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Recently there’s been much discussion and lively debate in the Master Sommelier community about the term “viscosity” as it’s used to describe
the appearance of wine as it moves in the glass.  Some feel the term is incorrectly used even to the extent that it should be wiped completely from the wine lexicon.  Mind you our intention in using the term has always been to help students assess the potential alcohol level and/or the presence of residual sugar in a given wine and to also help describe a wine to a guest at the table.  The concept behind our usage of the term is a wine with high alcohol and/or the presence of residual sugar such as a young vintage port will display thick, slowly moving tears/legs in the glass and thus have high viscosity.  A wine at the other end of the alcohol/residual sugar spectrum such as young Mosel Kabinett Riesling will do the opposite and simply sheet down the sides of the glass without forming any tears/legs whatsoever, thus offering low viscosity.  So far so good.  But the naysayers in our community
point out that according to a more strict scientific definition the term
viscosity does not and should not apply.  

At this point I’m tempted to roll my eyes and say, “really?” But given the pedigree and innate brilliance of my colleagues I’m willing to at least explore the definition of the term and even entertain a possible alternative.  With
that, Wikipedia, the free online encyclopedia and fountain of all truth and deep knowledge, defines viscosity by saying:

I. Viscosity

“The viscosity of a fluid is a measure of its resistance to gradual deformation by shear stress or tensile stress” (I’m thinking of sheep, occupational hazards and Prozac here).  “For liquids, it corresponds to the informal notion of “thickness. For example, honey has a higher viscosity than water.”  

Wikipedia further states that, “viscosity is due to friction between neighboring parcels of the fluid that are moving at different velocities.” 
(Frat party, anyone?) “When fluid is forced through a tube, the fluid generally moves faster near the axis and very slowly near the walls, therefore some stress (such as a pressure difference between the two ends of the tube) is needed to overcome the friction between layers and keep the fluid moving.” (I’m thinking of competitive eating contests here). “For the same velocity pattern, the stress required is proportional to the fluid’s viscosity. A liquid’s viscosity depends on the size and shape of its particles and the attractions between the particles.”  

II. Surface Tension

After reading the above slowly and out loud (multiple times) I’m not convinced it contradicts our use of the term viscosity.  But I also have to confess that several years ago when conducting a tasting seminar in Santa Fe I was corrected on my use of the term by a nuclear engineer from the Los Alamos labs.  Said scientist referred to the phenomenon as “surface tension.”  I’m quite positive he was (and still is) smarter than me so I once again turned to the uber-venerated Wikipedia for enlightenment.  The following definition was provided:

“Surface tension is a contractive tendency of the surface of a liquid that allows it to resist an external force.” (Water beds, anyone?) “It is revealed, for example, in the floating of some objects on the surface of water, even though they are denser than water, and in the ability of some insects (e.g. water striders) to run on the water surface. This property is caused by cohesion of similar molecules, and is responsible for many of the behaviors of liquids.” (As we don’t regularly invite denizens of the insect world into our glasses during tastings I think we may be off the hook here).

“Surface tension has the dimension of force per unit length, or of energy per unit area.” (My high school physics background is crumbling here so I’m just going to go with it.)  “The two are equivalent—but when referring to energy per unit of area, people use the term surface energy–which is a more general term in the sense that it applies also to solids and not just liquids. In materials science, surface tension is used for either surface stress or surface free energy.”  

At first glance I felt a strong need for a pie chart or some kind of visual aid here but reading the phrase, “surface tension is used for either surface stress or surface free energy” gave rise to vivid images of teenage angst and thus greater understanding.  

But I still wasn’t convinced we had to dump “viscosity” as a meaningful term used to teach students about the physical properties of wine in the glass.  That was until a third—and fourth—possible term was suggested by a fellow Master in the form of “flow inhibition” and “flow impedance.”  These two phrases were uttered during a conversation with the utmost seriousness and complete conviction.  Immediately visions of an impending trip to a urologist danced in my head.  But rest assured dear reader that I resisted all urges to snort, guffaw or allow various bodily functions to seize the moment in response. 

After that I almost allowed myself to pretend to feel a sort of catharsis over something I had been teaching for the better part of the last 20 years.  Was I wrong?  Have we all been wrong?  In that moment of darkness fellow Master and friend Steve Morey appeared with perhaps the most compelling piece of information of all.  It’s called the “Marangoni effect.” 

III. The Marangoni Effect                    

Wikipedia relates the following: “The Marangoni effect (also called the Gibbs–Marangoni effect) is the mass transfer along an interface between two fluids due to surface tension gradient. In the case of temperature dependence, this phenomenon may be called thermo-capillary convection (or Bénard–Marangoni convection). 

OK so we’re getting in deep here but stay with me.

“This phenomenon was first identified in the so-called “tears of wine” by physicist James Thomson (Lord Kelvin’s brother) in 1855.” (Aha!)  “The general effect is named after Italian physicist Carlo Marangoni, who studied it for his doctoral dissertation at the University of Pavia and published his results in 1865. A complete theoretical treatment of the subject was given by J. Willard Gibbs in his work On the Equilibrium of Heterogeneous Substances (1875-8).”

Further, “Since a liquid with a high surface tension pulls more strongly on the surrounding liquid than one with a low surface tension, the presence of a gradient in surface tension will naturally cause the liquid to flow away from regions of low surface tension. The surface tension gradient can be caused by concentration gradient or by a temperature gradient (surface tension is a function of temperature).”

Another aha!  Now surface tension is starting to make sense. Still further and utterly relevant to our present discussion was the following:

“As an example, wine may exhibit a visible effect called “tears.” The effect is a consequence of the fact that alcohol has a lower surface tension than water. If alcohol is mixed with water inhomogeneously,” (As in bad milk?) “a region with a lower concentration of alcohol (greater surface tension) will pull on the surrounding fluid more strongly than a region with a higher alcohol concentration (lower surface tension). The result is that the liquid tends to flow away from regions with higher alcohol concentration — along the tension gradient. This can also be easily demonstrated by spreading a thin film of water on a smooth surface and then allowing a drop of alcohol to fall on the center of the film. The liquid will rush out of the region where the drop of alcohol fell.”

After reading this many times and once again out loud it starts to makes sense. But yet another crisis of conscience immediately occurs.  How are we as wine educators possibly going to explain what we call legs and tears to students as related to that “Marangoni guy”?  I don’t know about you but the only thing I can see here is mass confusion and yet more wailing, gnashing of teeth and broken dreams. 

It all comes down to one of the major disconnects in learning anything about our beloved subject: wine has no inherent vocabulary.  We use the language of seeing, hearing and feeling to describe what we smell and
taste in wine and in doing so often appropriate, sometimes in error, nomenclature from other unrelated fields. Hence the above much ado about nothing concerning the term viscosity.  So often we in the wine community are guilty of trying to define and calibrate the physical and experiential properties of wine to the nth degree.  It’s a noble thing we do but ultimately impractical given the fact that so much of the wine experience is subjective (Is there any other kind of experience?).  It’s only through our collective hallucinations that we as professionals are able to arrive at commonly used terms such as “viscosity” and “fruit” and “finish.”  Never mind the fact that context in the wine experience is the ultimate trump card.  

So after much thought and careful deliberation I’ve come up with the perfect term to describe the properties of how wine moves in the glass.  It’s easy to understand for beginner and professional alike.  It’s called viscosity.  


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