Winetear

Winetear, an old hooded vintner, peers into his glass as the wine climbs the sides and falls back in tears.

The Physics

Swirl a glass of strong wine and a thin film climbs the wall above the liquid, gathers into a ridge and falls back as drops: tears, or legs. Alcohol evaporates from the film faster than water, so the film becomes more watery, and water has a higher surface tension than alcohol. The difference in surface tension pulls liquid up the glass from the bulk below, where there is more alcohol. James Thomson, Lord Kelvin's brother, described it in 1855 (“On certain curious motions observable at the surfaces of wine and other alcoholic liquors”, Phil. Mag. 10, 330). A flow driven like this is now called a Marangoni flow.

τ = ∇γ

• τ: shear stress on the liquid surface (Pa)
• ∇γ: gradient of surface tension along the surface (N/m²)

The flow runs from low to high surface tension. Gravity eventually wins over the thickened ridge at the top, which breaks into drops that run back down.

Honest caveat: Tears are not a sign of quality or sweetness, only of alcohol content and evaporation. A covered glass, which stops evaporation, soon stops crying.

On the sticker

Winetear holds up a glass of glowing red wine with a thin film on the glass above it. Printed text: WINETEAR with the line ∇γ DRIVES FLOW. Pixel-art kiss-cut vinyl sticker, white, in 2″, 3″, 4″ and 6″ sizes.