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Matthias Fuchs

Publications and source records attributed to Matthias Fuchs.

10 recordsLinked to original sources

Gender and age specific differences in exhaled isoprene levels.

The analysis of volatile organic compounds (VOC) in the human breath has attracted a considerable amount of clinical and scientific interest during the last decade. In our study, we turned our attention to gender and age specific differences of exhaled volatile compounds, particularly on isoprene which is one of the most abundant organic molecules found in human exhaled air. A total of 126 test persons were enrolled in the study: 66 females and 60 males. Moreover, the participants were classified into six groups with regard to their age. In a standardized setting all of them had to exhale the endexpiratory breath into a sample bag. The volatile compounds at m/z values from 21 to 229 were analyzed by using proton-transfer-reaction-mass-spectrometry. Isoprene (at m/z 69) was found to be highly significantly (p<0.001) elevated in the exhaled air of male subjects. Furthermore, it could be shown that 19-29 years old subjects exhale significantly lower levels of isoprene than older adults (p=0.002). No significant differences between groups were detected for any other measured mass. In conclusion, the present study demonstrates gender and age specific differences of isoprene levels in the exhaled air. These findings may be of potential clinical relevance regarding the multifaceted roles of isoprene, representing both indicator and effector molecule.

Adult↗

Mode coupling and dynamical heterogeneity in colloidal gelation: a simulation study.

We present simulation results addressing the dynamics of a colloidal system with attractive interactions close to gelation. Our interaction also has a soft, long-range repulsive barrier that suppresses liquid-gas type phase separation at long wavelengths. The new results presented here lend further weight to an intriguing picture emerging from our previous simulation work on the same system. Whereas mode coupling theory (MCT) offers quantitatively good results for the decay of correlators, closer inspection of the dynamics reveals a bimodal population of fast and slow particles with a very long exchange time scale. This population split represents a particular form of dynamic heterogeneity (DH). Although DH is usually associated with activated hopping and/or facilitated dynamics in glasses, the form of DH observed here may be more collective in character and associated with static (i.e., structural) heterogeneity.

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Flow curves of dense colloidal dispersions: schematic model analysis of the shear-dependent viscosity near the colloidal glass transition.

A recently proposed schematic model for the nonlinear rheology of dense colloidal dispersions is compared to flow curves measured in suspensions that consist of thermosensitive particles. The volume fraction of this purely repulsive model system can be adjusted by changing temperature. Hence, high volume fractions (phi</=0.63) can be achieved in a reproducible manner. The quantitative analysis of the flow curves suggests that the theoretical approach captures the increase of the low shear viscosity with increasing density, the shear thinning for increasing shear rate, and the yielding of a soft glassy solid. Variations of the high shear viscosity can be traced back to hydrodynamic interactions which are not contained in the present approach but can be incorporated into the data analysis by an appropriate rescaling.

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Quantification of abiotic reaction rates in mine tailings: evaluation of treatment methods for eliminating iron- and sulfur-oxidizing bacteria.

Effective treatment techniques for eliminating iron-oxidizing (IOB) and sulfur-oxidizing bacteria (SOB) are required for the comparison of abiotic and microbial sulfide oxidation rates and mechanisms in mine tailings. This study evaluates the effect of autoclaving, repeated heating, ethanol treatment, antibiotic treatment, gamma-radiation, and washing with deionized water on tailings characteristics and concentrations of IOB and SOB. Most probable number enumeration indicates that IOB and SOB were present at very low concentrations or below detection limits following treatment with all methods except rinsing and antibiotics treatment, where higher concentrations of IOB and SOB were present. The physical, chemical, and mineralogical characterization of the tailings indicated no changes in bulk mineralogy or bulk chemical composition as a result of treatment. However, an increase in oxidized sulfur species at the tailings surface, as determined by X-ray photoelectron spectroscopy, was observed for the heating, autoclaving, and antibiotics treatments. Batch weathering experiments, used to evaluate the effect of treatment on element release rates, indicated that the final element release rates (after >30 d) were similar between treated and untreated control samples. On the basis of the results of this study, experiments over relatively long periods (>30 d) are to be recommended forthe establishment of microbial and abiotic weathering rates in mill tailings samples. For the determination of abiotic reaction rates, treatment by gamma-radiation is suggested to be the most appropriate method for sulfide-rich tailings.

Anti-Bacterial Agents↗

Dynamical heterogeneities close to a colloidal gel.

