(316aj) The Onset of Turbulence in Drag-Reducing Aggregated Polymer Solutions

We employ gel permeation chromatography and static light scattering to show that high molar mass polyethylene oxide (PEO) chains can exist as molecular aggregates under quiescent conditions at very dilute concentrations. Subsequently, we quantify the onset of turbulent drag reduction (TDR) in PEO solutions as a function of polymer concentration. We identify three distinct types of onset behavior ?type I, II and III, depending upon the polymer concentration. In the type I onset behavior, the wall shear stress (Τ*_w) for onset of TDR is independent of polymer concentration (c). In the type II onset behavior, Τ*_w decreases with increasing polymer concentration as quantified by the universal scaling relationship, Τ*_w ~ c^{-0.31± 0.03}. With further increase in polymer concentration, the type II onset behavior transitions into a type III onset behavior where Τ*_w is again independent of polymer concentration. In addition we show that molecular aggregation and preshearing of the polymer solutions profoundly affects the onset behavior.