Control of transport in a chaotic lattice

We examine a mechanism for the elimination of vertical transport in a square, isotropic chaotic lattice. We find that applying a shear "jet" along a line of symmetry in the lattice can completely impede chaotic transport across a specified barrier. This is accomplished through a change in allegiance of heteroclinic connections within the lattice. This mechanism is relatively insensitive to details such as amplitude or shape of the applied shear. It depends quite sensitively, however, on the precise location of the shear. This suggests that the placement of flow control devices in physical situations may be much more critical to transport behavior than are other control parameters. In addition, we develop a necessary condition for the existence of boundary circles in non-twist maps. Using this condition we find that transport across a barrier can be eliminated for all practical purposes without evidence for an invariant boundary circle.