Haller G Karrasch D Kogelbauer F 2018 We seek transport barriers and transport enhancers as material surfaces across which the transport of diffusive tracers is minimal or maximal in a general, unsteady flow. We find that such surfaces are extremizers of a universal, nondimensional transport functional whose leading-order term in the diffusivity can be computed directly from the flow velocity. The most observable (uniform) transport extremizers are explicitly computable as null surfaces of an objective transport tensor. Even in the limit of vanishing diffusivity, these surfaces differ from all previously identified coherent structures for purely advective fluid transport. Our results extend directly to stochastic velocity fields and hence enable transport barrier and enhancer detection under uncertainties. application/pdf https://sonar.ch/global/documents/254155 eng info:eu-repo/semantics/altIdentifier/doi/10.1073/pnas.1720177115 info:eu-repo/semantics/altIdentifier/pmid/30150387 info:eu-repo/semantics/openAccess Proceedings of the National Academy of Sciences of the United States of America. - 2018 coherent structures diffusive transport stochastic transport turbulence variational calculus Material barriers to diffusive and stochastic transport. http://purl.org/coar/resource_type/c_6501