Aggregation and settling in aqueous polydisperse alumina nanoparticle suspensions

TL;DR

This study investigates the complex aggregation and settling behavior of polydisperse alumina nanoparticle suspensions in water near their isoelectric point, utilizing advanced SAXS techniques to monitor particle dynamics and validate findings with microscopy.

Contribution

It introduces the use of high-resolution SAXS to monitor real-time aggregation and settling in polydisperse alumina nanofluids, providing new insights into their instability mechanisms.

Findings

SAXS can distinguish rapid settling regimes in polydisperse suspensions.

Particle aggregation dynamics can be monitored via SAXS intensity changes.

Drying experiments validate SAXS predictions on particle size reduction over time.

Abstract

Nanoparticle suspensions (also called nanofluids) are often polydisperse and tend to settle with time. Settling kinetics in these systems are known to be complex and hence challenging to understand. In this work, polydisperse spherical alumina (Al2O3) nanoparticles in the size range of ~10-100nm were dispersed in water and examined for aggregation and settling behaviour near its isoelectric point (IEP). A series of settling experiments were conducted and the results were analysed by photography and by Small Angle X-ray Scattering (SAXS). The settling curve obtained from standard bed height measurement experiments indicated two different types of behaviour, both of which were also seen in the SAXS data. But the SAXS data were remarkably able to pick out the rapid settling regime as a result of the high temporal resolution (10s) used. By monitoring the SAXS intensity, it was further possible to record the particle aggregation process for the first time. Optical microscopy images were produced on drying and dried droplets extracted from the suspension at various times. Dried deposits showed the rapid decrease in the number of very large particles with time which qualitatively validates the SAXS prediction, and therefore its suitability as a tool to study unstable polydisperse colloids. Keywords: Nanoparticles, nanofluids, polydisperse, aggregation, settling, alumina, microscopy, SAXS