertension can be independently assessed. Besides immunological graft rejection and hypertension-induced damage to the renal graft, a genetic predisposition to hypertension in the kidney donor has been demonstrated to be associated with post-transplantation hypertension in the recipient. Thus, normotensive recipients of a renal graft from a genetically hypertensive donor consistently developed post-transplantation hypertension in four different animal models of genetic hypertension. Furthermore, in genetically hypertensive rats bilateral nephrectomy together with transplantation of a kidney from a normotensive donor has been shown to be associated with a decrease in blood pressure. CONCLUSIONS: Hypertension following renal transplantation may be due to a variety of factors, including immunological graft rejection, hypertension-induced damage to the renal transplant and a genetic predisposition to hypertension of the kidney donor. The finding that blood pressure is transplanted with the renal graft in genetic hypertension suggests a genetically determined alteration to the kidney as a major factor in the aetiology of primary hypertension. The nature of this factor is just beginning to be understood. Renal transplantation studies in rat models of genetic hypertension, combined with the tools of molecular biology, may help to provide further insights into the role of the kidney in primary hypertension.

Animals↗
PubMedDate not supplied

Application of intravascular brachytherapy: utilization of beta-irradiation source delivery systems within a community-based cardiac center.

PURPOSE: There are numerous clinical studies ongoing to assess the outcome, physics, and radiobiology of intravascular brachytherapy and its effect on the reduction of the rate of restenosis after balloon angioplasty procedures. The present study reports on the experience of two different delivery systems as utilized in the community hospital setting. METHODS AND MATERIALS: Patients were enrolled into one of four ongoing trials at our institution: the Novoste Beta-Cath trial, the Novoste Stents and Radiation Therapy trial (START), the Novoste START 40/20 trial, and the Guidant Intimal Hyperplasia Inhibition with Beta In-stent Trial (INHIBIT). The Novoste studies utilized the Novoste Beta-Cath System with 90Sr/Y, and the Guidant INHIBIT trial used 32P. Enrollments into the various trials were determined by inclusion and exclusion criteria specified by each protocol. Randomization was conducted per criteria as determined by the participating study protocol. RESULTS: Forty-two patients were enrolled in total. Thirty-four were included in the Novoste trials and eight in the Guidant study, according to availability of the trial. Assessment of practicality of treatment was dependent primarily on treatment duration and extension of time of catheterization procedures by the addition of intravascular radiation. Average dwell time within the Novoste trials was 3 min 40 s, and 7 min 46 s for patients in the Guidant study. No acute complications were observed in any of the trials. CONCLUSIONS: Intravascular brachytherapy can be performed in the community hospital setting without compromising the efficiency of balloon angioplasty procedures. Pending long-term outcome data and FDA approval for specific delivery systems, endovascular brachytherapy in community hospital cardiac catheterization laboratories can be realized in an efficient and timely manner.

Aged↗