Soft matter lubrication

Friction between sliding surfaces has a great economic and environmental cost, with it leading to a third of a car’s fuel use and wasting 23% of global energy production. This has spurred development of highly loaded contacts with liquid lubricants. Lightly loaded surfaces, with a thick liquid film are considered inefficient, and even trivial. This situation is common in soft matter, from swallowing food, or applying skin cream, to ceramic extrusion and even in synovial joints. However, there are additional complexities such as texture (tongue papillae), a non-Newtonian fluid (skin cream), deformability or a combination of sliding and squeezing.

In this talk, I will explore how scaling theories, alongside experiments, can disentangle soft matter lubrication. For a textured surface, the delicate balance between shear and pressure-driven flows is highlighted, while squeeze flow beneath a flexible surface isolates the coupling between pressure-driven flow and deformation. Finally, I will touch upon how such approaches lead to the design of optimised non-Newtonian fluids.

Apr 24, 2023 Leave a Comment

Time-dependent dynamics of 3D states in viscoelastic pressure-driven channel flow

Dilute polymer solutions do not flow like Newtonian fluids. Their flows exhibit in- stabilities at very low Reynolds numbers that are driven not by inertia, but rather by anisotropic elastic stresses. Further increase of the flow rate results in a chaotic flow, often referred to as purely elastic turbulence. The mechanism of this new type of chaotic motion is poorly understood.
In this talk we present the first coherent state in purely elastic parallel shear flows. We consider a model shear-thinning viscoelastic fluid driven by an applied pressure gradient through two- and three-dimensional channels. By starting from a linearly unstable mode recently discovered by Khalid et al.1 at very large flow rates and very low polymer dilution, we demonstrate that this instability sub-critically connects to significantly higher values of polymer concentration and lower flow rates2, rendering these structures experimentally relevant The dynamics becomes unsteady upon embedding the 2D coherent state in a 3D domain, see Fig. 1 for an instantaneous snapshot, and the time-dependent dynamics is discussed. The characterisation of those 3D states suggests their strong connection to purely elastic turbulence.

Apr 17, 2023 Leave a Comment

To the Centre of Titan and Beyond: 30 Years of Neutron Diffraction with The Paris-Edinburgh Cell

Apr 3, 2023 Leave a Comment