ROD CLIMBING AND NORMAL STRESSES IN HEAVY CRUDE OILS AT LOW SHEARS

This paper gives the results of a study of the nonlinear viscoelastic behavior of three heavy crude oils from California and Venezuela. A li near combination of normal stress coefficients at zero shear is expres sed in terms of the quantity (the climbing constant) used to measure t he height rise on a rotating rod. Measurements of the climbing constan ts are given for the crude oils. Values of both the first and second n ormal stress coefficients at zero shear are determined by the climbing constant when another combination of the two coefficients is known. I n principle, the required information can be obtained by back extrapol ation of the first normal stress difference, by back extrapolation of the dynamic modulus or by back extrapolation of the ratio of the secon d to first normal stress difference. Back extrapolation of data can be achieved when measurements are available at shear rates low enough to enter onto the second-order plateau of the functions generated by dif ferent instruments. Examination of previously published data for well- characterized solutions suggests that second-order rheology is most re adily obtained in rod climbing.