DESIGN, ASSEMBLY AND CHARACTERIZATION OF COMPOSITE ...

φsolids=0.32 and φsolids=0.44 suspensions are too agglomerated to allow a meaningful
viscosity measurement. An increasing ηapp is observed for φsolids=0.32 and φsolids=0.44 at
surfactant concentrations greater than 1.3 mg/m 2 and 1.5 mg/m 2 , respectively. In the
same surfactant concentration range ηapp of φsolids=0.11 and φsolids=0.17 suspensions
exhibits no significant change. An explanation for the observed ηapp behavior is likely to
include these considerations: i) at a low φsolids, less collision and friction between carbon
black particles result in a very low ηapp, provided that the agglomeration in suspension is
statistically insignificant. It is likely the case for the suspensions with φsolids = 0.11 and
φsolids = 0.17 at the surfactant concentration ≥1.25 mg/m
88
2 . When the surfactant
concentration is too low, as 0.9 mg/m 2 for the φsolids = 0.11 suspension and 1.1 mg/m 2 for
the φsolids = 0.17 suspension, the amount of agglomerated particles increases. These
particles collide with one another upon shearing and lead to higher ηapp. ii) As φsolids
increases, the ηapp of the suspension is expected to increase and the effect of
agglomerated particles on the magnitude of ηapp becomes significant. Suspensions with
less than 1.3 mg/m 2 surfactant concentration become dilatant. A higher surfactant
concentration is needed to achieve a low ηapp. This trend is shown by the minimal ηapp of
fully dispersed φsolids=0.44 suspension at surfactant concentration of 1.5 mg/m 2 , and of
φsolids=0.32 suspension at surfactant concentration of 1.3 mg/m 2 where insignificant
agglomeration could still exist. iii) An excessive surfactant addition (>1.5 mg/m 2 ) shows
a positive correlation with the ηapp of high φsolids suspensions due to the increase of
surfactant concentration in the aqueous medium; however, such increase in low φsolids
suspension is much less and insignificant to cause a rise in ηapp. Based on these analyses,