Pennsylvania, United States
Last year a team of physicists showed how to undo the “coffee-ring effect,” which occurs when drops of liquid with suspended particles dry, leaving a ring-shaped stain at the drop’s edges. The team discovered that different particles make smoother or rougher deposition profiles depending on their shape. The two deposition profiles of particular interest are “Poisson” and “Kardar-Parisi-Zhang” processes. Poisson processes arise when growth is random in space and time; the growth of one region is independent of neighboring regions. Kardar-Parisi-Zhang (KPZ) occurs when growth of an individual region depends on neighboring regions. A mathematical simulation of these growth processes might be a game of Tetris, but with single square blocks with the blocks falling at random into a series of adjacent columns, forming stacks. In a Poisson process a tall stack is just as likely to be next to a short stack as another tall stack. As such, Poisson processes produce a very rough surface, with large changes in surface height from one column to the next. On the other hand KPZ processes lead to blocks sticking to adjacent columns. When they fall into a column, they do not always fall all the way to the bottom but instead can stick to adjacent columns at their highest point. Thus short columns will catch up to their tall neighbors over time, and the resulting surfaces are smoother. There will be fewer abrupt changes in height from one column to the next.