Close-up of a glass of red wine on a bar counter.

The film of wine flowing down the inside of a glass can generate ‘tears of wine’. Credit: Alamy

Fluid dynamics

Why a glass of wine cries

Shock waves might explain the flow of wine droplets after a glass is swirled.

The mysterious phenomenon called ‘tears of wine’ is produced because of shock waves within the film of wine that climbs the side of a glass.

After wine is poured into a glass and swirled, a thin film of the liquid creeps up the glass.That happens because the alcohol in wine evaporates faster than the water, and the resulting difference in surface tension pulls the wine upwards. Gravity soon tugs the wine down in a flow of tear-shaped drops, whose formation has been unexplained.

Andrea Bertozzi and her colleagues at the University of California, Los Angeles, identified shock waves that might be the missing piece of the puzzle. As surface tension and gravity battle, they create unstable shock waves in the thin wine film. The shocks propagate upwards, pushing the liquid’s front into a scalloped shape, which culminates in the large droplets that trickle downwards as tears.

To reliably generate these tears: pour whisky or port into a conical glass, immediately cover the glass to stop evaporation, swirl the liquid slowly to coat the glass and remove the cover after a few seconds when the swirl disappears.