The rheology of bimodal mixtures of colloidal particles with long-range, soft repulsions

The rheological properties of dense suspensions of bimodal mixtures of coll oidal particles with long-range, soft repulsions were investigated. Suspens ions of particles suspended in 10(-4) M KCI with volume fractions ranging f rom 0.3-0.6 were studied for volume fraction ratios of large to small parti cles of 0, 0.25, 0.5, 0.75, and 1.0. Latex particles of diameters ranging b etween 105 to 544 nm were used. These particles were stabilized by a combin ation of electrostatic and short range steric repulsions. Four separate mix tures were investigated with size ratios (large/small) of 1.2-5. At volume fractions investigated, the suspensions displayed dynamic yield stresses, t au(y), and shear thinned with increasing stress or shear rate. The yield st ress was found to be proportional to the suspension's elastic modulus, with a constant of proportionality lying between 0.015 and 0.03 as has been rep orted for a wide range of monodisperse suspensions. The functional dependen ce of stress on shear rate could be reduced to a single master curve which was independent of volume fraction, particle size ratio, and mixing ratio b y scaling tau(y) on G, and the shear rate on G/eta(c) where eta(c) is the c ontinuous phase viscosity. In bimodal suspensions sheat thickening accompan ied by irreversible aggregation was observed at volume fractions substantia lly below that measured for monodisperse suspensions. The stress and sheat rate at thickening decreased rapidly as the volume fraction of the mixed su spension was increased. These results are substantially different than what has been reported for well-mixed suspensions of particles experiencing "ha rd" repulsions in that as the fraction of large particles is increased no v iscosity minimum is seen at low and intermediate shear rates. (C) 1999 Acad emic Press.