Simulation and visualization of nano SiO2 - water and decanoic acid-modified nano CeO2 - cyclohexane dispersions under a centrifugal field

This work examines simulation and visualization methods for describing settling characteristics of nanoparticles (NPs) of (60, 100 or 200) nm SiO2 NPs in water and 8.7 nm decanoic acid-modified CeO2 (C10CeO2) NPs in cyclohexane measured experimentally under centrifugal fields. Analytical centrifugation of NP dispersions at relative centrifugal fields (RCF) of up to 2325 g at 20 degrees C was used to obtain space and time-resolved concentration profiles (STCP) along with supernatant/suspension phase interface height (IH) changes with time. A simplified advection-diffusion (sAdDf) model (Antonopoulou et al., Physics of Fluids 30 (2018) https://doi.org/10.1063/1.5010735) was modified for polydisperse systems by using: (i) Gaussian (G) average of two single diameter simulations, (ii) effective axial dispersion coefficient (EADC) to replace diffusion coefficient (D) with (D-ax=D/k) and (iii) use of experimental maximum packing volume fraction (Maxpack). For SiO2 systems in water, the model with effective diameter (de) was able to quantitatively represent IH changes with time for 200 nm SiO2 NPs (d(e)=200 nm), 100 nm SiO2 NPs (d(e)=110 nm) and 60 nm SiO2 NPs (d(e)=90 nm), where smaller particle sizes required larger d(e). STCP of SiO2 NP systems could be represented quantitatively with G or AD modifications for 200 nm SiO2 NP systems, but simulations became qualitative as particle size decreased. STCP of C10CeO2 NPs in cyclohexane did not show clear IH boundaries as each profile seemed to show continuous hindered settling of collected particles. The model with G or AD modification of were able to provide qualitative representation of the STCP. Remarkably, Maxpack (=0.359) modification with the model allowed quantitative representation of the apparent IH for 7 wt% (1.89 vol%) C10CeO2 NPs in cyclohexane. Transmittograms of the STCP show that the settling characteristics of C10CeO2 NPs in cyclohexane are fuzzy without clear boundaries, which indicates settling is probably dictated by smaller NPs and their solvation shells that interact in a collective manner to cause long-range (mu m order) interactions.