Physicists explore the behavior of microscopic filaments

April 21, 2023

(Nanowerk News) Recently published research from an international team of physicists reveals how the three-dimensional shape of a rigid microscopic filament determines its dynamics when suspended in water, and how that control of shape can be used to engineer solid-like behavior even when the suspension is more than 99% water .

The paper was co-authored by Georgetown physics professors Peter Olmsted and Jeffrey Urbach and graduate student Matthew Sartucci. Has been published in Proceedings of the National Academy of Sciences (“straight and helical bound flagellar filaments form ultra-low-density glasses”).

“This work shows that we can use particle shape to create glass-forming materials at very low densities,” said Urbach. “We’ve engineered thin filaments whose geometry creates solids that get frustrated and actually get stuck even though there’s no chemical linking between the filaments, and they only occupy a fraction of the space” Fluorescent microscopy images of straight (left), helical (center), and chimeric fusion (left) bacterial flagella. (Image: Yardimci et al)

The study exploits an extraordinary property of the bacterial flagellum – the microscopic “tail” that organisms use for propulsion. For most bacteria, flagella are rigid helical filaments, which the bacteria twist to produce a propulsive force, like the propeller of a very small ship. There is a mutant bacterial strain that grows flagella that are straight, not helical, and using a new synthesis technique, the group at UC Santa Barbara, led by Prof. Zvonimir Dogic, is capable of producing chimeric filaments, which consist of straight segments of flagella fused into a helix (see figure). Other team members include Ph.D. student at Georgetown and Brandeis Universities, and research scientist at the Center national de la recherche scientifique (CNRS) in Lyon, France.

When suspended in water, these tiny filaments exhibit Brownian Motion, the random fluctuations in position described by the botanist Robert Brown in 1827 when observing pollen grains under a microscope, and quantitatively described by Albert Einstein in 1905, giving one of the first widely accepted. demonstration of the existence of atoms and molecules.

That PNAS studies investigated the dramatic changes that can occur when a group of filaments is added to a suspension (see image below). Straight filaments are free to spread along their length, and thus remain in motion even if they are partially hemmed in by their neighbours. The helical threads can also spread along their length, but only with a “corkscrew” to exit their cage.

For the chimeric filament, on the other hand, the straight tail compresses the corkscrew, and as a result the filament is completely stuck, even though the filament only takes up a fraction of the volume of the suspension (panel C). Long term filament dynamics (A) The labeled straight filaments fluoresce in a background suspension of the unlabeled filaments preferentially diffusing along their long axis. (b) The helical filaments showing the dynamics of the corkscrew. (C) Helix rod fusion filaments are permanently confined. In all panels, the time sequence of fluorescently labeled filaments is shown on Top. Schematics of various filament shapes with background filaments are shown Below. (Image: Yardimci et al)

The group investigated how this confinement affected the mechanical properties of suspensions, and in particular they made precise measurements of material stiffness. They showed that a suspension of straight and helical filaments behaved like a viscous liquid, whereas chimeric filaments produced a glassy, ​​albeit very soft, solid.

This work demonstrates that shapes can be used to control dynamics and mechanics, taking advantage of the extraordinary properties of self-assembled natural materials. Emerging technologies will allow researchers to create similar forms of synthetic materials, providing a powerful avenue for engineering adaptive materials with new properties.

“The dramatic influence of shape on mechanics can be matched by another topical area, namely dynamically deforming metamaterials,” says Olmsted. “In the future, one could design materials from analog objects that can switch between liquid and solid (glass) by chemical, optical, or electrical triggering, in applications such as robotics, protective devices and equipment, and (re)structuring fabrics.”

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