  We systematically investigated the impact of boundary confinement on the
shear-thickening rheology of dense granular suspensions. Under highly confined
conditions, dense suspensions were found to exhibit size-dependent or even
rarely reported non-monotonic ($S$-shaped) flow curves in steady states. By
performing in-situ boundary stress microscopy measurements, we observed
enhanced flow heterogeneities in confined suspensions, where concentrated
high-stress domains propagated stably either along or against the shear
direction. By comparing the boundary stress microscopy results with macroscopic
flow responses, we revealed the connection between non-monotonic rheology and
stress heterogeneity in confined suspensions. These findings suggest the
possibility of controlling suspension rheology by imposing different boundary
confinements.
