Gas bubbles are perfectly spherical because this shape offers a minimal surface for any given volume. Even when two bubbles are merging, the results is a larger sphere. But now, researchers from Harvard University have found a way to build stable 'armored' bubbles with non-spherical shapes. They can obtain peapod-, doughnut- and sausage-shaped bubbles by "coating ordinary gas bubbles with a tightly packed layer of tiny particles and then fusing them." This new process might lead to new textures for ice creams or shaving foams and could even be used for delivering drugs.
"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," says Anand Bala Subramaniam, a research associate in Harvard's Division of Engineering and Applied Sciences.
"The bubble wants to reduce its surface area by going back to a spherical shape, but the strong anchoring of the particles on the surface prevents their expulsion," Bala Subramaniam says. "The particles end up tightly packed, and eventually push against each other strongly, allowing the bubble surface to carry forces to support a non-spherical shape."
Below is a close-up of a single armored bubble showing the closepack arrangement of the colloidal armor (a). And on (b), the "triangulation of the shell reveals several 5-7 paired dislocations (red and green circles respectively) on a six-fold coordinated lattice, consistent with theoretical predictions of ground states of spherical crystals" (Credit: Harvard University).
And for your viewing pleasure, below is the Janus colloidal armor, with two differently labeled particles on an air bubble. On (a), you can see how particles on an air/water interface are producing Janus crystals. "The yellow particles are 4.9-m-diameter polystyrene particles dyed with rhodamine, and the green particles are 4.0-m particles dyed with fluorescein." And on (b), you can see an example of the Janus shell, with approximately two hemispheres of different size particles and fluorescence (Credit: Harvard University).
What kind of applications can we expect from these non-spherical bubbles? Here is the answer of Howard Stone, Professor of Engineering and Applied Mathematics at Harvard University.
"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."
And for more information about these 'armored' bubbles, the research work has been published by 'Nature Materials' under the name "Controlled Assembly of Jammed Colloidal Shells on Fluid Droplets" (Volume 4, Issue 7, Pages 553-556, July 2005). Here are two links to the first paragraph and to additional figures. The above illustrations have been picked from this last document.
Sources: Harvard University news release, via EurekAlert!December 14, 2005; and various web sites
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