  We show that a suspension of non-Brownian calcite particles in glycerol-water
mixtures can be tuned continuously from being a yield-stress suspension to a
shear-thickening suspension--without a measurable yield stress--by the addition
of various surfactants. We interpret our results within a recent theoretical
framework that models the rheological effects of stress-dependent constraints
on inter-particle motion. Bare calcite particle suspensions are found to have
finite yield stresses. In these suspensions, frictional contacts that constrain
inter-particle sliding form at an infinitesimal applied stress and remain
thereafter, while adhesive bonds that constrain inter-particle rotation are
broken as the applied stress increases. Adding surfactants reduces the yield
stress of such suspensions. We show that, contrary to the case of surfactant
added to colloidal suspensions, this effect in non-Brownian suspensions is
attributable to the emergence of a finite onset stress for the formation of
frictional contacts. Our data suggest that the magnitude of this onset stress
is set by the strength of surfactant adsorption to the particle surfaces, which
therefore constitutes a new design principle for using surfactants to tune the
rheology of formulations consisting of suspensions of adhesive non-Brownian
particulates.
