Droplets string themselves together

When designing products such as car bumpers, engineers often call for polymers that mix as poorly as vinegar and oil. Materials scientists can circumvent this problem by blending the ingredients so that one polymer forms droplets that disperse in the other.

New research has revealed that in very small spaces, these blends can yield unexpectedly well-organized microstructures that might prove useful for making such novel items as all-plastic electrical wires. In the Feb. 5 Physical Review Letters, Kalman B. Migler of the National Institute of Standards and Technology in Gaithersburg, Md., reports his observations of polymer droplets lining up to form microscopic strings.

“There’s a growing industry of making smaller-and-smaller-scale parts,” says Migler. But in the case of polymer blending, researchers haven’t understood the physics and chemistry that unfold on these small scales, he says.

While examining flows of polymer droplets in blends, Migler made a surprising observation. After placing a polymer blend between two clear, quartz disks, he rotated the top disk to generate twisting, shearing forces. Peering through a microscope as he slowed the disk’s rotation, Migler could see polymer drops of one of the ingredients spontaneously assuming unexpected arrangements within the surrounding polymer. First, the drops grew until just one layer of them could fit in the space between the disks, and then they lined up like pearls in a necklace. Next, the individual drops coalesced into a long string. Finally, neighboring strings combined to form ribbon-like structures.

Migler can select drop sizes by choosing an appropriate shearing rate. “Think about when you’re shaking salad dressing,” he says. “If you shake it extremely slowly, the drops of oil and vinegar will be pretty big.” The harder you shake, the smaller the drops become.

Migler can also fix the distance between the quartz disks to roughly the same size as the desired drops. “That tends to force [the drops] into a line, like army ants that get into ranks,” says Ronald Larson of the University of Michigan in Ann Arbor.

Usually, large drops and strings that form in a blend of unmixable polymers break up under shearing forces. Migler suspects that two factors contribute to the unexpected stability of the strings in his experiments. For one, the quartz disks provide surfaces that maintain the structures. Moreover, he says, the shear forces from the turning disk may help keep the strings from fragmenting.

“It’s a very interesting phenomenon,” says Larson, although he suspects applications remain a long way off.

One possibility, Migler says, is that a polymer with conductive properties could eventually be mixed into a polymer with insulating properties to make entirely plastic wires. He also speculates that researchers might be able to make intricate polymer microstructures that could serve as scaffolds for growing artificial tissues.