Like black coffee? If you have a cup regularly then you may have observed the presence of a wispy membrane on the surface of your coffee. What is this membrane? Japanese researchers took a closer look to find the answer.

The membrane in question was first identified by Torahiko Terada, a Japanese physicist, in the 1920s, noting a similar occurrence in hot tea. Vincent Schaefer, an American chemist, hypothesized during the 1970s that the membrane was made out of tiny water droplets held by an electric charge above the liquid's surface. In a new study, researchers from the Kyoto University added that the phenomenon also happens to hot water.

To closely study the phenomena of the coffee membrane, Takahiro Umeki at Kyoto University in Japan, and colleagues used a transparent container to heat water to around 122 degrees Fahrenheit. They then placed a camera under the water's surface to observe the phenomena, preventing the lens from misting over which would impede their view.

Observing the mist, researchers found out that it was indeed made out of tiny water droplets, each around 10 micrometers in radius and floating about 10 to 100 micrometers above the hot liquid's surface. As they float, the droplets of water come together, forming a mist in a manner that results into a triangular lattice. Like rain, the mist eventually falls from above the liquid's surface, assumed to have been condensed because of cold air from above.

Additionally, the researchers also spotted another phenomenon, where droplets in specific parts of hot liquid vanished so quickly that it was captured in consecutive frames in the video. This vanishing event, it turns out, is what leads to "cracks" in the membrane being originally observed.

While the study has finally uncovered how the mysterious wispy membrane over hot coffee forms, it also raised a few other questions. Like, what's responsible for the droplets of water levitating (which allowed the membrane to hover over the coffee) and why is a droplet of water vanishing in the first place.

The researchers disclosed that the study has no real technological applications but they pointed out that it was simply remarkable that something as simple as a cup of coffee was capable of intriguing phenomena. Maybe if further research was done, the dynamics behind the wispy membrane over a cup of coffee would yield surprising uses.

Aside from Umeki, Masahiko Ohata, Masatoshi Ichikawa and Hiizu Nakanishi also contributed to the study.