If you've noticed a thin white mist clinging to the surface of your coffee—not the crema, northe steam, but a ghostly white layer just on the surface—you're not alone. Now, Japanese scientists are beginning to understand where it comes from.
It's long been a mystery. First identified in the 1920s, it's also been shown to be present on top of tea and plain old hot water—it's just that coffee makes it show up rather nicely. While it's understandably been hypothesized that it's a mist of water droplets, it's until now been unclear exactly what it is.
Fortunately a team from Kyoto University in Japan has been taking the subject seriously, carrying out the first modern-day study of the white mist. To understand it, they placed a camera beneath the surface of cup of hot water, facing up and out from the surface, to see what happened as the temperature ramped up. Turns out, a series of tiny droplets of water—each with a radius of around 10 micrometers—rise up and out of the cup to a height of 10 to 100 micrometers. There, they form a triangular lattice and create the mist you've probably seen.
The team has also observed the droplets occasionally disappearing, causing a crack in the surface of the mist to propagate across the entire surface. Imaged at 8000 frames per second, it's also been observed that these events are accompanied by a wave that moves across the surface of the liquid, too.
But with all these answers come some more questions. First, it's not clear why the disappearance events occur. Perhaps even more puzzling, though, is that the team in Japan isn't entirely sure what causes the droplets to levitate above the surface in the first place.
The scientists speculate that the drops are likely electrically charged, given that they forms a neat triangular lattice. That suggests that an electrostatic force may develop that causes them to float above the surface and repel each other into their neat matrix. But why they'd become charged in the first place? Nobody knows.
Image by Daniel Go under Creative Commons license