Tears of wine
Church window (or tears / wine tears ) as technical term for the wine tasting are spontaneously formed structures on the inner wall of a cognac or wine glass.
Importance in the wine tasting
The stained glass windows provide information on the concentration of various ingredients. The viscosity, for example, is the wine of an alcoholic strength - influenced by the sugar and the extract content of wine - especially from the higher alcohols such as glycerol. The higher these values, the viscous / viscous, the wine, the slower and larger tears. About the quality of a wine, give the tears not provide direct information.
Alcohol rich wines produce more tears and pointed arch stained glass windows, low-alcohol wines weaker tears and round-arched church window.
Origin of the effect
Tears caused by circular swirling the glass, making the wine wets the inner wall. After a portion of the layer of liquid has evaporated, it flows not further reduced, but also against the force of gravity above. The reason is that alcohol evaporates faster than water, which increases the surface tension. The effect is greater than the top down, not only because there the layer is initially thin, but also because of there better ventilation - particularly pronounced in glasses that taper upwards. With the above higher surface tension " flows " up the surface and pulls with it the thin film ( Marangoni effect). Given below fresh fluid is drawn from the volume in the layer, while overhead tracking alcohol evaporates, the Marangoni convection does not come to a standstill.
After a while, collects at the top of the wetted surface, a ring of a residual fluid that is rich in sugar and other high-boiling constituents and becomes harder. First, the surface tension acting transversely and keeps the lower edge of the ring more or less smooth. The liquid but will eventually not run down area, because where the lower edge of the ring accidentally slightly "hang", he takes quicker than next more fluid from the layer, causing differences in the surface tension, conveying the initial difference ( instability). Finally, drops from lace. The rivulets, they leave behind the process remain clearly visible, because now acting in the transverse direction of Marangoni convection feeds them more liquid and it restricts the same time. The narrow, hanging threads are reminiscent of the skeletons Gothic church windows.
The closer the drops come the liquid level, the more violent is the " headwinds " of the Marangoni convection. Therefore, they initially continued to grow before they are emptied at the meniscus. Sometimes the thread can then withdraw slightly and form another drop.