Unusual behavior of polyelectrolyte threads: New phenomena in rheology and a step toward innovative fibers with permanent electrets
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Particulate matter poses serious threats to people’s quality of life, facilitates the spread of infectious diseases, and can have a global impact on entire ecosystems and the climate.
The filters on the market today only work to a certain degree and come up short when it comes to removing fine particulate matter.
This shortfall could all change as UIC Distinguished Professor Alexander Yarin, director of the Multiscale Mechanical and Nanotechnology Lab at UIC, and his collaborator Professor Eyal Zussman from The Technion-Israel Institute of Technology, have had a significant breakthrough. The researchers are gaining a better understanding of how rod-like electrical polymeric dipoles, known as polyelectrolytes, behave in thin threads subjected to axial electric fields.
The research was recently published in the journal “Proceedings of the National Academy of Sciences” under the title “Capillary self-thinning of threads of polyelectrolyte solutions with axial electric fields.”
The research examines the behavior of polyelectrolytes in aqueous solutions. The researchers treat each polyelectrolyte macromolecule as a minuscule electric dipole — meaning it has positive and negative ends.
When the solution forms a thin thread stretched and sustained between two disks and an electric field is applied along the thread, the polyelectrolyte macromolecules align in the direction of the electric field. This alignment creates strong axial elastic forces along the length of the thread. These forces alter how the thread thins over time—slowing it down, stopping it completely, or even oscillating as it cycles between thin and thick.
“Normally, threads made of ordinary Newtonian fluids or solutions of flexible or rod-like polymers shrink in predictable ways — either linearly or exponentially or nonlinearly. But when the electric field is applied, even thinning of polyelectrolyte threads deviates from known track, and when the electric field is strong, the polyelectrolyte thread behaves unusually. At high electric field strengths, the forces from the aligned dipoles become so strong that the thread’s cross-sectional radius starts to go up and down in time, revealing oscillations. It is shown in this work that the oscillations reveal a peculiar case of the Hadamard instability characteristic of the so-called ill-posed mathematical problems, which this one appeared to be,” Yarin said.
“It should be emphasized that strong elongational flows of polymer solutions and melts are fundamental for producing fibrous nonwovens. Then, polyelectrolyte-containing fibers will have permanently embedded charges, the so-called electrets, which can be used as the most effective filters for fine particulate matter removal using additional electric forces,” Yarin said.
In addition to advancing new technologies, the investigation has an environmental component.
“The incorporation of natural polymers (anionic cellulose nanocrystals adjacent to polyelectrolyte macromolecules, as in the present case) in fibrous filters is of colossal importance to enhance their biodegradability and compostability to diminish the content of petroleum-derived polymers and the accompanying planetary impact of microplastics resulting from them,” Yarin said. “This research opens a new chapter in the field of electro-elastocapillary phenomena.”