Nanoparticle aggregation influenced by magnetic fields

Colloidally stable nanoparticle suspensions of iron hydroxide FeO(OH) at pH 2 have been prepared and shown to be non-magnetic. However, when the pH of the dispersion was raised to destabilise the suspension, aggregates near 200 nm diameter were detected by Photon Correlation Spectroscopy (PCS), and these were influenced by magnetic fields in the 50 kA m−1 range. These aggregates were unusual in that they were colloidally stable and did not behave as flocs. Consequently they were called nucleags to distinguish them from conventional flocculated agglomerates. As the pH was raised further towards the isoelectric point, flocculation of the nanoparticles was observed and brown sludge deposited on the base of the container. This material was not influenced by magnetic fields. A preliminary theory has been proposed to account for the observations. The experimental results fitted the curve with a one parameter adjustment. The conclusion was that magnetic fields can influence the behaviour of metastable aggregates in nanoparticle dispersions under certain colloidal conditions. The results may be relevant to deposition of scaling or fouling films in pipes carrying fluids.