title: Mixing of discontinuously deforming media.
abstract: Mixing of materials is fundamental to many natural phenomena and engineering applications. The presence of discontinuous deformations-such as shear banding or wall slip-creates new mechanisms for mixing and transport beyond those predicted by classical dynamical systems theory. Here, we show how a novel mixing mechanism combining stretching with cutting and shuffling yields exponential mixing rates, quantified by a positive Lyapunov exponent, an impossibility for systems with cutting and shuffling alone or bounded systems with stretching alone, and demonstrate it in a fluid flow. While dynamical systems theory provides a framework for understanding mixing in smoothly deforming media, a theory of discontinuous mixing is yet to be fully developed. New methods are needed to systematize, explain, and extrapolate measurements on systems with discontinuous deformations. Here, we investigate "webs" of Lagrangian discontinuities and show that they provide a template for the overall transport dynamics. Considering slip deformations as the asymptotic limit of increasingly localised smooth shear, we also demonstrate exactly how some of the new structures introduced by discontinuous deformations are analogous to structures in smoothly deforming systems.
authors.affiliation: Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: L D
authors.initials: LD
authors.lastname: Smith
authors.affiliation: Department of Mechanical and Aerospace Engineering, Monash University, Clayton, Victoria 3800, Australia.
authors.forename: M
authors.initials: M
authors.lastname: Rudman
authors.affiliation: School of Civil, Environmental and Chemical Engineering, RMIT University, Melbourne, Victoria 3000, Australia.
authors.forename: D R
authors.initials: DR
authors.lastname: Lester
authors.affiliation: CSIRO Manufacturing, Highett, Victoria 3190, Australia.
authors.forename: G
authors.initials: G
authors.lastname: Metcalfe
mesh_terms: D008962:Models, Theoretical; D017711:Nonlinear Dynamics; D012212:Rheology