Focus: Bursting the Bubble Barrier
Blowing bubbles has finally made the leap from kindergarten pastime to precision science. A group of researchers pushed both liquid and gas through a tiny opening and created a stream of extremely regular and evenly spaced microbubbles–a difficult task for even the most sophisticated devices. These bubbles, described in the 31 December print issue of PRL, could be used in pharmaceuticals, foams, and even food processing.
Scientists and engineers are interested in homogenizing bubbles for all kinds of applications. But bubbles are difficult to control because they are formed by the chaotic interactions of surface tension, viscosity, and turbulence. To date, researchers have had little luck regulating bubbles, and those techniques they have developed usually involve complex machinery or chemical reactions.
Now Alfonso Gañán-Calvo and José Gordillo of the University of Seville in Spain have found a simple way to create identical bubbles. The team immersed a metal plate containing a 110-µm hole in a tank of liquid. They then brought a glass pipe filled with gas near one side of the hole. On that same side, they raised the pressure of both the gas and the liquid. The increased pressure forced both the liquid and the gas to flow through the hole in the form of a liquid jet with a narrow gas filament at its core. The lower density gas filament traveled through the hole much faster than the surrounding liquid, and a small “blob” of gas built up on the far side of the plate. As the blob grew, it began to block liquid flow through the hole, until the liquid snipped the filament that fed it–creating a bubble a few micrometers in diameter. Because these micro-traffic-jams occurred at extremely regular intervals, the technique created a string of evenly spaced, indistinguishable bubbles.
“I think this is a pretty ingenious method to make micron-sized bubbles,” says Gustavo Larsen of the University of Nebraska in Lincoln. Larsen believes that this benign technique could eventually be used to encapsulate living microbes for use as medicine. Wendy Zhang of the University of Chicago adds that it could be extremely useful for scientists studying foam because it would allow them to make a perfectly homogenous mass of bubbles. This kind of foam is also extremely stable, explains Gañán-Calvo, and could be used to add new textures to foods, a prospect he is currently investigating.