= Flow-alignment angle in planar shear
{title2=$\cos2\theta=1/\xi$}
For a uniform <polar order parameter> in <simple shear flow> $\mathbf v=(gy,0)$, the angular equation is $\dot\theta=g(\xi\cos2\theta-1)/2$. A fixed orientation for nonzero shear therefore requires $|\xi|\ge1$. For $|\xi|>1$, its angular stability condition is $g\xi\sin2\theta>0$; equality at $|\xi|=1$ requires nonlinear analysis. The ordered magnitude must separately satisfy the radial stationarity equation and be nonzero.
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