Transparency on scientific instruments

Scientists and commercial scientific instrument makers have a shared incentive against discloseing an instrument maker's contributions to research. Stricter rules to encourage reporting of such collaboration would help to improve transparency and reproducibility

Colloidal gelation and non-ergodicity transitions

Within the framework of the mode coupling theory (MCT) of structural relaxation, mechanisms and properties of non-ergodicity transitions in rather dilute suspensions of colloidal particles characterized by strong short-ranged attractions are studied. Results building on the virial expansion for particles with hard cores and interacting via an attractive square well potential are presented, and their relevance to colloidal gelation is discussed.Comment: 10 pages, 4 figures; Talk at the Conference: "Unifying Concepts in Glass Physics" ICTP Trieste, September 1999; to be published in J. Phys.: Condens. Matte

The effect of antifibrinolytic agents on the healing of modified Widman flaps in monkeys

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/65632/1/j.1600-0765.1984.tb00810.x.pd

Glasses in hard spheres with short-range attraction

We report a detailed experimental study of the structure and dynamics of glassy states in hard spheres with short-range attraction. The system is a suspension of nearly-hard-sphere colloidal particles and non-adsorbing linear polymer which induces a depletion attraction between the particles. Observation of crystallization reveals a re-entrant glass transition. Static light scattering shows a continuous change in the static structure factors upon increasing attraction. Dynamic light scattering results, which cover 11 orders of magnitude in time, are consistent with the existence of two distinct kinds of glasses, those dominated by inter-particle repulsion and caging, and those dominated by attraction. Samples close to the `A3 point' predicted by mode coupling theory for such systems show very slow, logarithmic dynamics.Comment: 22 pages, 18 figure

Simulation study of Non-ergodicity 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 modelled and the results were compared with mode coupling theory expectations and with the results for other systems (hard spheres and Lennard Jones). 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 non-ergodicity transition within MCT. The leading mechanism for this novel non-ergodicity transition is identified as bond formation caused by the short range attraction. The time scale and diffusion coefficient also show qualitatively the expected behaviour, although different exponents are found for the power-law divergences of these two quantities. The non-Gaussian parameter was also studied and very large correction to Gaussian behaviour 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 non-ergodicity transitions still hold.Comment: 13 pages, 15 figure

Dynamic heterogeneities in attractive colloids

We study the formation of a colloidal gel by means of Molecular Dynamics simulations of a model for colloidal suspensions. A slowing down with gel-like features is observed at low temperatures and low volume fractions, due to the formation of persistent structures. We show that at low volume fraction the dynamic susceptibility, which describes dynamic heterogeneities, exhibits a large plateau, dominated by clusters of long living bonds. At higher volume fraction, where the effect of the crowding of the particles starts to be present, it crosses over towards a regime characterized by a peak. We introduce a suitable mean cluster size of clusters of monomers connected by "persistent" bonds which well describes the dynamic susceptibility.Comment: 4 pages, 4 figure

Theory and simulation of gelation, arrest and yielding in attracting colloids

We present some recent theory and simulation results addressing the phenomena of colloidal gelation at both high and low volume fractions, in the presence of short-range attractive interactions. We discuss the ability of mode-coupling theory and its adaptations to address situations with strong heterogeneity in density and/or dynamics. We include a discussion of the effect of attractions on the shear-thinning and yield behaviour under flow.Comment: 17 pages, 6 figure

Higher order glass-transition singularities in colloidal systems with attractive interactions

The transition from a liquid to a glass in colloidal suspensions of particles interacting through a hard core plus an attractive square-well potential is studied within the mode-coupling-theory framework. When the width of the attractive potential is much shorter than the hard-core diameter, a reentrant behavior of the liquid-glass line, and a glass-glass-transition line are found in the temperature-density plane of the model. For small well-width values, the glass-glass-transition line terminates in a third order bifurcation point, i.e. in a A_3 (cusp) singularity. On increasing the square-well width, the glass-glass line disappears, giving rise to a fourth order A_4 (swallow-tail) singularity at a critical well width. Close to the A_3 and A_4 singularities the decay of the density correlators shows stretching of huge dynamical windows, in particular logarithmic time dependence.Comment: 19 pages, 12 figures, Phys. Rev. E, in prin

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 non-ergodicity 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