ECFP launches in-house technical workshops with the theme of Dense Suspensions  

To extend this workshop beyond the internal attendees, we have done a short Q&A with John and James.

Q: What is a dense suspension?

A: A suspension is a mixture of solid particles, usually from just below a micrometre in size and up so that thermal motion does not dominate, in a liquid background. This becomes “dense” or concentrated when the particles become surrounded and start to feel the influence of multiple neighbouring particles. At this point how the particles come into direct contact and pack closely together really matter. This makes dense suspensions a real challenge to understand as sometimes details can really matter.

Q: What types of applications are dense suspensions found within?

A: Both – Dense suspensions are found all around us in a wide variety of settings, from food products like melted chocolate to house paints to building materials like cement and concrete. There are even more examples where suspensions are used in intermediate industrial processing steps, for example the solid electrodes in Li-ion batteries are typically formed from dense suspensions of the active material mixed with conductive fillers. On a larger scale, dense suspensions even show up in earth science, notably mudslides and magma/lava flows. While the specific solid particles in these different systems are quite diverse, the general principles around particle contacts and packing limits tie them all together.

Q: How would you go about characterising the behaviour of a dense suspension?

A: Both – When used in applications how a dense suspension flows is often critical – paint needs to flow onto the roller but stay on the wall just like fresh cement needs to be pumped but hold up heavy aggregates in concrete while setting. This means the flow behaviour, or rheology, is absolutely critical and is a key technique for the Soft Matter Group and ECFP where mechanical properties are measured in well-defined small-scale flows. In application, you would want a low high-shear viscosity and large yield stress, measured from a flow curve that measures stress with changing flow speed or shear rate. However, to understand where this comes from on a microscopic level additional techniques are vital, for example confocal microscopy (great for translucent “refractive index matched model systems) or cryo-SEM for challenging opaque samples.

Q: What do you find most interesting about studying dense suspensions?

A: John – while researchers have been studying suspensions for nearly a century, we only recently settled on the right framework to understand and describe them. This finally allows us to move from the rheology of ‘simple’ systems to tackle realistic materials and flows. Its feels we are only beginning to take full advantage of this and translate academic understanding to practical impact.
A: James – how insight from one application area can inform something that on the surface seems completely unrelated. I think this really underlines the value of taking a physics-based model system approach to messy problems, even if can be difficult to keep all the relevant complexity and not throw the baby out with the bathwater.