Machine Design

Superthin Flow Research Could Bring Breakthroughs In Polymers, Biotech

Have you ever wondered where you will be and what you will be doing when the next, great brilliant idea hits?

How about eating pancakes? That's what happened to physicists at the University of Chicago.

These scientists believe the behavior of air bubbles in ordinary breakfast syrup may demonstrate how to make vanishingly thin tubes and fibers for biomedical, electronics and other applications.

Experiments conducted in Sidney Nagel's laboratory at the University of Chicago showed how to make liquid threads that measure only 10 m in diameter (approximately one-fifth the diameter of a human hair). Now his Chicago colleague Wendy Zhang reports that it is theoretically possible to make much thinner threads by slightly altering experimental procedures. If proven in the laboratory, the technique has potential use in fiber optics, electronics and other industries.

"There are many people who are trying to use this idea, or ideas like this, to make very thin wires," said Zhang, an assistant professor in physics at the University of Chicago. "They're very interested to know what's the smallest size they can achieve."

The calculation Zhang devised to answer that question indicates there is no theoretical limit to the thinness of a thread produced via fluid flow. But the calculation doesn't account for the microscopic building blocks of matter. In reality, a thread cannot be thinner than the molecules of which it is made.

This work will open new avenues for controlled production of extremely thin and long holes in materials like polymers, glasses, and ceramics. It could also have an impact in biomedicine, biotechnology, and the hot nanotech industry.

This line of research began for Zhang in the laboratory of Nagel, the Stein-Freiler Distinguished Service Professor in Physics. Nagel has made what he calls "physics at the breakfast table" one of the hallmarks of his research. When Nagel introduced Zhang to his experiments in the flow of viscous fluids several years ago, "I was completely enchanted," she said.

Despite the whimsical breakfast theme, serious issues lurk behind Nagel's work. He, along with Milan Mrksich, professor in chemistry, and Marc Garfinkel, assistant professor in surgery, both at the University of Chicago, are working on a precisely controlled method of fluid flow to protect transplanted insulin-producing cells from the body's immune system.

The team has successfully coated small clumps of cells, but Zhang wondered if it would be possible to coat individual cells.

An ordinary bottle of syrup demonstrates the principle of viscous entrainment, the topic of Zhang's recent paper. If you turn a bottle upside down, a large air bubble slowly rises to the top. "With a long-enough bottle, the rising movement can distort the bubble so severely that it takes on a tear-drop shape, with a thin tendril of air being drawn out from the rear," she explained.

The phenomenon illustrates how the flow in syrup naturally creates small, extended structures. In this case, the extending structure is the trailing tendril of air. To make this tendril smaller than the 10-m limit Nagel and his associates have observed in the laboratory, they would need to constantly adjust the pressure on the back of the tube to keep the shape of the interface between the fluid and the air at the front always the same.

An existing method called electrospinning can make microscopically thin fibers. But Zhang noted that viscous entrainment has advantages over electrospinning. For example, electrospinning was recently adapted to create hollow fibers. This requires the use of two liquids, one for the sheath and one for the core, then remove the core. There is a possibility that viscous entrainment would work directly with air so that you can do the entire processing in one step.

Today her idea exists only as a calculation in a scientific journal. It remains to be seen whether it will work in practice. "You don't know until someone's actually used it," she said.

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