RESUMO
Many biological materials, consumer products and industrial formulations are colloidal suspensions where the suspending medium is itself a complex fluid, and such suspensions are effectively soft matter composites. At rest, the distortion of the microstructure in the suspending fluid by the particles leads to attractive interactions between them. During flow, the presence of a microstructure in the viscoelastic suspending medium changes the hydrodynamic forces due to the non-Newtonian and viscoelastic effects. However, little is known about the structural development, the rheology and the final properties of such materials. In the present study, a model flocculated suspension in both a Newtonian and a viscoelastic medium was studied by combined rheological and rheo-confocal methods. To this extent, micrometer-sized fluorescent PMMA particles were dispersed in polymeric matrices (PDMS). The effect of fluid viscoelasticity is studied by comparing the results for a linear and a branched polymer. Stress jump experiments on the suspensions were used to de-convolute the rate dependence of the viscous and elastic stress contributions in both systems. These results were compared to a qualitative and quantitative analysis of the microstructure during flow as studied by fast structured illumination confocal microscopy, using a counter-rotating rheometer. At comparable interaction strength, as quantified by equal Bingham numbers, the presence of medium viscoelasticity leads to an enhanced densification of the aggregates during steady-state flow, which is reflected in lower limiting high shear viscosities. Following a strong preshear, the structural and mechanical recovery is also altered between the Newtonian and viscoelastic matrix with an increase in the percolation threshold, but with the potential to build stronger materials exploiting the combination of processing history and medium rheology at higher volume fractions.
RESUMO
Shearing lyophobic colloidal suspensions can lead to aggregation, followed by gelation, if the formed clusters grow to sizes large enough to percolate. If the temperature is set over the glass transition temperature of the suspended material, the particles embedded in the same aggregate start to coalesce with one another. Coalescence occurs to the finite viscosity of the particles' material, which leads to material diffusion from particle to particle. The driving force of this process is the reduction of the particle-dispersant interface and, as a consequence, the decrease the center-to-center separation of the particles. This leads to decreased cluster size, and hence a delayed gelation. Simultaneously, coalescence reinforces the particle-particle bonds formed upon aggregation, leading to clusters that are able to resist higher hydrodynamic forces before breaking up, hence leading to faster gelation. These two competing effects, combined with the natural complexity of colloidal aggregation makes it rather difficult to understand and predict which trend becomes dominant. In the present work, the shear-induced gelation of model polymeric colloidal systems with different glass transition temperatures has been studied. Starting with their interaction potential we investigate the impact of temperature on the gel time in concentrated suspensions (φ = 5%) under steady shear, followed by the effect of temperature on the stress-resistance of fully destabilized clusters under agitation. The results of the present work allow for a systematic view and deepened understanding of the factors governing shear-induced gelation in the presence of coalescence.
