Effects of solvency and interfacial nanobubbles on surface forces and bubble attachment at solid surfaces

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