Direct Observation of Alternating Stretch-Coil and Coil-Stretch Transitions of Semiflexible Polymers in Microstructured Flow

Analyzing the behavior of semiflexible polymers experiencing hydrodynamic forces is an important step toward a better understanding of polymer dynamics in microfluidic applications as well as in living cells. In particular, studying conformational changes of fluorescently labeled, semiflexible actin filaments in flow fields of spatially varying flow strength will significantly contribute to this goal. The experimental situation is realized in flows through structured microchannels with alternating high- and low-velocity segments. While entering the wider channel segments, the semiflexible filaments undergo a buckling transition under compression whereas they are stretched with a suppression of thermal fluctuations in the extensional regime when reentering the narrow segments. The nature of these nonequilibrium and nonstationary conformational transitions is characterized by analyzing the evolution of the end-to-end distances, center-of-mass velocities, and bending energies along the passage of the filaments through the channels.