Written by Eric Ng

Tears of wine is a phenomenon identified by physicist James Thompsonn in 1855. This effect was later name after an Italian Physicist Carlo Marangoni, who studied this phenomenon in his thesis.

This effect refers to the mass transfer along an interface due to the gradient of surface tension and the tears of wine refers to ring of clear liquid, near the top of a glass wine, from which droplets form and flow back into the wine. (See figure 1 & 2) In layman term, some called it wine legs, curtains or church windows.

It is mostly observed in wine of high alcohol content. It had also been used in ancient time to judge the quality of a wine. However we do not intend to have a long discussion on the quality of wine or beverage here, not even teaching  the skills of judging here (though the best way is to taste it), we are indeed impressed by the science behind this Marangoni effect.

As a researcher working in colloidal science and assembly, I came across this interesting paper [1] depicting the forgotten effect of Maragoni on Coffee-Ring deposition. The science of Marangoni flow was long forgotten due to common use of water-based colloids in daily life. This effect was found to have significant effect on the mass transfer of particles along the free surface of liquid, provided no surface-active agent present [2].

Conventional understanding of coffee-ring deposition occurs at which particles tends to deposits at the edge of water droplet where the evaporation rate is the fastest. The author Hu brought us a new insight that alkane-based colloids without surfactant tends to be different. Deposition occurred at the center of the droplet instead of the edge, despite higher evaporation at the edge. The question here is: “Does the directional convective flow towards the edge stop?”

The answer is definitely no. He revealed a recirculatory flow of particles, which started from the convective flow of particles to the side, and then followed by surface-tension driven flow along the free liquid surface, from edge towards the center top of the droplet. This flow was driven by the gradient of surface tension caused by the temperature difference along the liquid surface. The particles then sinked down to the bottom and continued with the convective flow to the edge. As drying time is short, particles tends to adhere to the center of the substrate, resulting in the deposition at the center of the droplet.

The following shows the flow field of liquid droplet (Marangoni Flow) as depicted by Hu et al.

Things to know: Surface-active agent

Supporting Video by Hu et al on “Marangoni Effect Reverses Coffee-Ring Depositions”

http://pubs.acs.org/doi/suppl/10.1021/jp0609232/suppl_file/jp0609232si20060213_090622.avi

References:

1. Hua Hu and Ronald G. Larson, “Marangoni Effect Reverses Coffee-Ring Depositions,” The Journal of Physical Chemistry B 110, no. 14 (April 1, 2006): 7090-7094, doi:10.1021/jp0609232.
2. Savino et al, “Buoyancy and Marangoni Effects in an Evaporating Drop,” Journal of Thermophysics and Heat Transfer 60, no. 4, (Oct-Dec 2002): 562-574.