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.
