We discuss and review a generalization of the usual hydrodynamic description of smectic A liquid crystals motivated by the experimentally observed shear-induced destabilization and reorientation of smectic A like systems. We include both the smectic layering) and the director \(\hat{n}\) of the underlying nematic order in our macroscopic hydrodynamic description and allow both directions to differ in non equilibrium situations
In a homeotropically aligned sample the nematic director couples to an applied simple shear, whereas the smectic layering stays unchanged. This difference leads to a finite angle between \(\hat{n}\) and \(\hat{p}\), which we find to be equivalent to an effective dilatation of the layers
We include the couplings of the velocity field with the order parameters for orientational and positional order and show how the order parameters interact with the undulation instability
We discuss pathways to higher instabilities leading to the formation of onions via cylindrical structures and/or the break-up of layers via large amplitude undulations
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