June 02, 2008

Permanent nanoscale bubbles created

Engineers at Harvard's School of Engineering and Applied Sciences (SEAS) have whipped up, for the first time, permanent nanoscale bubbles---bubbles that endure for more than a year---from batches of foam made from a mixture of glucose syrup, sucrose stearate, and water. Future applications of these microbubbles could significantly extend the lifetimes of common gas-liquid products that experience rapid disintegration, such as aerated personal-care products and contrast agents for ultrasound imaging

Stone, Vicky Joseph Professor of Engineering and Applied Mathematics and associate dean for applied physical sciences and engineering, was in the audience when Bee projected an image of a micrometer-size bubble with a distinctive polygonal geometry. The bubble surface appeared to be faceted with regular pentagonal, hexagonal, and heptagonal domains that intersected to form a soccer ball-like structure. None of the faces spanned more than 50 nanometers.

"Small bubbles on that scale never last because of surface tension---they instantly disappear. What Rodney showed on that screen was extraordinary," said Stone. "It was impossible; we all thought it was impossible."

Smaller bubbles have a greater surface tension and a higher gas pressure than larger ones. As a result, larger bubbles usually grow at the expense of smaller ones, which have very short lifetimes.

The experimental study, conducted by SEAS graduate student Emilie Dressaire in collaboration with Unilever colleagues, revealed that when the bubbles were covered with the chosen surfactant mixture, the surfactant molecules crystallized to form nearly impermeable shells over the bubble surfaces.