Fluid dynamics and porous media
Nano and microporous materials are ubiquitous across many industrial sectors including construction, food and pharmacueticals, petrochemicals and agriculture. We study the distribution and transport of liquids (usually water) within the pore structure of these materials, these phenomena impact many of the processes central to these industries.
The group carries out fundamental experimental and modelling studies of relevance to porous media. Experimental work, led by Professor Peter McDonald focuses on the development and application of a suite of nuclear magnetic resonance imaging and relaxation analysis (NMR / MRI) instruments and techniques using facilities some of which are unique to the University of Surrey. NMR and MRI are powerful, non-destructive and non-invasive techniques able to probe fluid dynamics in porous media over a very wide range of time and length scales ranging for nano-metric / pico-second to centimetres and days.
Theoretical work is led by Dr Faux. Like experiment, it too is multi-length and timescale ranging from molecular dynamics simulations of diffusion and surface adsorption at the molecular level all the way through to Lattice Boltzmann and Monte Carlo simulations of transport in complex geometries. A second and equally important strand of the theoretical work focusses on proper interpretation of NMR relaxation times of fluids in porous media with a view to learning both about the confining microstructure (pore size distribution; pore connectivity) and of the fluids within (surface wetting; surface adsorption; inter-pore exchange rates etc).
Applications work in the group focusses primarily construction materials including cements and concrete; wood (both living trees and felled timber) and, with Joe Keddie, coatings materials.
NMR and MRI of cement and concrete
Concrete is the most massively produced man-made material on the planet – estimated at 4 tonnes per person per year globally. The binder of concrete is cement and the active component of cement is calcium-silicate hydrate, a highly disordered, nano-porous material. Although cement is an inherently low CO2 material, arguably as environmentally friendly as wood on a kg for kg basis, the sheer volumes involved mean that cement production is responsible for circa 5-8% of man-made global CO2 emissions. Our work seeks to impact this environmental cost and has two principal drivers. The first is that water is intimately linked to cement hydration and hardening. The second driver is that water transport underlies nearly all forms of concrete degradation.
Current work is directed towards understanding the nano-scale morphology and water dynamics in cement with a view to enabling predictive incorporation of supplementary cementitious materials with lower CO2 impact. The group was the first to identify and measure the exchange of water between nano-scale gel pores in cement, has more recently proposed a quick and efficient means to characterise the C-S-H morphology, density, composition and pore size distribution using widely available bench top NMR equipment and most recently has shown how cement microstructure is dynamic and changing in response to water drying / ingress. At the other end of the scale, a portable MR system for the in-situ characterisation of the curing and degradation of concrete in the built environment has been developed undergone “field-trials” in Germany and UK. A “Best Practice” guide to making these measurements has been written in collaboration with NPL.
The group works closely with members of the industry / academic Nanocem consortium providing much of the NMR / porous media know-how input into, and an increasing fraction of the modelling work that is going on in that consortium. Work has been and / or is currently funded by the UK Engineering and Physical Sciences Research Council (EPSRC); The Royal Society; Industry and the EC. In particular the EC H2020 Marie Sklodowska-Curie Initial Training Network ERICA – Engineered Calcium Hydrates for Applications – that is funding 13 research students (4 in Surrey) is managed by Surrey.
NMR/MRI of wood
Understanding cell-water interactions and water transport in wood is important to many aspects of the timber industry, including the optimisation of the wood drying process. NMR and MRI are uniquely powerful tools for obtaining experimental data.
To that end, in recent years, the group has systematically designed, tested and commissioned a tree hugger magnet for magnetic resonance imaging (MRI) of water distribution in living trees and drying timber. This magnet is able to visualise water distributions and is now at Forest Research (Edinburgh). In addition we have used multi-scale X-ray CT scanning to obtain multi-scale digital maps of wood micro and macro structure in order to directly link wood microstructure to water transport.
Current studies with wood focus on
(i) understanding of macroscopic shrinkage, warping and cracking of wood that result from microstructural and molecular changes in wood upon drying and rewetting. These stress-induced changes are of critical importance to the forest products industries due to product loss. Our improving understanding of water in wood enables us to begin to quantify these problems using novel Lattice Boltzmann methods that combine effective media and multi-fluid (liquid / vapour) approaches.
(ii) introducing pits into the analysis. Pits are pores in wood cell walls that open and close in response to a relative humidity gradient across the wall. Counter intuitively it is currently believed that these only open in the absence of a pressure gradient like an inverse airlock.