Biomedical subjects

J Mewis

Publications and source records attributed to J Mewis.

3 recordsLinked to original sources

PubMed2002

Structure and rheology during shear-induced crystallization of a latex suspension.

Microstructure and rheology of a concentrated sterically stabilized colloidal suspension undergoing flow-induced ordering was studied by combined small-angle x-ray scattering and rheometry. This system is known to form bundlelike structures at high stress values in continuous shear flow. Under large amplitude oscillatory flow, hexagonal close-packed crystalline domains are formed within 1 sec of the inception of shear. In the intermediate range of frequencies and amplitudes, a nearly perfect hexagonally close-packed layer structure was observed after the cessation of flow. Lower frequencies or stress amplitudes resulted in polycrystals and, on the other hand, high frequencies or stress amplitudes led to partial melting of the layered structure. During the oscillatory flow, the intensity of the Bragg peaks showed pronounced oscillations.

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PubMed2000

Flow-Induced Anisotropy in Mixtures of Associative Polymers and Latex Particles.

The effect of associative polymers on the structure and rheological behavior of colloidal suspensions is discussed. Adding associative polymer is known to increase the viscosity of the suspensions. At high shear rates the increase is close to what could be expected on the basis of the hydrodynamic effects of the added polymer. At low shear rates the viscosity increases much more. Small-angle light scattering (SALS) during flow is used here to investigate the underlying structural mechanisms. The SALS patterns indicate that the associative polymer changes the particulate structure: characteristic butterfly patterns appear even at relatively low particle volume fractions. They are not present in the suspensions without associative polymer. The patterns indicate that fluctuations in particle concentration are more pronounced in the flow direction than in the vorticity direction and that anisotropic particulate structures with an orientation along the vorticity direction develop. The evolution of their characteristic length scale during flow has been followed over time. Changing the hydrophilic part of the polymer from polyacrylamide to polyacrylic acid induces stronger associative interactions. In the suspensions this results in a reduction of the relative viscosity rather than an increase. The difference in degree of associativity between the polymers also has an effect on the SALS patterns in the suspensions both at rest and during flow. The rheology as well as the SALS suggest the presence of a strong polymer network in the second system. The competition between adsorption of the associative polymer on the particles with the intermolecular associations between the polymer chains seems to be responsible for the observed differences. Copyright 2000 Academic Press.

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PubMed1999

Large-Scale Bundle Ordering in Sterically Stabilized Latices.

Time-resolved small-angle light scattering and linear conservative dichroism measurements are presented for concentrated, sterically stabilized, aqueous latices under simple shear flow. At low stress levels, flow causes a mild distortion of the liquid-like structure in colloidally stable dispersions, which is quite well understood. In this paper flow-induced structures are investigated in concentrated dispersions when the system is brought far from equilibrium by means of hydrodynamic forces. At high stress levels various structural changes have been predicted by numerical simulation, among others string phases oriented in the flow direction. Here, experimental results are reported on a bundle-like ordering in very dense systems, which involves a length scale much larger than that of a single string of particles. Two latices, with different particle sizes and different thicknesses of the stabilizing layer, are compared. The occurrence of the bundle-like ordering is related to the rheological behavior: it causes a significant decrease in viscosity. It is shown that the presence of this phase results in a structural hysteresis, which explains a thixotropic behavior that is encountered in some stable colloidal suspensions. Also the relaxation behavior of the bundle-like phases has been studied. Interparticle forces are found to have a very strong effect on the relaxation time scales. Copyright 1999 Academic Press.

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