Hindrance in MWCNT nanofluid flow through porous media observed by PET

We studied the behaviour of a concentrated multi-wall carbon nanotube (MWCNT) nanofluid injected into a synthetic porous media. The experimental conditions were chosen for optimal particle mobility: strongly repulsive interactions between particles and between particles and the glass matrix, inhibiting aggregation and adsorption. Furthermore, a large glass bead size rendered filtration unlikely. However, hindrance effects in the concentrated, though stable against sedimentation, nanofluid due to particle- particle interactions showed to have a decisive influence on the fluid flow characteristics, in particlular, the particle transport.
Using positron emission tomography (PET) as imaging modality we could observe the transport of the MWCNT nanofluid through the glass bead packing in situ. During pulse injection of the suspension into the porous media the suspension spread out as a horizontal plume at the bottom of the column by “flooding” the respective pore volume. After this pulse injection the subsequently injected water seemed to penetrate the suspension only mobilizing MWCNTs from the top of the suspension plume rather than displacing the bulk of injected nanofluid. This mobilization from the top of the plume is most effective in the central parts of the column where the flow velocity is highest while most of the nanofluid is trapped in more stagnant zones of the pore space at the bottom edges of the column with minimal MWCNT displacement. The mobilized MWCNTs are higly diluted and mainly visible via the continuous diminishing of the plume from its central top boundry.
These observations can be explained by a pronounced hindrance of the particle transport due to particle-particle interactions in the concentrated suspension which is only overcome at the edges of the nanofluid plume, primarily in the central zones of highest flow velocity.
In order to achieve the PET measurements the MWCNTs used in this experiment were oxidzed by oxidative acid treatment (Wang et al. 2006) and radiolabeled with the positron emitter I-125. In an uncomplicated one-pot synthesis the CNTs were labeled by an electrophilic attack of I+ on the electron-rich CNT side-wall catalyzed by the so-called iodogen 1,3,4,6-Tetrachloro-3α-6α- diphenylglucoluril.