Visualization of Molecular Fluctuations near the Critical Point of the Coil−Stretch Transition in Polymer Elongation

Abstract

The coil−stretch transition in the extension of polymers in two-dimensional flows was investigated near a critical boundary defined by simple shear flow. Visualization of individual molecules revealed a sharp coil−stretch transition in the steady-state length of the polymer with increasing Weissenberg number (Wi) in flows where the magnitude of the elongational component (||E||) slightly exceeded the rotational component (||Ω||). However, unlike in pure elongational flow, large fluctuations in the length of the polymer were observed near the critical point of the transition. These fluctuations result in a “softening” of the phase transition between coiled and extended states of the polymer. In flows where ||Ω|| is slightly greater than ||E||, significant transient polymer deformation was observed. However, the average length of the polymer as a function of Wi increased much more slowly than in simple shear flow.