Influence of particle modulus on the rheological properties of agar microgel suspensions

The linear viscoelastic and steady shear flow properties of high phase volume suspensions of a range of agar microgel particles have been measured and are found to depend upon the deformability (or modulus) of the particles. Agar concentrations in the range 0.5-5 wt % are utilized, giving a range of particle modulus spanning 2.4-185 kPa. On increasing the particle modulus, in suspensions with phase volumes above maximum packing, the storage modulus increases by two orders of magnitude although the loss tangent (tan delta) also increases due to increasing viscous dissipation. The flow properties of the suspensions at high shear stresses also showed significant differences due to changing particle rigidity. The suspensions containing the hardest particles are found to display limited evidence of shear-thickening behavior at high stresses, while those containing the softest particles continue to shear thin. A high-shear plateau in the viscosity is observed for suspensions with particles of medium rigidity. The suspensions containing the stiffer particles also have a considerably higher viscosity for the same degree of space filling. Empirical expressions linking the viscoelastic and flow properties to the particle modulus have been derived. (C) 2004 The Society of Rheology.