Physicists explored microscopic filament behavior

Particle shape can be used to create a material that forms an extremely low-density glass.

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The dynamics and mechanics of materials made of polymers are significantly influenced by geometry and topology. For instance, the characteristics of flexible polymers with ring, branching, or star shapes differ noticeably from those of their linear counterparts. Comparatively, little is known about how the microscopic dynamics and macroscopic rheological characteristics of the suspensions of rigid filaments are affected by their three-dimensional shape.

An international team of physicists studied how the 3D shape of rigid filaments determines the microscopic dynamics when suspended in water. They also examined how control of that shape can be used to engineer solid-like behavior even when the suspension is more than 99% water.

They found that the particle shape can be used to create a material that forms a highly low-density glass. What’s more, they also engineered thin filaments whose geometry creates a frustrated, completely jammed solid even though there is no chemical linking between the filament. They take up only a tiny fraction of the space.

For the study, the team mainly used the remarkable properties of the flagella of bacteria—the microscopic “tails” that the organisms use for propulsion. The flagella of most bacteria are rigid helical threads that the bacteria rotate to produce propulsion.

By using novel synthesis techniques, the team at UC Santa Barbara, was able to generate chimeric filaments composed of a segment of straight flagella fused to a helical one.

fluorescently labeled straight filament
(A) A fluorescently labeled straight filament in a background suspension of unlabeled filaments preferentially diffuses along its long axis. (B) Helical filament exhibits corkscrewing dynamics. (C) Helix-rod fusion filaments are permanently caged. In all panels, a time sequence of fluorescently labeled filaments is shown on the Top. Schematics of different filament shapes with the background filaments are shown on the Bottom. Credit: Yardimci et al.

When suspended in water, these tiny filaments exhibit Brownian Motion. The study found that dramatic changes can occur after adding a bunch of filaments to the suspension.

Even though they are partially confined by their neighbors, the straight filaments are free to disperse along their length and remain mobile. Only by “corkscrewing” out of their prisons can the helical filaments disperse throughout their length. Contrarily, even though the chimeric filaments only occupy a small portion of the suspension volume, the straight tails suppress the corkscrewing, causing the filaments to become stuck.

The team specifically carried out accurate measurements of the material’s stiffness to better understand how this caging affected the mechanical characteristics of the suspension. They demonstrated that the chimera filaments generate a glass-like solid, although a very soft one, whereas the straight and helical filament suspensions act like viscous liquids.

Scientists noted, “This work shows that shape can be used to control dynamics and mechanics, leveraging the remarkable properties of self-assembled natural materials. Emerging technologies will enable researchers to fabricate similar shapes from synthetic materials, providing powerful avenues for engineering adaptive materials with novel properties.”

Georgetown physics professors Peter Olmsted, co-author of the study, said, “The dramatic influence of shape on mechanics could dovetail with another topical area, that of dynamically shape-changing metamaterials. In the future, one could design materials from analogous objects that can switch between fluid and solid (glass) by a chemical, optical, or electrical trigger in applications such as robotics, protective devices, and equipment, and (re)structuring fabrics.”

Journal Reference:

  1. Sevim Yardimci et al., Bonded straight and helical flagellar filaments form ultra-low-density glasses, Proceedings of the National Academy of Sciences (2023). DOI: 10.1073/pnas.2215766120