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Apparent wall slip (AWS) is an interphase phenomenon which describes a specific boundary conditions when a dispersion flows along a wall. AWS is caused by dispersion-wall interaction (spatial reasons, electrostatic interactions) and can be either positive (formation of depleted layer apparently amplifies flow intensity) or negative (formation of stagnant layer apparently suppresses the flow). The electrostatic interactions between the dispersion and the wall can be influenced by the addition of electrolytes. In this work, the influence of different electrolytes on AWS of aqueous kaolin suspensions is studied experimentally. The fluidity and AWS characteristics of purely aqueous and deflocculated kaolin suspensions are measured by gap-dependent rotational viscometry using unconventional cone-cone geometry. The applied sensors are made of different materials: stainless steel (smooth and sandblasted), titanium, and duralumin (with anodized surface).<\/p>\n
Both the quality of the sensor surface and the presence of electrolytes strongly influence the observed AWS behavior. In the case of purely aqueous 40% kaolin suspension, positive AWS (depleted layer formation) is measured on the stainless steel and titanium sensors, while negative AWS (stagnant layer formation) is observed on the anodized duralumin sensor. In the case of fully deflocculated suspensions, Newtonian flow behavior is observed with almost no measurable AWS effects. In the case of partially deflocculated suspensions, the type of deflocculant becomes important. While the presence of Na2<\/sub>CO3<\/sub> or NaOH does not qualitatively change the AWS trends and only slightly increases them, the presence of SHMP (sodium hexametaphosphate) leads to positive AWS also on anodized duralumin. On the other hand, the addition of NaCMC (sodium salt of carboxymethylcellulose) induces negative AWS on all the surfaces studied.<\/p>\n <\/p>\n
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