Attraction between hydrophobic surfaces studied by atomic force microscopy

Attraction between hydrophobic surfaces, known as the hydrophobic force, is critically important for attachment of particles to air bubbles in flotation. However, the origins and models for this attractive force between hydrophobic surfaces have been a source of debate since the first direct measurements of this force in the early 1980s. Using an atomic force microscope (AFM) we studied the attraction between an AFM hydrophobic probe and a flat hydrophobic surface in water, in water-ethanol mixtures, and in water saturated by gases with different solubility. The strong attractive force with long-range jump-in attachment positions decreases with an increase in the ethanol content and disappears in pure ethanol. The size of steps on the force curves depends on the gas solubility. However, the measured forces do not depend on the gas solubility significantly. The influence of surface roughness and heterogeneity appear to be significant. Experimental results indicate the role of surface stabilized submicron-sized bubbles in the hydrophobic attraction. This is in line with recent direct and indirect evidences for the presence of gaseous bubbles at hydrophobic surfaces as well as with the early insights of flotation scientists. (C) 2003 Elsevier B.V. All rights reserved.