The slow retraction method (SRM) for the determination of ultra-short relaxation times in capillary breakup extensional rheometry experiments

We monitor the capillary thinning and breakup of low viscous liquid filaments with high speed imaging to determine the relaxation time of dilute polymer solutions in extension. The induction of filament thinning by a slow extension of a liquid bridge beyond the static stability limit enables one to create axially symmetric thinning profiles with minimized inertial oscillations from acceleration of the liquid. The minimized disturbance of the capillary thinning process by this slow retraction method (SRM) allows the observation and quantitative fitting of the visco-capillary and inertio-visco-capillary balance as well as the potential flow regime for a series of Newtonian liquids covering a viscosity range from 350 to 27 mPa s. For dilute solutions of polyethylene oxide in water the SRM allows the reliable determination of relaxation times in extension of as low as 240 mu s. A lower limit for the polymer concentration c(low) below which an elasto-capillary balance cannot be observed is introduced, based on the finite extensibility limit L-2 of the polymer chain. (C) 2010 Elsevier B.V. All rights reserved.