Flow of nanofluids through porous media: Preserving timber with colloid science

Understanding the flow of particles through accessible paths in timber is important to optimising the timber preservation process. In this paper, we identify inconsistencies between the previously established flow paths in timber for simple liquids, and those for particulate systems. We show that the flow paths of nanofluids are through the rays of Pinus sylvestris (Scots pine) sapwood, then into adjoining tracheids through ruptured cross-field pits with effective pore size of 1.75–3.0 μm. We then present data from a custom-designed apparatus, with which we have studied the effect of size, charge and concentration of silica nanoparticles on their flow through pine sapwood. Our results show that particles smaller than 60 nm passed well through timber irrespective of their zeta potential. The flow of positively charged particles was significantly reduced when particle diameter exceeded 100 nm; whereas negatively charged particles with diameter of 250 nm still passed through timber reasonably well, provided the concentration of particles was below 0.5% (w/w). Furthermore, we rationalise such flow data with AFM and sessile drop contact angle measurements, which gauge the interactions between the nanofluids and a functionalised silica surface as a model timber surface. Whilst negatively charged nanofluids showed better wettability on the model surface that the positive nanofluids, the wettability did not show any particle-size dependence. We suggest that such contact angle measurements, performed under quiescent conditions, could not fully predict the flow and deposition of nanofluids through timber, which would be more complex due to the presence of an applied external pressure that could affect inter-particle and particle-surface interactions.