The famous Henri Matisse painting La Danse. Photo: Anna Pakutina.The spiral is pervasive throughout the universe – from the smallest DNA molecule to ferns and sunflowers, and from fingerprints to galaxies themselves. In science, the ubiquity of this structure is associated with parsimony – the way objects will organize themselves in the simplest or most economical way.
Researchers from the University of Pittsburgh and Princeton University have unexpectedly discovered that this principle also applies to some non-biological systems that convert chemical energy into mechanical action. Specifically, that two-dimensional polymer sheets can rise and rotate in spiral helices without the application of external power.
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 researchers report their findings in a paper in PNAS Nexus.
“Through computational modeling, we placed passive, uncoated polymer sheets around a circular, catalytic patch within a fluid-filled chamber,” said lead author Raj Kumar Manna, a post-doctoral associate at the University of Pittsburgh. “We added hydrogen peroxide to initiate a catalytic reaction, which then generated fluid flow. While one sheet alone did not spin in the solution, multiple sheets autonomously self-assembled into a tower-like structure. Then, as the tower experienced an instability, the sheets spontaneously formed an interweaving structure that rotates in the fluid.”
“The whole thing resembles a thread of twisted yarn being formed by a rotating spindle, which was used to make fibers for weaving,” added Anna Balazs, professor of chemical and petroleum engineering at the University of Pittsburgh. “Except, there is no spindle; the system naturally forms the intertwined, rotating structure.”
Further analyzing the results, Manna found that tiny random fluctuations in the local concentration of the reactant create sufficient torque for the four sheets suspended in the fluid to be dragged upward, intertwine and rotate. This interlinking phenomena occurs innately when the 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’ reactant – such as a teaspoon of glucose – into the chamber,” Balazs said. “Doubling the number of sheets to eight increased the complexity of the spiral. We wondered if this was the tip of the iceberg – if we pressed further, could we develop the parameters to achieve various dynamic movements, which would be critical in programming soft robotics.”
“Devices are typically three-dimensional and not two, so by creating the design rules, we could increase the complexity of the rotating structures formed by the sheets,” explained Oleg Shklyaev, another post-doctoral associate at the University of Pittsburgh. “Anna provided the inspiration with the painting La Danse by Henri Matisse and asking if we could replicate the dancers’ poses.”
Manna and Shklyaev then added extensions to the four sheets, making T-shaped structures resembling outstretched arms. Changing the shape of the sheets allowed them to ‘tune’ the sheets’ movement to resemble a coordinated circle dance.
According to Balazs, the researchers can quantify how the interlinked sheets should be designed and arranged, thereby allowing others to further develop more robust, scalable systems. Additionally, varying the shape of the container 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 transduced into mechanical energy, which can perform specific actions. And although the process dissipates energy, simply adding another small amount of reactant reactivates it,” she said. “The next stage in our study is to program passive and active sheets to form other interwoven, three-dimensional structures, now that we know how to control their responses.”
This story is adapted from material from the University of Pittsburgh, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.