Abstract
Shear thickening is a widespread phenomenon in suspension flow that, despite sustained study, is still the subject of much debate. The longstanding view that shear thickening is due to hydrodynamic clusters has been challenged by recent theory and simulations suggesting that contact forces dominate, not only in discontinuous, but also in continuous shear thickening. Here, we settle this dispute using shear reversal experiments on micron-sized silica and latex particles to measure directly the hydrodynamic and contact force contributions to shear thickening. We find that contact forces dominate even continuous shear thickening. Computer simulations show that these forces most likely arise from frictional interactions.
Highlights
Shear thickening, the increase of viscosity with shear rate, is ubiquitous in complex fluids [1,2,3,4]
The longstanding view that shear thickening is due to hydrodynamic clusters has been challenged by recent theory and simulations suggesting that contact forces dominate, in discontinuous, and in continuous shear thickening
We find that contact forces dominate even continuous shear thickening
Summary
Which the shear is reversed and a negative shear rate is imposed to accumulate the same amount of strain. Contact forces in hard-sphere systems will drop to zero immediately after reversal, Fig. 1(b). This qualitative difference between these two forces allows us to disentangle their separate contributions by measuring the transient stress upon reversal. We monitor the viscosity after shear reversal ηrev as a function of accumulated strain after reversal γ at four representative shear rates γ_ 1⁄4 0.50, 1.00, 2.00, and 5.00 s−1, in the shear thickening regime, Fig. 2(b). The viscosity immediately after reversal drops to a value η0rev that remains constant, to within experimental error, until γ ≳ 0.1. While the contact contribution increases with γ_, the hydrodynamic contribution remains constant as the suspension shear thickens. Relative viscosity Η Relative viscosity Η RelatiVivsecvoisistcyoΗsitΗy0 Η (a) .. . =0.74 .0 . =0.20 .0 . =0.13 .0
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