The coffee-stain effect is a well-known effect in physics and daily life in which a dark-colored edge remains when a fluid containing particles evaporates. This is caused by an "avalanche" of particles moving to the outer edge, University of Twente scientists showed in a past study. In inkjet and 3-D printing, this is an undesired effect. Now, researchers have demonstrated that the effect can be suppressed by modifying the surface using an oily layer, according to results published in the Proceedings of the National Academy of Sciences (PNAS).