Extensional Relaxation Times of Dilute, Aqueous Polymer\nSolutions

Abstract

We show that visualization and analysis\nof capillary-driven thinning\nand pinch-off dynamics of the columnar neck in an asymmetric liquid\nbridge created by dripping-onto-substrate can be used for characterizing\nthe extensional rheology of complex fluids. Using a particular example\nof dilute, aqueous PEO solutions, we show the measurement of both\nthe extensional relaxation time and extensional viscosity of weakly\nelastic, polymeric complex fluids with low shear viscosity <i>η <</i> 20 mPa·s and relatively short relaxation\ntime, λ < 1 ms. Characterization of elastic effects and extensional\nrelaxation times in these dilute solutions is beyond the range measurable\nin the standard geometries used in commercially available shear and\nextensional rheometers (including CaBER, capillary breakup extensional\nrheometer). As the radius of the neck that connects a sessile drop\nto a nozzle is detected optically, and the extensional response for\nviscoelastic fluids is characterized by analyzing their elastocapillary\nself-thinning, we refer to this technique as optically-detected elastocapillary\nself-thinning dripping-onto-substrate (ODES-DOS) extensional rheometry.