If you ever enjoy a cup of black coffee in the morning, you may have noticed that the surface of the liquid is often covered by a wispy white membrane that almost seems to stick to the surface. This membrane ripples with any passing breeze and sometimes breaks apart, as though cracked, leaving curious fractures.

This membrane is something of a mystery. It seems to have been first identified in the 1920s by the Japanese physicist Torahiko Terada, who noticed a similar phenomenon on the surface of hot tea. In the 1970s, the American chemist Vincent Schaefer discussed the effect, hypothesising that the white membrane must be made of tiny droplets of water, possibly held above the surface by an electric charge. He also suggested that the droplets may levitate as a result of the flow of evaporated water from the surface.

Despite this historical interest, the true nature of this white mist has never been properly understood. That’s why today’s work by Takahiro Umeki at Kyoto University in Japan and a few pals will be so exciting for coffee lovers. These guys have carried out the first modern study of the white mist and have discovered for the first time why cracks appear so suddenly in its fabric.

Umeki and co begin by pointing out that the white mist is not a phenomenon that is confined to tea and coffee but occurs above the surface of hot water in general. To study it in detail, they setup a transparent container of hot water at about 50 degrees centigrade and placed a camera underneath rather than on top to prevent the lens from misting over. They then filmed the mist on the surface.

It turns out that the mist is indeed made up of tiny droplets of water, each with a radius of around 10 micrometres. The droplets float between 10 and 100 micrometres above the surface and form a triangular lattice when they come together to form a mist. Curiously, the droplets tend to fall from above the water surface, presumably having condensed in the cool air above, like rain.

The videos also show another puzzling phenomenon. Every now and again, the droplets in a specific region vanish so quickly that this process occurs between consecutive video frames in an ordinary 30 frame per second camera. It is these vanishing events that create the characteristic “cracks” in the misty membrane on hot coffee.

To find out more about this process, Umeki and co used a camera capable of 8000 frames per second at shutter speeds of 1/16000 seconds. This shows that the vanishing event propagates across the surface at a speed of around one metre per second. It is also accompanied by a wave that moves across the surface beneath the mist.

All this video evidence raises a number of puzzling questions. First, what is the force that causes the droplets to levitate? Umeki and co conclude that the drops are probably charged because they form a triangular lattice in densely populated regions, suggesting that they must repel each other. It is well known that small particles can easily become charged.

Whether this creates an electrostatic force that is responsible for the levitation is less clear. This force should depend on the electrical status of the surface of the water but this in turn should be influenced by impurities in the water such as salt. But the evidence gathered by Umeki and co is that the mist phenomenon is insensitive to variations in the water temperature, humidity and purity.

Just as puzzling is the possibility that the drops could be levitated by the evaporating flow from the water surface, since this too should be influenced by factors such as temperature and humidity. For the moment, the nature of the levitating force is still a mystery.

Umeki and co have had better luck determining the cause of the vanishing events. One clue is that the vanishing events spread at the same speed as capillary waves on the surface of water. Umecki and co say this suggests that the droplets are being swamped by a wave underneath as it moves across the surface.

Their calculations show that the peak of such a wave would be high enough to catch the droplets as it moves and could also be powered by the energy input from the swamped droplets.

Indeed, the videos suggest that the vanishing events can be triggered by the interaction between a single droplet on the surface of the water which then creates a wave that spreads.

However, yet another mystery is why a single droplet would vanish in the first place. One idea that Umecki and co put forward is that a cosmic ray hitting the droplet could change its electrical charge causing it to collide with the surface. But again this is an idea that will need more work to confirm.

Umecki and co make no pretensions towards any technological applications of their work. They simply point out how remarkable it is that a cup of hot brew can be the source of such puzzling phenomena. “Very few people appear to have imagined such fascinating phenomena happening in a teacup,” they say.

Perhaps there are important applications that will come from the study of the white mist on the surface of hot coffee. If any readers can help, please post your suggestions here, after enjoying your morning shot of caffeine.

Ref: arxiv.org/abs/1501.00523 : Dynamics Of Microdroplets Over The Surface Of Hot Water