Influence of dissolved air on bubble attachment to highly oriented pyrolytic graphite

The effect of air, dissolved in 0.1 M KCl solution, on bubble attachment to the smooth hydrophobic surface of highly oriented pyrolytic graphite was studied. The stability of a wetting film in such a system is governed by surface forces, i.e. electrostatic and van der Waals interactions. At the high ionic strength investigated, the electric double layer forces are both weak and of short range, therefore the stability of the wetting film is dominated by van der Waals interactions. The Hamaker coefficient for the highly oriented pyrolytic graphite-KCl aqueous solution-air system is negative and hence van der Waals interactions are repulsive. A repulsive force should stabilize the wetting film, preventing its rupture and bubble attachment to the highly oriented pyrolytic graphite surface. Many experimental studies have found that wetting films are not stable at graphite or coal surfaces, and air bubbles attach. In the present experiments, the stability of the wetting films decreased with increasing amount of dissolved air. The time required for film drainage, rupture, and air bubble attachment was shortened by two orders of magnitude when the experiments were performed in air saturated 0.1 M KCl solution. This instability was attributed to an increasing number of nano-and submicron-bubbles nucleated at the graphite surface. The Hamaker coefficient across the air-KCl aqueous solution-air system is positive and hence van der Waals interactions are attractive, resulting in wetting film rupture and macroscopic air bubble attachment to a highly oriented pyrolytic graphite surface decorated with resident nano-and submicro-metre bubbles.