Fluid shearing experiments play a crucial role in understanding the flow behavior of matter, including liquids and soft solids. One important concept in this field is the shearing of fluids, where fluid layers slide over each other under shear forces. This phenomenon is studied in rheology, the science that focuses on the flow behavior of materials. Shear forces are lateral forces applied parallel to a material, inducing deformation or slippage between its layers.
Recent research has introduced a novel approach in fluid shearing experiments, focusing on the consideration of polymer topology. Polymer topology refers to the spatial arrangement and structure of molecules within a polymer. In this study, researchers used ring polymers, which are macromolecules composed of repeating units forming closed loops without free ends. Two types of connected ring pairs were considered in the experiments: bonded rings (BRs) with chemical linkages and polycatenanes (PCs) with mechanical linkages via a Hopf link.
Special emphasis was placed on taking into account hydrodynamic interactions through appropriate simulation techniques. These interactions are crucial as they play a significant role in the emerging patterns observed in the experiments. The delicate interplay between fluctuating hydrodynamics and polymer topology was found to govern the dynamic behavior of the ring polymers under shear.
The results of the study yielded surprising findings. The response of the two components, BRs, and PCs, exhibited significant differences compared to other polymer types such as linear, star, or branched polymers. The dominant dynamic pattern observed in other polymers under shear, known as “vorticity tumbling,” was either suppressed in BRs or absent in PCs. Instead, the researchers identified new dynamic patterns in the ring polymer types, namely gradient-tumbling and slip-tumbling.
The unique dynamic patterns observed in the ring polymers have implications for their mechanical properties. BR molecules were found to tumble around the gradient direction under shear, releasing internal stresses. In contrast, PCs maintained a fixed, stretched, and non-tumbling conformation, storing stresses permanently. This difference in behavior resulted in a much higher viscosity for PCs compared to BRs, indicating a potential influence on the shear viscosity of highly concentrated solutions or polymer melts.
The study emphasizes the importance of considering the interplay between hydrodynamics and polymer architecture in fluid shearing experiments. The unexpected modes of motion observed in the ring polymer types highlight the unique signatures of polymer topology on the dynamic behavior of fluids. Further experimental and theoretical studies are needed to explore the impact of these findings on the mechanical properties of polymer solutions. This research was conducted by a collaboration between the University of Vienna, the Sharif University of Technology in Iran, and the International School of Advanced Studies (SISSA) in Italy.