TINY BUBBLES An atomic force micrograph reveals that air bubbles roughly 30 nm tall coat the surface of hydrophobic materials immersed in water.

Dispersions of tiny hydrophobic particles play key roles in a number of areas of industrial and technological importance such as paints, inks, and emulsions, since their performance depends on interactions between the colloid particles from which they are made.

Several researchers have proposed that nanometer-sized air bubbles that collect on the surfaces of hydrophobic materials immersed in water control the particles' interactions and govern their properties. Yet no direct evidence had been presented for the nanobubbles. They are too small to be imaged via optical methods and too fragile to be probed using other techniques. And on the basis of thermodynamics, some experts argued against the existence of nanobubbles--asserting that their lifetimes would be immeasurably small.

But now, associate professor Phil Attard and postdoctoral fellow James W. G. Tyrrell of the Ian Wark Research Institute of the University of South Australia have recorded atomic force micrographs of particles coated with nanobubbles--directly proving that the bubbles exist and are stable for hours [Phys. Rev. Lett., 87, 176104 (2001)].

Attard says one focus of his institute is froth flotation for mineral separation, a technique based on selective interactions between hydrophobic mineral particles and air bubbles. The work "advances our understanding of these interactions and should lead to better control of important processes."

Surprisingly, the group observes that hydrophobic surfaces immersed in water are almost entirely coated with bubbles. The finding suggests that the drag of water flow near hydrophobic materials, such as the fibers used in some Olympic swimsuits, is lower than that of bubble-free surfaces.

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