Softening of shear-thickening in suspensions by the addition of large particles

This study investigates the rheological behavior of shear-thickening suspensions made with different types of small particles upon the addition of much larger particles referred to as granules. The size ratio ranges from 20 to 120. We examine the effects of granule size, volume fraction, and surface properties on shear-thickening characteristics. Starting from a fumed silica suspension exhibiting discontinuous shear thickening (DST), the addition of granules at different volume fractions shifts the onset of thickening to lower shear rates. Concomitantly, the strength of the thickening, quantified by the thickening index, decreases, transitioning from DST to continuous shear thickening (CST). Comparison with suspensions of silica spheres reveals a similar trend, suggesting generality across different systems. These results contrast with the results of prior work on cornstarch-based and nanosilica sphere suspensions, where granule addition was found to enhance thickening. We discuss possible origins of these differences and propose a mechanism for the observed softening: when granules are much larger than the surrounding particles, they induce local variations in shear rate and disrupt the formation of an extended network of force chains. These findings highlight the critical role of particle size ratio in determining the rheology of complex suspensions, paving the way for tailoring material properties for industrial and scientific applications.