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Durham Centre for Soft Matter

Publication highlights

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.

Topological inversions in coalescing granular media control fluid-flow regimes

Physical Review E 96 033113 (2017)

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

Journal of Rheology 61 931 (2017)

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

Physical Review Letters 119 028006 (2017)

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).

Sticking and sliding of lipid bilayers on deformable substrates

Soft Matter 13 181 (2017) (Emerging Investigators special issue)

Biological lipid membranes are commonly bound to deformable structures, such as the cell cytoskeleton or the extracellular matrix. Having negligible elasticity, however, it is not obvious how the membrane conforms to the area deformation of the substrate without rupturing. To explore this question, Margarita Staykova and her coworkers confine artificial membranes to silicone substrates that can be bi-axially stretched and compressed with a well-defined rate and magnitude. We find that the membrane stress response depends substantially on the membrane-substrate interaction. Strongly adhered membranes follow the substrate area changes by absorbing (on stretch) and expelling (on compression) lipid protrusions, providing explanation for analogous observations on cells. Weakly adhered membranes on the other hand are able to slide over the deforming substrate, thus preserving their surface area constant. This novel system is expected to significantly extend the applications of the current lipid technologies beyond solid supported membranes, and contribute to our understanding of the mechanisms of membrane area regulation in cells.

Pharmaceutical polymorph control in a drugmimetic supramolecular gel

Supramolecular gels can exhibit drug-specific molecular recognition that change the polymorphic outcome of a pharmaceutical crystallization process entirely in ways that can be explicitly understood and designed for. In this paper the Steed and Day groups report the crystallization of the olanzapine precursor ROY from a gel of compound 1 (Scheme 1) which was explicitly designed to have the same peripheral chemical functionality as the ROY molecule itself. ROY is a very well explored model pharmaceutical which has ten polymorphic forms. The name comes from the Red, Orange and Yellow colours of the crystals. Gels of 1 (and ONLY 1) selectively result in the crystallization of the metastable red (R) form of ROY. A range of other gels and crystallization from solution under exactly the same conditions give the thermodynamically most stable yellow (Y) form (Scheme 1).

Gels are very difficult to structurally characterise, however, the group used computational crystal structure prediction methodology in combination with molecular conformational modelling to show that the role of the active gel surface gel is in stabilising the conformer most closely related to the R form of ROY and promoting its epitaxial deposition on a locally ordered segment of gel fibre.

Scheme 1. Crystallization of the Y form of ROY from a gel of a control compound and the metastable R form from a gel of the ROY-mimic gelator 1. The thermodynamic Y form is also obtained from solution under the same conditions.