|Coffee stain typeface by Mark Mustaine|
What he discovered was that the particles in a liquid came in two shapes: round like in coffee and more oblong in other liquids. The thin edge of a liquid dries more quickly than the center of the spill, and as it dries liquid flows towards the drying edge. If the particles in the suspension are round they will more easily be carried along by the fluid, and deposited at the edge when evaporation occurs. More oblong particles will tend to clump and so they will be more evenly distributed when the water evaporates and leaves its stain.
All of this, aside from being vaguely interesting, has a practical application. Paints and inks are just controlled stains after all. Understanding the physics that cause smoother and more evenly distributed stains means that ink and paint can be made to dry more evenly. As Yunker points out, "In inkjet cartridges, the ink costs more than expensive champagne per volume, If we can reduce the solid content in inkjet ink by a small amount, we could potentially decrease the cost by a large amount."
It never ceases to amaze me how many small bits and pieces of ingenuity go into every facet of our lives. Even the small, mundane and ignored details like the structure of a stain can and do lead to flashes of insight and a world that is made incrementally better.
You can read more about it in the article Physics of Coffee Stains Explained.
(HT: Real Clear Science)