Normal stresses in extremely shear thickening polymer dispersions

Abstract

Measurements of the first and second normal stress differences of an extremely shear thickening polymer dispersion have been performed at imposed shear stress both by means of a Rheometrics RDS and a home-made Normal Force Cell using cone-plate geometry. The sample is a 58.7 vol.% dispersion of styrene/ethyl acrylate copolymer particles with 280 nm average diameter in glycol. The spheres are electrostatically stabilized by negative charges due to carboxylic groups. In the regime of strong shear thickening a negative first normal stresses difference N 1 is found, the absolute value of which is equal to the absolute value of the shear stress τ: N 1 = -|τ|. Measurements of the normal force acting on a disk of smaller radius than the full plate radius yield for the second normal stress difference N 2 the relation N 2= - N 1/2. The experimentally observed relation of the apparent shear stress τ a and apparent first normal stress difference N 1a in plate-plate geometry, N 1a= -|τ a|, can be explained by assuming that only the sample part near the rim is shear thickening whereas the inner part remains in the low viscosity regime. In the latter regime N 1a is positive and smaller than τ a. It is further explained why small deviations of the concentricity in Searle geometry create a strong lateral force that pulls the bob towards the cup.