Polymer chains show unexpected phase change when stretched by flow
Hidden Phase Transition in Single-Polymer Stretching under Elongational Flow
Machine Learning
Summary
Stretching polymer chains with strong flows causes them to change shape in known ways, like stretching out from coils. This paper finds a new kind of change that happens after the usual stretching, detected by machine learning analyzing the polymer’s movements. This new change means the polymer can adopt two different folded shapes when partially stretched. The authors confirmed their discovery with mathematical models and found some supporting evidence in DNA stretching experiments.
What this means in practice
- •For materials engineers: Design polymer processing methods that control molecular shape changes by exploiting the discovered hidden phase transition under flow.
- •For biophysics instrument developers: Enhance single-molecule stretching devices to detect subtle folding dynamics indicated by new phase transitions in polymers like DNA.
Authors
Aiqing Zhu, Fushuai Wang, Eleonore Vissol-Gaudin, Qianxiao Li, Beatrice W. Soh
Abstract
Using machine learning, we identify a second transition in polymer chain stretching driven by planar elongational flow, beyond the well-known coil-stretch transition. We achieve this by developing a conditional extension of Universal and Identifiable Dissipative Dynamics (UIDD) that jointly learns reduced stochastic dynamics from microscopic trajectories across flow strengths and yields the corresponding stationary nonequilibrium free energy landscapes. As the flow strength increases, the evolving landscape exhibits the expected coil-stretch transition, and then a previously unrecognized dynamical transition. In the latter, the stretched states persist, but the distribution along a folding coordinate becomes bimodal when conditioned on small end-to-end separation. Large-deviation analysis of a simplified analytical chain model reproduces both transitions and shows that they are phase transitions in the long-chain limit. Single-molecule DNA experiments provide qualitative support for the dynamical transition through the predicted reversal of the folding response.