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 but not for studying the details of the very thin ink layer. So Timonen and coworkers then used a laser to gradually erode the less-than-one- micron-thick ink layer and, after each laser ablation step, obtained a depth profile with 70-nm resolution. The figure shows a 3D visualization of the inked paper’s topography with the toner thickness superimposed. The researchers found that the thickness of the toner layer was dependent on the roughness of the coated paper rather than on variations of the coating. In particular, the ink was thinner where fibers protruded, even if those protrusions were in valleys rather than on peaks. (M. Myllys et al., J. Appl. Phys. 117, 144902, 2015, doi:10.1063/1.4916588 .)