Credit: © 2008 AAAS

The formation of bubbles is important in applications as diverse as ultrasound imaging, food preparation, personal care products and construction. However systems of bubbles on the micrometre scale are highly unstable and tend to evolve to lower-energy configurations of larger bubbles. Now, Howard Stone and co-workers1 at Harvard University and Unilever have made bubbles with diameters of less than 1 micrometre that remain stable for at least a year.

Stone and co-workers mixed thick glucose syrup and water with sucrose stearate, and then added air by mechanical entrainment. The sucrose stearate molecules act as a surfactant, with hydrophilic parts that stay in the liquid shell and hydrophobic parts that project into the bubble. The researchers found that their bubbles were covered in hexagonal patterns around 50 nanometres in size. The patterns — which can be predicted from molecular thermodynamic models — appear to be an elastic surface response that prevents further shrinkage of the bubble.

The bubbles remained stable for a year, although the average bubble size increased slightly. The size of the bubbles can be tailored by changing the experimental conditions, but the hexagonal pattern size remains the same.