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Watching the molecular dance of ring polymers in shear flow

10/26/2020

Professor Charles Schroeder and PhD student Michael Tu
Professor Charles Schroeder and PhD student Michael Tu
New research from the Schroeder group has reported on the dynamics and tumbling motion of ring polymers in shear flow. In this paper, the team directly visualized the molecular motion of individual ring DNA polymers stretching and tumbling in flow.

The findings are published in ACS Macromolecules.

Ring polymers are large molecules that have no beginning or end. “Imagine a rubber band that is both stretchy and pliable. At the molecular scale, these materials behave very differently than common linear polymers that are ubiquitous in everyday materials and are more akin to cooked spaghetti noodles,” said Michael Tu, a Ph.D. student who was the primary author on the work.

Due to their closed-loop structures, ring polymers are thought to have unique molecular scale properties, which could lead to interesting and fundamentally different bulk material properties. Although ring polymers often appear in nature, such as in the genomic DNA of bacteria, they have been difficult to synthesize and systematically study in the lab.

In this work, Tu et al. study the dynamics of ring polymers in shear flow using single molecule imaging. “Shear flow is commonly encountered in materials processing applications and occurs anytime a fluid moves past a solid boundary, making it an important flow field to study polymeric systems in,” explained Tu.

Using a custom micromechanical device coupled with fluorescence microscopy, the team directly visualized the molecular motion of individual ring DNA molecules in shear flow. “The motion revealed the molecular dance of rings continually tumbling in flow,” said Tu. Single molecule experiments were complemented by modeling and computer simulations, providing a quantitative understanding of ring polymer dynamics in flow.
Single molecule images of fluorescently-labeled ring DNA viewed in the flow-gradient plane of shear flow. The flow field direction is given by the yellow arrows. These experiments directly reveal the tumbling motion of individual ring polymers in shear flow.
Single molecule images of fluorescently-labeled ring DNA viewed in the flow-gradient plane of shear flow. The flow field direction is given by the yellow arrows. These experiments directly reveal the tumbling motion of individual ring polymers in shear flow.

“We were excited to provide a new molecular-scale understanding of ring polymer dynamics. Moreover, our experiments confirmed recent theoretical predictions from the team of Christos Likos at the University of Vienna (Liebetreu et al., ACS Macro Lett. 2018),” said Charles Schroeder, professor of materials science and engineering. “Our experiments showed that rings swell into a dynamic 'open loop' conformation in the vorticity direction, which is the direction perpendicular to the flow and gradient axes of shear flow, which confirms the modeling results from the Likos group.”

Using a combination of single molecule experiments and simulations, Schroeder’s team found that linear and ring polymers stretch differently in shear flow, resulting in qualitatively different distributions of polymer stretch in flow. Such unexpected differences in distributions warrant further investigation into the role of the ring topology compared to linear polymers, especially in non-dilute solutions.

The paper “Direct Observation of Ring Polymer Dynamics in the Flow-Gradient Plane of Shear Flow” can be found here: DOI: 10.1021/acs.macromol.0c01362

This work was financially supported by the National Science Foundation (NSF) by Awards CBET-1604038 and CBET-1603925 and the Joint Center for Energy Storage Research (JCESR), an Energy Innovation Hub funded by the U.S. Department of Energy, Office of Science, Basic Energy Sciences.