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Flatly opposed: Squeezing through a channel turns objects sideways

Image of a channel with a narrow section. In the blue regions, the particles are oriented parallel to the flow direction; in the orange regions, they are oriented perpendicular to the flow direction. Credit: Stephan Förster research group, University of Bayreuth.

Image of a channel with a narrow section. In the blue regions, the particles are oriented parallel to the flow direction; in the orange regions, they are oriented perpendicular to the flow direction. Credit: Stephan Förster research group, University of Bayreuth.

One might naturally assume that whatever is within a fluid lines up with that fluid as it pours through a channel. However, scientists now find disks, tubes, and many other particles don’t always go with the flow at all, sometimes lining up perpendicular to the way the fluid is flowing. These findings could have important consequences for many phenomena, such as the clotting of blood or the spinning of fibers.

Chemist Stephan Förster at the University of Bayreuth in Germany and his colleagues were doing what they thought were standard measurements with the new X-ray source PETRA III at the research center DESY in Hamburg, Germany. Particle accelerators there known as synchrotrons can supply very tight beams of X-rays, helping them image the orientation of particles flowing in microscopic channels at high resolutions.

This research included anisotropic particles—that is, ones that look different depending on direction, such as corkscrews, fibers, rings, and pyramids. The way these particles flow down narrow pipes is critical in many different areas, such as silk from spinnerets, cells in capillaries, and nylon from nozzles.

“My group was doing the tough job, spending day-and-night sessions at the synchrotron beamline and collecting lots of data,” Förster says. “Only when they came back to the university and when we looked at the data, we discovered that there was something odd.”

In one set of experiments, the scientists looked at worm-like cylinders made of organic polymers that were 25 nanometers wide and several microns long.

After these cylinders passed through narrow sections in these channels, the researchers discovered the particles in the middle of the channels often lined up perpendicular to the direction of flow. In contrast, particles near the walls of these channels stayed lined up with the direction of flow after passing through the same narrow sections.

“Common understanding and textbook knowledge would suggest that particles always align parallel to the flow direction, like trees floating in a river,” Förster says. “We did not expect this result.”

These findings also hold true with cylinders of different compositions, in different concentrations, and with channels and narrow points of varying widths. Ongoing experiments suggest this phenomenon is the case for disk-like particles as well, and for more rigid cylinders.

Computer simulations and direct analysis of fluid behavior in these channels reveal that after fluid exits a narrow point in the channel, the liquid in the middle wants to fill the widened space. This adds a major sideways component to its flow, and particles line up with this motion, explaining why they move the way they do.

“The observation now explains why fibers always have to be pulled after passing nozzles,” Förster says. “Even spiders have to pull silk fiber—they do this with their legs.”

This research also provides a new view on proteins and cells passing narrow sections in blood capillaries—the reorientation that happens afterward might trigger agglomeration, which in turn might lead to blood clotting, or thrombosis.

“And, when one thinks about it, if tree logs float through narrow sections of rivers, one also observes that they rotate, stick together, and stay as such when floating further down,” Förster says.

Armed with this knowledge, “we could now better design pipes and nozzles to produce high-performance fibers, meaning we could make better fibers and use fiber materials more efficiently to save resources,” Förster says. “We may also be able to make better blood flow improvers against thrombosis.”

The scientists detailed their findings online April 8 in the Proceedings of the National Academy of Sciences.

[youtube http://youtu.be/sM98HMMnHHw]

Image of a microscopic channel with a narrow section. In the blue regions, the particles are oriented parallel to the flow direction; in the orange regions, they are oriented perpendicular to the flow direction. Credit: Stephan Förster research group, University of Bayreuth.

Categories: Applied Physical Sciences
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