  Immiscible contaminants are commonly involved in naturally occurring
suspensions. The resulting variations of their flow behavior has rarely been
evaluated. Here, we investigate the variation of the viscosity of the oil-based
two-phase suspension over a period of two years, which is exposed to the
ambient air at the production stage. We find that the air's absolute humidity,
which strongly varies with the 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 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.
