Tuesday, May 29, 2018

Research Revisited: Knotted Hearts, Boson Stars, and Magnetic Particles

Sometimes, science news coverage can package research a little too neatly—with a clear beginning, middle, and end. In reality, research is a messy process with lots of back-and-forth, frustrations, and surprises. Scientists publish journal articles that highlight their results, but these are more like trail markers than final destinations. With this in mind, we’re introducing a new occasional feature on Physics Buzz, getting back in touch with scientists whose work we’ve profiled to see the twists and turns their research is taking.

Science research is anything but a straight road.
Image Credit: Photo by Lukasz Szmigiel on Unsplash.
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Knots: From Heart Arrhythmia Models to Magnetic Skyrmions

Two years ago, we highlighted a surprising connection between erratic patterns of electrical activity that can lead to heart attacks and mathematical models of knotted vortices, in Untangling Knots in Heart Arrhythmia Model. This work, by Paul Sutcliffe and Fabian Maucher from Durham University, shed light on the physics of vortices—an area that could have implications in medical treatments.
In a more recent paper in the American Physical Society’s journal Physical Review Letters, Sutcliffe took knots to a completely different realm. Our longtime readers might remember magnetic skyrmions—particle-like features in magnetic materials that react to tiny amounts of current, which could someday form the basis of next-gen electronics systems.

Essentially, a skyrmion is a point in a magnet where the magnetization is reversed, which can move around like a particle as the "reversedness" is transferred from one atom to another.
Image Credit: American Physical Society/Alan Stonebraker
If you take a thin slice of some magnetic materials, you can twist the magnetization to create skyrmions capable of moving around within the two-dimensional world of the material, explains Sutcliffe. He continues, “Here we show that numerical computations predict that, in certain magnetic materials, skyrmions can be liberated from their two-dimensional world to create new fully three-dimensional stable nanoparticles formed by tying a knot in a skyrmion string.”

In other words, Sutcliffe has shown that it’s theoretically possible to tie magnetic skyrmions into stable knots with different magnetic properties—and that doing so brings skyrmions out into the three-dimensional word. It’s likely this could lead to fascinating technological applications, but the first step is to figure out how to actually knot skyrmions in the lab. Sutcliffe is presenting this work at a few conferences this summer, hopefully igniting experimental collaborations.
For more details, check out Magnetic nanoknots evoke Lord Kelvin’s vortex theory of atoms on Phys.org.

Boson Stars: The Theory Grows

Exotic boson stars played the leading role in Rich Physics in Models of Hypothetical Boson Stars, another post from 2016. According to the laws of physics, these super dense stars could exist—but we don’t know yet if they do. If they do, their existence could explain active galactic nuclei or even dark matter. Our post highlighted research by a trio of scientists at the University of Delhi and Iowa State University, whose theoretical work showed that if charged boson stars exist, they are likely to be places of exciting new physics.

The same researchers, Sanjeev Kumar, Usha Kulshreshtha, and Daya Shankar Kulshreshtha have since expanded the theory behind these fascinating hypothetical stars. Two years ago, we reported on points of interest they identified while studying boson stars in a particular theoretical framework. Those points occurred in two-dimensional phase diagrams of the stars. The team followed up this work with a deeper analysis of their model, resulting in a set of three-dimensional plots that described the conditions under which boson stars could exist—and their properties—in greater detail.

To view these stars from another perspective, the team then joined forces with researchers Jutta Kunz and Sarah Kahlen at the University of Oldenburg in Germany. This time, they studied charged boson stars using a variation of the previous theoretical framework. Again, they examined the conditions under which boson stars could exist and analyzed their properties. And again, points of interest emerged in two-dimensional phase diagrams of the stars that indicate rich physics. After publishing these results, the team is now taking things to the next level—analyzing this model in three dimensions.

Changing Scales: Magnetic Particles at the Nano, Micro, and Milli

Mountains Made of “Magnetic Honey” highlighted new insight on ferrofluids, mixtures of liquid and ferromagnetic nanoparticles that are really fun to play with (and have important practical applications too). Normally, ferrofluid acts like a regular liquid, but turn on an external steady magnetic field and the fluid takes on crazy shapes with mountainous features arranged on a hexagonal lattice. This happens because the magnetic moments of the nanoparticles, which are usually randomly aligned, align within each "mountain" and cause the mountains to repel one other.

Two of the ferrofluid researchers, Bayreuth University’s Reinhard Richter and Ingo Rehberg, have since applied their expertise to tiny swimmers in tiny swimming pools. Drug delivery and surgeries could someday be carried out by robots or microorganisms that swim through the body, but first we need to know how micro-sized things swim. Richter and Rehberg are part of a team exploring how, under an external alternating magnetic field, magnetic micro-particles and a nonmagnetic disk self-assemble and swim on the surface of water. Their recent results demonstrate how to change a swimmer’s speed by adjusting the magnetic field. The team also explored the impact of pool shape—curved walls influence a swimmer’s path just as the curved edges of a pool table influence a ball’s path. For a stadium-shaped pool, chaotic trajectories appear.

On the millimeter scale, Richter and Armin Kögel, another Bayreuth physicist, are part of an Austrian-German-Siberian team studying networks of magnetic balls that form in a “carrier liquid” of nonmagnetic balls. Last year the team characterized the emerging networks and identified why they form. Among other applications, this work helps explain similar behavior that occurs on the nanoscale but is hard to study directly. Nanoparticles may be small, but understanding their behavior is no small effort.
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Leave a comment if we’ve covered something you would like an update on—maybe you’ll see it in a future Research Revisited!

Kendra Redmond

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