Experimental uncertainties in extensional rheometry of liquids by fiber drawing

Abstract

Drawing of liquid filaments from a circular die is the most successful method to date for determining the response of polymer solutions in uniaxial extension. Reported here is an analysis of the experimental uncertainties in fiber‐drawing extensional rheometry and the effect they have on the results. In fiber drawing, measurements of the liquid flowrate, the variation of the fiber diameter along its length, and the force exerted on the die are used to determine the extension rate and the corresponding extensional stress. Experimental uncertainty in the flowrate measurement ordinarily is of little consequence, but the same is not true of the uncertainties in the diameter profile and the force measurement. The optical measurement of the diameter profile is limited by diffraction and chromatic aberration. The precision of the force measurement is limited by uncertainty in the nulling procedure. In addition, competition among contending forces limits the operating range of reliable extensional measurements. At low stress levels (∼103 dynes/cm2), inertial, gravitational, and surface tension forces dominate over viscous forces and destroy the precision of the extensional stress measurement. At high stress levels (∼106 dynes/cm2), cavitation may be limiting. The magnitude of the gravity force determines minimum attainable extension rates (∼10 s−1) whereas instabilities limit the maximum ones. These operating limits are supported by experiments on glycerine and solutions of several water‐soluble polymers; polyacrylamide, hydroxymethylcellulose, xanthan, and guar.