Forces measured between hydrophobic surfaces due to a submicroscopic bridging bubble

Atomic force microscopy on hydrophobic microspheres in water reveals a strong attraction with a range of 20-200 nm, following an initial steep repulsion at long range. The data are consistent with a single submicroscopic bubble between the surfaces, with the attraction due to its attachment and lateral spread, and the repulsion dependent on film drainage and the electric double layer. The results provide direct experimental evidence of the existence of long-lived submicron bubbles, and of their bridging as the cause of the measured long-range attractions between macroscopic hydrophobic surfaces.