Polymers are large molecules that are divided into smaller, repeating subunits called monomers. Some polymers occur naturally as a circle or ring, while others are a long strand of protein. For the purposes of this article, we’re focusing on ring polymers. If you submerge a polymer in a solvent that is contained between two parallel plates and then pull those plates apart, something called “shearing” occurs.
When a polymer is under a shear force, this flexible chain displays different dynamic modes, such as “tumbling” and “tank-treading.” Tumbling means that the polymer is flipping and moving just like a coin would when tossed into the air. Tank-treading refers to the rotation, like the wheels of a moving car. Because polymers are flexible, in addition to the different dynamic modes, polymer rings under shear force also stretch in the direction in which they’re being pulled, just like a rubber band.
Until now, rotation and stretch were thought to be the only shear effects on polymeric rings. A group of physicists from the University of Vienna discovered another “phase,” which they called the “inflation phase.” When the shear force increased to a certain level, the scientists noticed that there was a swelling of the polymer, not only in the flow direction, but also in an orthogonal one. In other words, the stretched polymeric ring seemed to open and stabilize (no tumbling). If the scientists increased the shear force, the tumbling began again, and the polymer aligned with the flow.
What these scientists observed is important for future study of separation methods for ring polymers and polymers of different topological forms. Understanding that there are hydrodynamic interactions at play when analyzing ring polymers under shear is critical.