Single Molecule Polymer Physics in 3D Flows

Single polymer dynamics is a powerful approach to understand the nonequilibrium behavior of polymers in flow, revealing molecular subpopulations that are otherwise hidden in ensemble-level measurements. Despite recent progress, prior work has nearly exclusively focused on polymer dynamics in two-dimensional (2D) flows generated in planar microfluidic geometries. Here, we extend nonequilibrium dynamics of single polymers and soft materials to 3D flow fields, including uniaxial and biaxial extensional flow. Single polymers and colloidal particles are trapped and manipulated in 3D using automated flow control. Trap stiffness is experimentally determined by analyzing the power spectral density of particle position fluctuations in 3D flows. In addition, single colloidal particles are precisely manipulated along long-distance user-defined trajectories in 3D using active feedback control. Using a simultaneous dual orthogonal-plane imaging technique, the center-of-mass position of single particles is precisely tracked in space during active manipulation in 3D. We further apply the active 3D flow control method to study the nonequilibrium dynamics of single polymers in uniaxial and biaxial flow. Our results reveal fascinating dynamics of single ring polymers and linear polymers in biaxial extension, including both transient and steady-state stretching dynamics in flow. Ring polymers are found to exhibit differences in the coil-to-stretch transition in planar 2D versus 3D extensional flows due to a coupling between chain architecture, flow character, and intramolecular hydrodynamic interactions (HI). Overall, this work extends the nonequilibrim dynamics of single polymers, particles, and soft materials to three dimensions, enabling quantitative analysis of soft materials in complex flow fields.