Dynamics at Soft Interfaces

This thesis presents six investigations into some of the dynamics that can ensue at soft interfaces. The interfaces investigated are fluid, elastic and adhesive in nature, and the means by which the interfaces are brought into contact varies, from thermal and mechanical agitation to inertial forces. Chapter 2 is a theoretical and numerical investigation, motivated by Xu et al. (2005), into the inertial impact (≈1 ms -1) of a droplet (≈ 1 mm) on a dry solid substrate in a gaseous atmosphere, producing a splash. In particular we investigated the role of the air and present an analysis on the dominance or neglect of several physical effects as contact is approached. Chapter 3 presents an investigation into the nature and evolution of contact between a rigid colloid and a bed of adhesive bonds. To gain insight into a system where bonds possess a distribution of mean rest lengths we consider systems with one and two families of bonds in the presence and absence of detachment. In chapter 4 we consider the dynamics that ensue when a colloid is attached to a fluid-fluid interface, but distant from its equilibrium position. After deriving force balance from an energetic basis we study the dynamics by appealing to a balance of power. We conclude with a comparison to experiments conducted in the Manoharan Lab at Harvard. Chapter 5 is an investigation into the behaviour of an adhesive elastic pellet confined and forced in a fluid-filled pipe. In particular,we investigate the enhancement of the detachment rate by the translational motion of the pellet and vice-versa. In chapter 6 we present a theoretical and experimental investigation into the unidirectional response of a mechanical ratchet (certain common species of barbed grass) to unbiased mechanical agitation in an inertial and viscous limit. Chapter 7 provides the theoretical underpinning to the self-assembly of macro-scopic magnetic monomers, which are agitated by a turbulent flow. The central result being that Langevin dynamics ensue in the limit of large Reynolds and Stokes numbers, allowing us to calculate the yield curve, finding agreement with experiments