  The dynamics of a passive scalar gradient experiencing fluctuating velocity
gradient through the Lagrangian variations of strain persistence is studied. To
this end, a systematic, numerical analysis based on the equation for the
orientation of the gradient of a non-diffusive scalar in two-dimensional flow
is performed. When the gradient responds weakly its orientation properties are
determined by the mean value of strain persistence. Statistical alignment of
the scalar gradient with the direction defined by the opposed actions of strain
and rotation, by contrast, requires the gradient to keep up with strain
persistence fluctuations. These results have been obtained for both strain-and
effective-rotation-dominated regimes and are supported by relevant experimental
data. Consequences of the unsteady behaviour of the scalar gradient on mixing
properties are also analyzed.
