Abstract

For over a century, sub-micron colloidal particles, now often referred to as nanoparticles, have been formulated into water-based suspensions and then coated and dried for use as a medium to preserve and display images. For the majority of that time, the dominant technology for both capture and display of images was silver halide photography. Following the recent rise of digital photography, image generation by inkjet and thermal dye transfer printing have become increasingly popular. This presentation will describe some ways in which colloid and interface science has contributed to both silver halide and inkjet coatings. Silver halide crystals and liquid-crystalline dye assemblies capture light, while the colours are produced by interfacial reaction of developer organic molecules that have been coated as an oil-in-water emulsion. Naturally, the roles of surfactants in emulsion formation and coating control are crucial. Polymers too have many functions, providing colloidal stabilisation, rheology control, physical and chemical gelation, and a reversibly swellable medium that protects the image. In contrast, the colours in inkjet printing are not created from the coated layer but are delivered separately in the form of liquid microdroplets. Here, one of the major role of colloidal particles is to provide a quick-drying layer that gives sharp, glossy images. The processing of colloidal materials that are well suited to give glossy, porous layers can be challenging, as illustrated by the extremely unusual behaviour exhibited by a commercially available boehmite. This material disperses readily in water to give aggregates of, apparently inter-grown, crystallites. The dispersions are colloidally stable at low pH (optimally, 2 4), where they have a strong surface charge. However, concentrated suspensions of this material undergo irreversible shear-thickening and subsequent reversible switching between jammed and high-viscosity states.