Wada dye boundaries in open hydrodynamical flows

Dyes of different colors advected by two-dimensional flows which are asymptotically simple can form a fractal boundary that coincides with a chaotic saddleʹs unstable manifold. We show that such dye boundaries can have the Wada property: every boundary point of a given color on this fractal set is on the boundary of at least two other colors. The condition for this is the nonempty intersection of the saddleʹs stable manifold with at least three differently colored domains in the asymptotic inflow region