Shear thinning of non-Brownian suspensions and its variation at different ambient conditions

Immiscible contaminants are commonly involved in naturally occurring suspensions. The resulting variations in their flow behavior have rarely been evaluated. Here, we investigate the variation in the viscosity of an oil-based two-phase suspension over a period of 2 years, which is exposed to the ambient air at the production stage. We find that the absolute humidity of air, which strongly varies with seasons, causes exchanges of water droplets with the suspension, substantially altering its shear-thinning behavior. Only in winter, when the humidity is low, is the latter close to that of the ideal two-phase suspensions. Our measurements suggest that when the surface roughness of the suspended solid particles is sufficiently low, immersed droplets remain in a free state, effectively increasing repulsion between particles, weakening shear thinning. In contrast, when the roughness is sufficiently high, immersed droplets become trapped on the particle surfaces, inducing an attractive particle interaction via water bridging, enhancing shear thinning.