Mixing of discontinuously deforming media

TL;DR

This paper introduces a novel mixing mechanism combining stretching with cutting and shuffling, leading to exponential mixing rates in discontinuously deforming media, demonstrated through fluid flow experiments.

Contribution

It develops a new framework for understanding mixing in media with discontinuous deformations, linking cutting and shuffling with stretching to explain enhanced mixing.

Findings

Exponential mixing rates achieved with combined stretching and cutting.

Webs of Lagrangian discontinuities serve as templates for transport dynamics.

Analogies between discontinuous and smooth deformation structures are established.

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.