An apparatus for measuring the elastic recovery, exhibited by viscoelastic fluids after the cessation of steady flow, is designed. This is a coaxial double cylinder type viscometer, in which the shaft of the inner cylinder is set free to rotate by an elastic force of the sample immediately after the revolution of the outer cylinder is stopped. By using the concept of internal strain, the following equation can be derived.
_??_(1)
θ_e: angular displacement of inner cylinder caused by the elastic recovery of sample after the cessation of flow, σ_i: shearing stress at the inner cylinder surface, R_i: radius of inner cylinder, R₀: radius of outer cylinder, n, k: constants for the sample.
For Na-PAA solutions eq.(1) agrees with experimental results.
Concentration and temperature dependence of θ_e at various σ_i for Na-PAA was measured.
For the range of 1-2%, n=1/2 and for the region smaller than 1%, n approached to zero.
n=0 means that the strain obeys the Hooke's law in shear.
It is considered that the contribution of orientation entropy to the fluid elasticity of Na-PAA solutions is remarkable, because θ_e is depressed with increasing temperature.
Elastic recovery after the cessation of steady flow, which was a kind of non-linear creep recovery, was observed as a function of time.