Dynamical heterogeneities in a colloidal fluid close to gelation are studied by means of computer simulations. A clear distinction between some fast particles and the rest, slow ones, is observed yielding a picture of the gel composed of two populations with different mobilities. Analyzing the statics and dynamics of both sets of particles, it is shown that the slow particles form a network of stuck particles, whereas the fast ones are able to move over long distances. Correlation functions show that the environment of the fast particles relaxes much faster than that of the slow ones, but at short times the bonds between fast particles are longer lived due to the flexibility of their structure. No stringlike motion is observed for the fast particles, but they occupy preferential sites in the surface of the structure formed by the slow ones.

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Static properties of a simulated supercooled polymer melt: structure factors, monomer distributions relative to the center of mass, and triple correlation functions.

We analyze structural and conformational properties in a simulated bead-spring model of a nonentangled, supercooled polymer melt. We explore the statics of the model via various structure factors, involving not only the monomers, but also the center of mass (CM). We find that the conformation of the chains and the CM-CM structure factor, which is well described by a recently proposed approximation [Europhys. Lett. 58, 53 (2002)]], remain essentially unchanged on cooling toward the critical glass transition temperature T(c) of mode-coupling theory. Spatial correlations between monomers on different chains, however, depend on temperature, albeit smoothly. This implies that the glassy behavior of our model cannot result from static intrachain or CM-CM correlations. It must be related to interchain correlations at the monomer level. Additionally, we study the dependence of interchain correlation functions on the position of the monomer along the chain backbone. We find that this site dependence can be well accounted for by a theory based on the polymer reference interaction site model. We also analyze triple correlations by means of the three-monomer structure factors for the melt and for the chains. These structure factors are compared with the convolution approximation that factorizes them into a product of two-monomer structure factors. For the chains this factorization works very well, indicating that chain connectivity does not introduce special triple correlations in our model. For the melt deviations are more pronounced, particularly at wave vectors close to the maximum of the static structure factor.

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Simulation study of nonergodicity transitions: gelation in colloidal systems with short-range attractions.

Computer simulations were used to study the gel transition occurring in colloidal systems with short-range attractions. A colloid-polymer mixture was modeled and the results were compared with mode coupling theory (MCT) expectations and with the results for other systems (hard-spheres system and Lennard-Jones system). The self-intermediate scattering function and the mean squared displacement were used as the main dynamical quantities. Two different colloid packing fractions have been studied. For the lower packing fraction, alpha-scaling holds and the wave-vector analysis of the correlation function shows that gelation is a regular nonergodicity transition within MCT. The leading mechanism for the novel nonergodicity transition is identified as the bond formation caused by the short-range attraction. The time scale and diffusion coefficient also show qualitatively the expected behavior, although different exponents are found for the power-law divergences of these two quantities. The non-Gaussian parameter was also studied and a very large correction to Gaussian behavior was found. The system with higher colloid packing fraction shows indications of a nearby high-order singularity, causing alpha scaling to fail, but the general expectations for nonergodicity transitions still hold.

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Schematic models for dynamic yielding of sheared colloidal glasses.

The nonlinear rheological properties of dense suspensions are discussed within simplified models, suggested by a recent first principles approach to the model of Brownian particles in a constant-velocity-gradient solvent flow. Shear thinning of colloidal fluids and dynamical yielding of colloidal glasses arise from a competition between a slowing down of structural relaxation, because of particle interactions, and enhanced decorrelation of fluctuations, caused by the shear advection of density fluctuations. A mode coupling approach is developed to explore the shear-induced suppression of particle caging and the resulting speed-up of the structural relaxation.

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Theory of nonlinear rheology and yielding of dense colloidal suspensions.

A first-principles approach to the nonlinear flow of dense suspensions is presented which captures shear thinning of colloidal fluids and dynamical yielding of colloidal glasses. The advection of density fluctuations plays a central role, suppressing the caging of particles and speeding up structural relaxation. A mode coupling approach is developed to explore these effects.

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Comparative simulation study of colloidal gels and glasses.

Using computer simulations, we identify the mechanisms causing aggregation and structural arrest of colloidal suspensions interacting with a short-ranged attraction at moderate and high densities. Two different nonergodicity transitions are observed. As the density is increased, a glass transition takes place, driven by excluded volume effects. In contrast, at moderate densities, gelation is approached as the strength of the attraction increases. At high density and interaction strength, both transitions merge, and a logarithmic decay in the correlation function is observed. All of these features are correctly predicted by mode coupling theory.

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