Duncan Hewitt (University of Cambridge)

Numerous natural and industrial materials, ranging from lava and mud to cosmetics, foodstuffs, blood and mucus, exhibit both fluid-like and solid-like behaviour: they can flow like a viscous fluid under sufficient force, but withstand deformation at lower stresses. Clogging and flow localisation in such materials is a generic feature of their mechanics: it underlies the triggering of mudslides, catastrophic liquefaction of mining waste, blood jamming in sickle cell disease, and numerous issues in industrial processing and cleaning. The problem of locomotion through such materials – encountered, for example, by worms moving through soil or sediment – is also controlled by these features of flow and stress localisation.

In this talk, I will explore some basic fluid mechanical behaviour of idealised viscoplastic materials, illustrating some ways in which channelling and clogging can generically occur. To further illustrate some of these features, I will particularly focus on the problem of undulatory ‘swimming’ through such materials, with the development of a viscoplastic slender-body theory and an explanation of how the rheology allows for a ‘burrowing’ mechanism of locomotion.