Optical observation of gas bridging between hydrophobic surfaces in water

The very strong and long-ranged interaction force between hydrophobic surfaces in water has been a debated issue for a long time. Recent studies suggest that the long-range attraction is attributable to the bridging of nanoscopic bubbles attached on the surfaces. However, it is still unclear at present whether such a bridging is able to exit stably or not. To clarify the existence of the gas bridge, we conducted the optical observation and the force measurement between the hydrophobic glass particle and plate in water simultaneously, using a combined apparatus of an atomic force microscope and an optical inverted microscope. It is found that (i) the image of the dark ring of ca. 1 μm in diameter appears in the region where the existence of the bridge is confirmed by force curves, but disappears when the separation between surfaces becomes shorter than the low limit of the wavelength of visible light, and (ii) the sudden disappearance of the image coincides well with the breakage of the bridge estimated from the separating force curve. The results obtained here are consistent with the above-mentioned mechanism for the long-range attraction between hydropobic surfaces in water.