Members of the Durham Centre for Soft Matter pursue research that covers a wide range of disciplines. A small selection of publication highlights is provided here to give a sense of the scope. You can search for publications from the Centre by author, year or keyword by clicking here.
Blooming of smectic surfactant/plasticizer layers on spin-cast poly(vinyl alcohol) films
Recently published work by Richard Thompson and coworkers has been featured as a research highlight by the ISIS Neutron & Muon Source. The article nicely explains how the team used neutron reflectometry to study surfactant self-organisation in the context of laundry detergent pods, with some surprising results. This work is jointly funded by one of the UK Research Councils (EPSRC) and by industry (Procter and Gamble).
Continuum percolation of polydisperse rods in quadrupole fields: Theory and simulations
Carbon nanotubes can be used as a "filler" in composite materials to make the material electrically conductive. To do this, a network of nanotubes must percolate through the material, providing a contiguous conducting path. Conditions for percolation are optimal when the nanotubes point isotropically in all directions. However, formulation of nanotube composites tends to induce partial alignment of the particles, for example if flow of processing involves flow. This effect tends to raise the density of particles needed for percolation, potentially to the detriment of other material properties. Furthermore, the extent of alignment is sensitive to the length of the nanotubes, and real materials always contain a mixture of lengths. In this article, Mark Miller and Mihail Kotsev from Durham Chemistry have collaborated with Paul van der Schoot and Shari Finner at Eindhoven University of Technology to investigate how alignment affects percolation in nanotube mixtures. They make the remarkable prediction (based on both analytical theory and numerical simulations) that the length polydispersity of carbon nanotubes can counteract the undesirable increase in percolation density caused by alignment, making it possible for the mixture to percolate at a lower density than any of its individual pure components. This suggests a way to optimise the formulation of such materials.
Topological inversions in coalescing granular media control fluid-flow regimes
Sintering — or coalescence — of viscous droplets is a fundamental process that occurs in many natural and industrial scenarios, such as the manufacture of ceramics, and the welding of volcanic ash. Sintering is driven by surface tension, and causes compacts of coalescing droplets to densify and to become less permeable. In this paper, Fabian Wadsworth, Ed Llewellin, and Kate Dobson use synchrotron-source, 4D x-ray computed tomography to image the evolution of the internal geometry of a sintering compact of molten glass beads, and use lattice-Boltzmann simulations of gas flow through the deforming pore space to determine the evolution of permeability. They find that changes in permeability are linked to a topological inversion — from droplets in a continuous gas, to discrete bubbles in a continuous liquid — allowing them to construct a unified physical description of the sintering process.
Theoretical prediction and experimental measurement of isothermal extrudate swell of monodisperse and bidisperse polystyrenes
Extrudate swell is a phenomenon encountered in an industrial extrusion process and involves a polymer melt flow expanding perpendicular to the process flow direction. In this paper Ben Robertson, Richard Thompson, Tom McLeish and Ian Robinson show that this phenomenon is dependent on the stretch of polymer chains induced by flow within and at the exit from the melt extruder. We show that parameters such as molecular weight are not important when the process flow speed is normalised by the stretch relaxation time of the polymer. We use our Multi-Pass Rheometer to perform extrusion experiments and compare these to viscoelastic flow simulations, showing a good agreement between the two for monodisperse melts. We discuss the origins of disagreements for bidisperse blends and discuss methods of improving the simulations for these systems.
Edge Fracture in Complex Fluids
Edge fracture is a free surface instability that can develop when viscoelastic fluids are subjected to large shear-rates in common experimental geometries (e.g., cone and plate). The instability manifests as a disturbance that propagates inwards from the sample edge, and disrupts rheological measurements. By analysing the linear stability properties of a free surface between a bulk polymeric fluid under shear and the air, Ewan Hemingway, Halim Kusumaatmaja, and Suzanne Fielding explore the mechanisms that drive the edge fracture instability and characterise the critical shear-rate at which it first appears. These analytics also provide insight into how edge fracture might be avoided experimentally, for example by bathing the rheometer in a viscous fluid. The predictions are complemented by full 2D hydrodynamical simulations using a diffuse-interface approach which correctly capture the dynamics of the contact line (where the air/fluid interface meets the solid wall of the rheometer).