(283e) Single Polymer Dynamics in Large Amplitude Oscillatory Extension (LAOE)

Over the past two decades, advances in fluorescence imaging and particle manipulation have enabled the direct observation of single polymer dynamics in model flows such as shear flow and planar extensional flow. The vast majority of single polymer studies, however, has focused on chain dynamics using simple transient step forcing functions. In order to study single polymer dynamics in non-idealized â??modelâ?? flows, there is a clear need to implement more complicated flow types and transient flow forcing functions. For example, large amplitude oscillatory shear flow (LAOS) has been widely used in recent years to study the linear and nonlinear viscoelasticity of materials in bulk rheology but complementing molecular techniques have not yet been applied.

In this work, we directly probe single polymer dynamics using large amplitude oscillatory extensional flow (LAOE) in precisely controlled microfluidic devices. We are able to generate both small amplitude and large amplitude sinusoidal oscillatory extensional flow in a cross-slot microfluidic device while imaging the conformational dynamics of a single DNA polymers trapped at the stagnation point. Using this experimental technique combined with Brownian dynamics (BD) simulations, we study the single chain dynamics of a λ-DNA molecule as a function of both the Weissenberg number (Wi, flow strength) and Deborah number (De, probing frequency). In LAOE, polymer chains undergo repeated cycles of compression, rotation, and stretching between extensional and compressional axes in a highly dynamic and transient manner. To our knowledge, this work is the first demonstrated molecular rheology experiment utilizing two-dimensional time-dependent control over the entire x-y flow plane. Using this approach, we characterize transient chain dynamics by constructing single polymer Lissajous curves, which are defined based on chain conformation and stretching in flow. We further construct a series of Lissajous plots as a function of Wi and De using experiments and BD simulations. Our results reveal four distinct characteristic shapes of the Lissajous curves, each representing a unique behavioral regime for a single polymer chain. We further study the average chain dynamics in LAOE over an ensemble of polymer chains. Based on the average projected extension, our results show the existence of a critical Wi at the coil-stretch transition that is a function of the dimensionless cycle frequency (De). In this way, we map out a wide range of the Pipkin space for a single polymer chain ranging from linear steady-state conditions at low Wi and De to non-linear unsteady states at high Wi and De. We use a conformational space construction based on experimental data to visualize average chain dynamics in LAOE in Pipkin space both below and above the coil-stretch transition.