Tears of wine

Following is the movie that is taken at 300 times of real speed. The diameter of the evaporation dish was about 8cm. The red liquid was 70 percent ethanol aqueous solution, red ink was also added.

This is the experimental setup of the above movie.

The movie was taken from the above of the evaporation dish. The procedure from (i) to (iv) is seen in the movie.

In fact, if you pour wines (or Japanese SAKE is also OK :)) into the glass, you can find this penomenum occur in your glass.

More vivid tear formation can be seen by using watch glass as a container. The solution used was the same as the above.

How can we understand this phenomenon?

At first, we should note that the solution (fake wine) is mixture of alcohol (ethanol) and water. As you know, alcohol evaporates from water spontaneously.

Assuming the evaporation is homogeneous at the surface of solution, the alcohol concentration at the region near wall is lower than that far from wall. This is due to the smaller volume of solution near the wall.

The surface tension of the solution at the region near the wall is higher than the rest, since alcohol reduces surface tension.

Here, Marangoni effect is a key for tear formation. The Marangoni effect is a phenomenon that a flow is produced from the place with lower surface tension to the place with higher surface tension.

Therefore, a flow is constantly generated to the thin film on the wall, and the lifted solution forms tears.