Deposition of rutile titanium dioxide particles onto oppositely charged surfaces: A comparison of stagnation point flow and quiescent conditions and the formation of two-dimensional ''rafts''

The rate of deposition of paint-grade rutile titanium dioxide particles (similar to 0.3 mu m diameter) onto model substrates, under a variety of conditions, has been monitored. The model substrates used were optically-flat glass plates, whose surfaces were made cationic by reaction with (aminopropyl)trimethoxysilane, APTMS. A flow geometry was arranged such that the particles impinged, through a jet, normally onto the substrate. This creates a stagnation point region near the substrate surface, opposite the mouth of the jet. In this region, at relatively low mass transfer rates, it is supposed that particles reach the surface by a diffusion process. The deposition (rate) of the particles was monitored visually using both a microscope and also a CCD camera linked to an image-analysis system. This allowed the number of particles deposited per unit area to be monitored. Deposition rates, and maximum coverages, have been established at various particle concentrations and various background electrolyte (NaCl) concentrations. The results obtained agreed well with experiments in which APTMS-coated glass plates were simply immersed (''vertically'') in a suspension of titania particles, and particles were allowed to deposit under Brownian motion conditions. In the flow experiments, provided the NaCl concentration was below a critical value, the particles remained stable in dispersion and ''isolated'' on the substrate surface. However, over a certain salt concentration range, when the particles were precovered with an adsorbed layer of poly(vinyl alcohol), two-dimensional aggregation (''raft formation'') was observed on the substrate surface.