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Spinning cylinders recreate nature's patterns
Some of nature's most exquisite patterns; leaves around a plant's stem, scales on a pine cone, even the tail of some viruses, consist of small objects attracked to the center of a cylindrical, rotating chassis in specific patterns.
Taking inspiration from nature, scientists at the Center for Soft and Living Matter, Institute for Basic Science (IBS), in South Korea, identified the conditions necessary for dynamically building large structures from small objects within spun cylinders. While nature offers us beautiful examples of patterns, such as strands of DNA, recreating the same tubular structures in the laboratory has been difficult.
Nature prefers building through self-assembly, a process where individual components spontaneously organize into ordered structures.
Researchers devised a method to compact particles placed within a cylinder, even in some experiments using bubbles, by exploiting centripetal* force of a rotating fluid. Due to this force, higher density fluid is pushed out while lower density material is driven to the center. As denser (heavier) liquid rotates, the lighter particles within the cylinder arrange in a tubular assembly. *centripetal force - (from Latin centrum, "center" and petere, "to seek" is a force that makes a body follow a curved path. Not to be confused with centrifugal force, an inertial force (also called a "fictitious" or "pseudo" force) directed away from the axis of rotation.
Creating tubular crystals under non-equilibrium conditions — in a rotating suspension — is a conceptually new attempt at particle self-assembly. By using this method, it is possible to make tubular crystals out of two kinds of particles, which had not been done before. The work appears in the journal Advanced Materials.
The first author, Lee Tae-hoon, a graduate student, adds: "This study can be extended to various systems, including soft entities, such as bubbles or even living cells." Researchers believe their work will contribute to the creation of variously shaped microcomposites that can be equally dispersed throughout other materials. This would make them useful in photonic applications. Photonic applications use the photon in the same way that electronic applications use the electron. Devices that run on light have a number of advantages over those that use electricity.
Conducting this kind of experiment has traditionally had to deal with issues caused by gravity. When gravity is present, sedimentation occurs, which has led to research being conducted on the International Space Station to remove gravity from the equation.
"What we managed to do by using rotating liquids is to effectively switch off the gravity because we beat it against the buoyant force. Gravity is always there, but we introduced a force which exactly matches it. In some sense, we are able to do an experiment on Earth that would normally require outer space, zero gravity conditions," explains Bartosz Grzybowski, study leader.
Now that the scientists are able to control groups of particles using rotation, they will focus on controlling individual particles. It is possible to move a single particle in a 3D space using lasers (optical tweezers) or magnets (magnetic traps) — but both methods require bulky facilities.
Bartosz Grzybowski explains: "If you want to catch a particle and move it to a desired location in 3D it usually requires quite a lot of equipment. Now we know how to manipulate small objects by fluid flow in a rotating frame of reference, and how to manipulate particles in 3D to actually position them as if using a pair of tweezers."
In addition to studying the effects on solid particles, bubble shape studies are leading to experiments on increasingly smaller units — living cells. The ability to gently apply forces to soft objects could potentially lead to controlling the function of cells while still keeping these cells alive.
When suspended in a denser rotating fluid, lighter particles experience a cylindrically symmetric confining potential that drives their crystallization into either monocomponent or unprecedented binary tubular packing. These assemblies form around the fluid's axis of rotation, can be dynamically interconverted (upon accelerating or decelerating the fluid), can exhibit preferred chirality, and can be made permanent by solidifying the fluid. The assembly can be extended to fluids forming multiple concentric interfaces or to systems of bubbles forming both ordered and “gradient” structures within curable polymers.
Authors: Taehoon Lee, Konrad Gizynski, Bartosz A. Grzybowski
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Single component tubular assemblies — in a closed system — change
from one type of structure to another by change of rotation rate.
Image credit: IBS.