‘Armored’ bubbles can exist in stable nonspherical shapes

Researchers at Harvard University have demonstrated that gas bubbles can exist in stable non-spherical shapes without the application of external force. The micron- to millimeter-scale peapod-, doughnut-, and sausage-shaped bubbles, created by coating ordinary gas bubbles with a tightly packed layer of tiny particles and then fusing them, are described on the Web site of the journal Nature.

“Particles have been used to stabilize emulsions and foams for over 100 years,” says lead author Anand Bala Subramaniam, a research associate in Harvard’s Division of Engineering and Applied Sciences who conducted much of the work before receiving his undergraduate degree from Harvard College last June. “However, we’ve demonstrated that not only are particles useful for making bubbles last longer, they fundamentally alter the properties of these bubbles. Instead of behaving like a fluid surface that flows to balance unequal stresses, the ‘armor’ of particles on the surface of the bubbles actually supports the unequal stresses inherent in non-spherical shapes.”

Surface tension gives all bubbles and drops their perfectly spherical shape by minimizing the surface area for a given volume. Ordinarily if two bubbles are fused, the product is a larger but still spherical bubble. But when particles are strongly anchored to the bubble surface and the bubbles are fused, a stable sausage shape is produced.

“We have provided a general explanation of why these non- spherical bubbles can be observed,” says co-author Howard A. Stone, Bala Subramaniam’s adviser and the Vicky Joseph Professor of Engineering and Applied Mathematics at Harvard. “Bubbles are engineered into many consumer products. The ability to alter the shapes of bubbles and liquid drops in products like ice cream or shaving foams or creams may provide a means to alter the consistency or texture of these products. The non-spherical bubbles could also find use as vessels for delivering drugs, vitamins, or flavors.”

Bala Subramaniam and Stone’s co-authors are Manouk Abkarian and Lakshminarayanan Mahadevan of Harvard’s Division of Engineering and Applied Sciences. Their work was supported by Unilever.