When a coffee mug leaves a ring, the outer edges of that ring are darker than the inside of the ring. That's because the solute is separated from the liquid during the evaporation process - the coffee ring effect. This same effect can happen with printers. When printing a text document, the text itself consists of an outline of the letters and the inside of the letters. While it may not be visible to the untrained eye, the outlines are actually darker than the inside. This happens during the drying process, just like the coffee ring effect. Efforts have been made to find a way to remove this difference in pigmentation.
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