Tears of wine: The dance of the droplets.

For a long time, the phenomenon known as the "tears of wine" was believed to be due only to the surface tension gradient (e.g., the Thomson-Marangoni stress on the fluid/fluid surface's dynamics of wetting) and gravity. We experimentally demonstrated that the wine tear formation is not solely due to the surface tension gradient; instead, the ridge instability triggers the wine tears. Pouring wine into a glass causes a wine film to form on the glass. The film drains down under gravity and a ridge forms at its upper part. Over time, the ridge becomes unstable. Under gravity, a necklace of droplets (tears) appears and slides down. Here, we present experimental evidence that the Plateau-Rayleigh-Taylor theory for the stability criterion of a horizontal annulus fluid column under small, exponentially growing capillary disturbances and the surface tension breakdown into droplets can also be applied to the stability of a horizontal fluid's (wine) ridge on a wetted solid, where gravity causes the droplets (tears) to slide down, resulting in the formation of a necklace of droplets ("tears"). The wine droplets ("tears") move down and up ("dance") due to the effects of the surface tension gradient and gravity. The process repeats itself for a while. Over time, the wine components (e.g., organic acids and tannins) adhere on the glass. The glass surface becomes less hydrophilic and the wine wets the glass less. The wine film on the glass becomes unstable, the ridge does not form, and the tears stop appearing. The knowledge gained from the present study will enhance our understanding of the wetting and spreading dynamics of fluid mixtures on solids. It will also benefit our understanding of fundamental phenomena (such as wetting and spreading) and applied technologies (such as painting, printing, cooling, and cleaning), as well as aid in the development of robust devices (such as the lab on a chip).