The topography of ink on paper

Dribble some ink or toner into water and it will diffuse uncontrollably. More sophisticated substrates are needed for controlled printing, and the interaction of ink with paper is of great importance to the ultimate quality and durability of the product. Coating paper with minerals or polymers is a common way to influence that interaction. Yet the microscopic three-dimensional structural characteristics of the ink–paper interface still remain mysterious. A group of researchers in Finland, led by Jussi Timonen (University of Jyväskylä), is working to clear that up. The team brought old techniques together in new ways for their analysis of 1-mm² samples of lightly coated paper—very heterogeneous substrates with almost no coating in some patches—covered with cyan toner. First, they used x-ray tomography on the printed paper to get a sample’s underlying topography at 0.8-μm resolution in all three dimensions. That resolution was fine for looking at the sample laterally...