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U Gasser

Publications and source records attributed to U Gasser.

11 recordsLinked to original sources
PubMed2005

Pair interaction of dislocations in two-dimensional crystals.

The pair interaction between crystal dislocations is systematically explored by analyzing particle trajectories of two-dimensional colloidal crystals measured by video microscopy. The resulting pair energies are compared to Monte Carlo data and to predictions derived from the standard Hamiltonian of the elastic theory of dislocations. Good agreement is found with respect to the distance and temperature dependence of the interaction potential, but not regarding the angle dependence where discrete lattice effects become important. Our results on the whole confirm that the dislocation Hamiltonian allows a quantitative understanding of the formation and interaction energies of dislocations in two-dimensional crystals.

Journal Article↗
PubMed2005

Time-dependent strength of colloidal gels.

Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure.

Journal Article↗
PubMed2005

Anisotropic mean-square displacements in two-dimensional colloidal crystals of tilted dipoles.

Superparamagnetic colloidal particles confined to a flat horizontal air-water interface in an external magnetic field, which is tilted relative to the interface, form anisotropic two-dimensional crystals resulting from their mutual dipole-dipole interactions. Using real-space experiments and harmonic lattice theory we explore the mean-square displacements of the particles in the directions parallel and perpendicular to the in-plane component of the external magnetic field as a function of the tilt angle. We find that the anisotropy of the mean-square displacement behaves nonmonotonically as a function of the tilt angle and does not correlate with the structural anisotropy of the crystal.

Journal Article↗
PubMed2004

Anisotropic defect-mediated melting of two-dimensional colloidal crystals.

The melting transition of anisotropic two-dimensional (2D) crystals is studied in a model system of superparamagnetic colloids. The anisotropy of the induced dipole-dipole interaction is varied by tilting the external magnetic field off the normal to the particle plane. By analyzing the time-dependent Lindemann parameter as well as translational and orientational order we observe a 2D smecticlike phase. The Kosterlitz-Thouless-Halperin-Nelson-Young scenario of isotropic melting is modified: dislocation pairs and dislocations appear with different probabilities depending on their orientation with respect to the in-plane field.

Journal Article↗
PubMed2004

Limits to gelation in colloidal aggregation.

We show that the dynamics of large fractal colloid aggregates are well described by a combination of translational and rotational diffusion and internal elastic fluctuations, allowing both the aggregate size and internal elasticity to be determined by dynamic light scattering. The comparison of results obtained in microgravity and on Earth demonstrates that cluster growth is limited by gravity-induced restructuring. In the absence of gravity, thermal fluctuations ultimately inhibit fractal growth and set the fundamental limitation to the lowest volume fraction which will gel.

Colloids↗
PubMed2001

Real-space imaging of nucleation and growth in colloidal crystallization.

Crystallization of concentrated colloidal suspensions was studied in real space with laser scanning confocal microscopy. Direct imaging in three dimensions allowed identification and observation of both nucleation and growth of crystalline regions, providing an experimental measure of properties of the nucleating crystallites. By following their evolution, we identified critical nuclei, determined nucleation rates, and measured the average surface tension of the crystal-liquid interface. The structure of the nuclei was the same as the bulk solid phase, random hexagonal close-packed, and their average shape was rather nonspherical, with rough rather than faceted surfaces.

Journal Article↗
PubMed1997

Clinical efficacy and tolerability of a new levodopa/benserazide dual-release formulation in parkinsonian patients. L-Dopa Dual-Release Study Group.

To improve the response of parkinsonian patients to L-Dopa treatment, a new preparation of L-Dopa/benserazide with dual-release properties was developed. The breakable three-layer tablets with biphasic dissolution kinetics combine the advantages of the standard and sustained-release formulations. The clinical efficacy of this new formulation was assessed in an open-label, pilot study for 14 weeks conducted by Swiss neurologists. Sixty-one parkinsonian patients were included: 5 (8%) patients were de novo, 39 (64%) had fluctuations, and 17 (28%) without fluctuations. The mean Hoehn and Yahr stage was 2.6 and the mean duration of disease was 7.4 years. The best prestudy treatment was kept stable for 2 weeks before entering the first 8-week period in which standard and/or sustained L-Dopa treatment could be either entirely or partially substituted by the dual-release formulation, which was as far as possible kept unchanged during the second 6-week period. During the substitution period, the overall dose of L-Dopa was significantly increased by 11.5%, probably reflecting some differences in the bioavailability of the various galenical formulations, and the mean daily drug intakes were reduced from 5.4 at baseline to 4.1 at week 8 (a 24% reduction, p < 0.001). Sixteen percent of the patients dropped out of the study because of unsatisfactory results, but none left for safety reasons. At the end of the study, complete substitution was attained in 71% of the patients. The remaining 27% combined the dual-release formulation with standard and/or sustained-release L-Dopa. The efficacy of treatment was assessed with the Webster Score and qualified with a mean decrease of 27% (p < 0.001) between baseline and week 14. A significant decrease of the reported adverse events such as dyskinesias and end-of-dose or wearing-off akinesias was also observed before (27 patients, 44%) and after (9 patients, 17%) the substitution (p < 0.02). These results infer that the dual-release formulation is as good as or superior to any other galenic form of L-Dopa. In conclusion, the dual-release formulation of L-Dopa either introduced or substituted for the best treatment available showed good clinical efficacy and tolerability in all stages of the evolution of idiopathic Parkinson's disease treatment in this 14-week open-label study.

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