The present work represents a part of a larger project devoted to the investigation of mixing processes in straight channels under laminar flow conditions. It is known that such processes can be significantly intensified through introduction of longitudinal vortices. The overall system efficiency depends on the additional pressure loss associated with vortex generators. The present investigation is focused on the development of vortex generators in the form of small surface corrugations. The corrugations modify the flow field and create a centrifugal force field that is capable of creation of vortices through an instability mechanism. Such vortex generators may potentially incur a lower pressure loss as they take advantage of flow instability rather then direct flow forcing. The "hydrodynamically active" corrugations are identified using methods of hydrodynamic stability theory. Detailed results are given in the case of a simple converging-diverging channel with the wall geometry described in terms of a sin function.