The Mysteries Of White Mist On The Surface of Black Coffee

Japanese physicists have finally begun to tease apart one of the more important cosmic conundrums.

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.

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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.

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