= Torque reversal of a linear Oldroyd fluid
{title2=$\Omega(t)=-\Omega_+\left[1+\frac{2G_0\tau}{\mu_0}e^{-t/\lambda_2}\right]$}
In an inertialess <cone-and-plate rheometer>, reverse a steady torque from T to -T at time zero. The polymer shear stress remains continuous while the positive solvent viscosity allows the rate to jump. The linear stress equation gives $\lambda_2=\mu_0\tau/(\mu_0+G_0\tau)$ and the displayed velocity for t>0, where $\Omega_+=3\alpha T/[2\pi a^3(\mu_0+G_0\tau)]$. The transient backward speed exceeds the final backward speed because stored polymer stress initially retains its former sign.
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