The forces between a carbon sphere and highly ordered pyrolytic graphite (HOPG) surface were measured in ethanol aqueous solutions. A long-range attractive interaction was observed in ethanol aqueous solutions with less than 60% ethanol on a volume basis. The magnitude and the range of the attractive interaction decreased with an increase in ethanol concentration in solution. An air bubble was found to attach to the HOPG surface in solutions with an ethanol concentration less than 20 vol %. The presence of nanobubbles at the HOPG surface, made possible through a surface washing process and confirmed through in situ imaging, extended the range of both the long-range attraction and the bubble attachment regime to ethanol concentrations 60-80 vol %. The shear yield stress of suspensions of carbon black in ethanol-aqueous solutions was used as an indicator of the presence of a net attractive interaction between particles. The yield stress was observed to increase rapidly for ethanol concentrations less than 60 vol %. Contact angle analysis for ethanol-aqueous solutions in air showed that an angle of greater than 40° was required to induce a long-range attraction. Modeling indicated that the range and form of the interaction was consistent with a capillary cavitation force. The data clearly shows that surface nanobubbles are not critical to the development of a long-range attractive force, bubble pickup on surfaces, or enhanced suspension rheology, but that their presence greatly aids these processes.

No abstract

Measuring yielding in cohesive suspensions is often hampered by slip at measurement surfaces. This paper presents creep data for strongly-flocculated suspensions obtained using vane-in-cup tools with differing cup-to-vane diameter ratios. The three suspensions were titania and alumina aggregated at their isoelectric points and polymer-flocculated alumina.The aim was to find the diameter ratio where slip or premature yielding at the cup wall had no effect on the transient behaviour. The large diameter ratio results showed readily understandable material behaviour comprising linear viscoelasticity at low stresses, strainsoftening close to yielding, time-dependent yield across a range of stresses and then viscous flow. Tests in small ratio geometries however showed more complex responses. Effects attributed to the cup wall included delayed softening, slip, multiple yielding and stick-slip events, and unsteady flow. The conclusion was that cups have to be relatively large to eliminate wall artefacts. A diameter ratio of three was sufficient in practice, although the minimum ratio must be material dependent.

No abstract

No abstract