Interpartical Attractions and the Mechanical Properties of Colloidal Gels
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INTERPARTICAL ATTRACTIONS AND THE MECHANICAL PROPERTIES OF COLLOIDAL GELS C. J. RUEB and C. F. ZUKOSKI, Department of Chemical Engineering, University of Illinois, Urbana, Illinois 61801 Abstract: The influence of the strength of interparticle attractions on the mechanical properties of colloidal gels is explored. As attractions increase, suspensions gel. At the gel point the zero shear rate viscosity diverges while the low frequency modulus becomes measurable. In addition at the gel point, storage and loss moduli were of the same order and displayed power law sealing on oscillation frequency. With much stronger attractions, the critical strain required to rupture the gel scales in a power law manner or elastic modulus. These observations are discussed in terns of recently developed models of gel fonnation and dynamics.
I. INTRODUCTION The density of green particle compacts plays an important role in determining the flaw density of fired ceramics. Higher initial densities result in lower shrinkage on sintering and a lower probability of there being large voids in the fired piece. Typically dense compacts are generated by using colloidally stable dispersions. If the particles are large, these suspensions easily form dense compacts. However, if small particles (i.e., less than 100 nm) are required, due to the large osmotic pressures produced at modest volume fractions, dense compacts are difficult to prepare. In addition if mixtures of particles with different densities are required, segregation may occur in stable suspensions. Recently Velunakanni et al. (1) have shown that these problems can be avoided using weakly agglomerated charge stabilized aqueous suspensions. By agglomerating aqueous alumina slurries by addition of salt to suspension's at pH's well away from the particle's isoelectric point, weak, easily compacted agglomerates are formed. If the same suspensions are agglomerated at the particle's isoelectric point, strong flocs that resist densification are produced as a result of particles residing in the primary van der Waals minimum. By working with charged particles, hydration shells of adsorbed ions produce a short range interparticle repulsion and the strength of the attraction is limited. As a result the agglomerated particles are held at small separations allowing for floc rearrangement. The work of Velamakanni et al. demonstrates that by controlling the degree of attraction, higher quality compacts can be formed. However, the influence of the strength of the attraction on floc rearrangement and floc structure are poorly understood, thus limiting application of these concepts to other systems. In this paper we address the influence of the strength of interparticle attraction on the mechanical properties of organic sterically stabilized colloidal gels. Uniform silica particles coated with close packed octadecyl hydrocarbon chains suspended in decaline are chosen as a model system. At room temperature, these suspensions display hard sphere characteristics where the pair potential is zero until contact
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