Beside being a sublime sensorial experience, tasting wine is a very instructive exercise. Indeed, the fluid dynamics of wine involves a variety of intriguing phenomena.
The wine is a mix of ethanol and water. Twirling the wine by a gentle circular movement of the glass creates a thin wine layer on the glass sides. This “orbital shaking” enhances the oxygenation of the wine and releases its “bouquet”. Understanding the underlying mixing process is important for many applications. As an example, it permits one to better design of large scale bioreactors for cell cultivation, as discussed by Reclari et al.
The coating layer on the glass sides displays a very more complex behaviour. The ethanol evaporates more quickly than water since it has the lower surface tension. The imbalance between the surface tension of the bulk wine and the water-rich liquid on the layer pulls wine in the glass to climb up the sides (this is an example of the Thomson-Marangoni effect). Then, the interplay between gravity and nonlinear surface phenomena leads to the formation of tears moving down the sides of the glass as discussed in detail by Nikolov et al. A more recent study by Dukler et al. suggests that under-compressive shockwaves caused by evaporation also play a major role in the formation of tears.
It is thus apparent that studying the fluid dynamics of wine is not just matter of curiosity. It enhances our fundamental understanding of many processes in fluids (e.g. wetting and spreading dynamics of fluid mixtures on solids), as well as having important practical applications. Therefore, let us keep tasting wine … if not for us, for the Science!