Rheology of colloidal suspensions flocculated by reversible bridging

Contact Networks Enhance Shear Thickening in Attractive Colloid-Polymer Mixtures

Increased shear thinning arising due to strong attractive interactions between colloidal particles is thought to obscure shear thickening. Here, we demonstrate how moderate attractions, induced by adding a nonadsorbing polymer, can instead enhance shear thickening. We measure the rheology of colloidal suspensions at a constant particle volume fraction of ϕ=0.40 with dilute to weakly semidilute concentrations of three polyacrylamide depletants of different molecular weights. Suspensions containing large polymer exhibit increased shear thickening and positive first normal stress differences at high shear stress, and increased heterogeneous fluctuations in the boundary stress. These results are consistent with a friction-based model for shear thickening, suggesting that the presence of large, extended polymers induces the formation of near-spanning networks of interparticle contacts.

Effective properties of suspensions/emulsions, porous and composite materials

A new method (a version of a mean field method) is suggested to calculate effective properties of suspensions/emulsions, porous and dispersed materials. The aim is to demonstrate a wide range of applicability of the new method. To show the idea of the new method the dependence of the effective diffusion coefficient in the porous medium on the porosity is deduced. Based on the same method the following dependences are deduced: the effective viscosity of suspensions and emulsions as functions of volume fraction of suspended particles or droplets, elastic modules of rubber/polymer sheets with cracks and elastic modules of composite materials on the volume fraction of inclusions in the case of an arbitrary number of different types of inclusions. In all cases the calculated dependences are compared with available experimental data and published theoretical models.

Polyvinylpyrrolidone modified bioactive glass fibers as tissue constructs: in vitro mesenchymal stem cell response

The sol-gel synthesis of a bioactive glass (BAG) sol with the incorporation of polyvinylpyrrolidone and the subsequent spraying of short, discontinuous fibers is reported. The incorporation of the polymer into the BAG sol allowed for increased control of the rheological properties and resulted in a more homogeneous fibrous material when sprayed through an air gun. Reaction kinetics and sol viscosity were monitored and analyzed during synthesis, and fibers were characterized using scanning electron microscopy and thermal analysis. Fibers were sintered at 900 degrees C and were examined for in vitro bioactivity in a simulated body fluid solution. The presence of hydroxyapatite crystals is confirmed by examination with scanning electron microscopy, energy dispersive X-ray spectroscopy (EDX), and X-ray diffraction (XRD). Both the proliferation rate and cell density of rat mesenchymal stem cells cultured on BAG fiber constructs of varying porosities were shown to be dependent upon fiber spacing.

Aplicação de um Modelo Reológico Não-Linear em Sistemas Poliméricos Dilatantes

A dilatância é um comportamento reológico comum a vários sistemas poliméricos embora não ocorra em polímeros puros. Existem várias teorias aplicáveis à explicação desse fenômeno e alguns modelos matemáticos para o ajuste de dados experimentais de quatro de suas seis formas de ocorrência. Neste trabalho é dado enfoque à dilatância antecedida por um comportamento newtoniano e sucedida pelo comportamento pseudoplástico. Para ajustar dados constantes da literatura para sistemas desse tipo, é apresentado um modelo híbrido e não-linear. Para o teste do modelo foram utilizados dados existentes na literatura para solução de alfa,ômega poli(t-butilestireno) em óleo mineral e suspensão de partículas do copolímero de estireno com acrilato de metila em solução aquosa de poli(ácido acrílico). O modelo proposto apresenta boa concordância com os dados experimentais.