Nanobubbles imaged
DOI: 10.1063/1.2405690
On hydrophobic surfaces. A hydrophobic surface not only sheds water but also attracts other such nearby surfaces when immersed in water. One idea to explain these interactions invokes bubbles—only 20-30 nm high—on the surfaces. As two hydrophobic surfaces approach each other underwater, the bubbles would eventually coalesce, drawing the surfaces together through capillary adhesion. A complete layer of bubbles also could serve as a kind of lubricant that allows water to slip smoothly over the surface—such as happens with the hydrophobic fabric of swimming suits worn by Olympic contenders. Such nanobubbles are too small to image with light and too fragile to probe with most contact techniques. Another difficulty is posed by thermodynamics: A 10-nm bubble would have a pressure of 14 MPa (140 atm) and should rapidly dissolve. Now, James Tyrrell and Phil Attard of the University of South Australia have gently examined clean glass surfaces underwater with a tapping-mode atomic force microscope. As shown here, they saw irregularities that formed closely packed networks, which covered the surfaces nearly completely. Because the irregularities were softer than the substrate, could be destroyed by pressing too hard, and reemerged after destruction, the researchers concluded that they imaged nanobubbles. In addition, they found that the nanobubbles are not spherical but are flattened like pancakes, with curvatures—and therefore pressures—much lower than previously expected. (
