Engineers Discover Spontaneous Intertwining of 2D Polymer Sheets in Self-Rotating 3D ‘Dancing’ Spirals

The spiral is ubiquitous throughout the universe – from the smallest molecule of DNA to ferns and sunflowers, and from fingerprints to galaxies themselves. In science, the ubiquity of this structure is associated with parsimony — that things will work out in the simplest or most economical way.

Researchers from the University of Pittsburgh and Princeton University have unexpectedly found that this principle also applies to certain non-biological systems that convert chemical energy into mechanical action – allowing two-dimensional polymer sheets to rise. and spin in spiral propellers without the application of an external power supply. .

This self-assembly into coherent three-dimensional structures represents the group’s latest contribution to the field of soft robotics and chemo-mechanical systems.

The research was published this month in Proceedings of the National Academy of Sciences (PNAS) Link. Lead author is Raj Kumar Manna with Oleg E. Shklyaev, postdoctoral associates with Anna Balazs, Emeritus Professor of Chemical and Petroleum Engineering and John A. Swanson Chair of Engineering at Pitt’s Swanson School of Engineering. Contributing author is Howard A. Stone, Donald R. Dixon ’69 and Elizabeth W. Dixon Professor of Mechanical and Aerospace Engineering at Princeton.

“Using computer modeling, we placed uncoated passive polymer sheets around a circular catalytic patch in a liquid-filled chamber. We added hydrogen peroxide to initiate a catalytic reaction, which then generated a fluid flow. While a single sheet did not spin in the solution, multiple sheets self-assembled autonomously into a tower-like structure,” Manna explained. “Then, as the tower experienced instability, the sheets have spontaneously formed an intertwined structure that rotates in the fluid.”

As Balazs pointed out, “The whole thing looks like a twisted thread of yarn formed by a rotating spindle, which was used to make fibers for weaving. Except there is no spindle; the system forms naturally the intertwined rotary structure.”

Flow affects sheet which affects flow

Analyzing the results in more detail, Manna discovered that tiny random fluctuations in the local concentration of the reactant create enough torque to cause the four sheets suspended in the fluid to be pulled upwards, intertwine and rotate. This phenomenon of interconnection occurs innately when reactants and products have different volumes, which creates a density gradient in the presence of gravity.

“It was a surprise to see these simple 2D sheets form a complex spiral just by ‘sprinkling’ a reagent – like a teaspoon of glucose – into the chamber,” Balazs said. “Double the number of leaves to eight increased the complexity of the spiral. We wondered if this was just the tip of the iceberg — if we took it a step further, could we expand the parameters to achieve various dynamic movements, which would be critical in software programming? robotics.”

Shklyaev added: “The devices are usually three-dimensional and not two-dimensional, so by creating the design rules, we could increase the complexity of the rotating structures formed by the leaves. Anna provided the inspiration with the painting La Danse d ‘Henri Matisse and asked if we could reproduce the poses of the dancers.’

Manna and Shklyaev then added extensions to the four leaves, creating T-shaped structures resembling outstretched arms. According to Shklyaev, changing the shape of the sheets allowed them to “tune” the movement of the sheets to resemble a coordinated circle dance.

According to Balazs, researchers can quantify how interconnected sheets should be designed and organized, allowing others to further develop more robust and scalable systems. Additionally, varying the shape of the vessel holding the fluid – which was rectangular in their modeling – provides another handle for tailoring the systems dynamic response.

“When you have a release of chemical energy into a fluid, it is then transformed into mechanical energy, which can perform specific actions. And although the process dissipates energy, just adding another small amount reactive reactivates it,” she said. “The next step in our study is to program passive and active sheets to form other intertwined three-dimensional structures, now that we know how to control their responses.”

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