Empirically modeling polymer collapse in a poor solvent via a non-equilibrium, granular chain experiment

We present a new empirical method for investigating the collapse of a semi-flexible, homogeneous polymer-like structure in a poor solvent. A stainless steel chain in a thin film of water that is vertically oscillated plays the role of the polymer in our model system. As the system is shaken, the chain collapses into a steady-state compact configuration. Collapse is largely dominated by the surface tension of the water; however other factors also contribute in a one-to-one correspondence with real homopolymers. This system may be tailored to investigate the physics of polymer folding in a fundamental manner. We use the normalized radius of gyration to compare collapse dynamics between experiments of different chain lengths. Free energy minimizing behavior is observed as the polymer passes through both “on-pathway” and “off-pathway” intermediate states. At all chain lengths, nucleation (formation of pearls) dominates the early stage of collapse dynamics. If the ends of the chain remain within the radius of gyration, collapse occurs quickly. However, when the ends of the chain stray far outside the radius of gyration, nucleation is impeded and collapse progresses more slowly.