Poly Ploys | Why Wine 'Cries'

Tom Bezigian explains the fundamental fluid science that gives wine its "legs."

Surface tension is an important consideration in all coating operations. This is discussed briefly here with an analogy to wine. Surface tension is an important phenomenon and a critical consideration in coating operations, including solution coating, adhesive laminating, extrusion coating and laminating, printing, and other related processes.

Surface tension is the force exerted by a fluid, of the force required to wet out a solid material. Different materials have different surface tensions. It’s almost intuitive that alcohol has a lower surface tension than water, which has a lower surface tension than “Play Doh,” the popular children’s putty.

When two liquids with different surface tensions are combined, there is movement in the form of unseen currents occurring as the forces due to chemical potential are balanced. This subject, chemical potential, deserves a book on its own merit; however, suffice it to say that chemical potential is when dialysis works. Fluids with different energy react in such a way that the forces are balanced, causing movement of the fluids from point A to B and vice versa.

With regard to solid materials, each requires a certain force to wet out (uniformly and completely cover) the surface with a fluid, that is, to cover the surface completely versus the fluid beading up into droplets. Compete coverage of a solid with a coating material or ink is normally needed to successfully produce industrial products.

As a point of reference, the opposite is true when you wax your car … a material is applied to the paint surface to reduce the surface tension below that of the surface tension of water so that water beads off the car, helping preserve the paint while giving an attractive appearance.

In the video seen here, as alcohol evaporates at the topmost part of the film formed on a wine glass, droplets are formed. The droplets are thicker than the film adhering to the wine glass (often called “clingage” in industry) and are then visually observable. When the force of gravity exceeds the adhesive surface tension of the wine, the droplets fall back into the body of wine, giving the appearance of crying, (also called “legs") so commonly observed by those who partake in the pleasure of wine.

In liquid coating applications, including printing, viscosity control is important for several reasons. The surface tension of a film is critical to the success of subsequent printing and adhesive operations. The actual phenomenon has been extensively described by many contemporary writers, such as Dave Markgraff, Rory Wolf, Tom Gilbertson, and many others. Fundamental work on this topic goes back to the 1960s and can be found in various scientific journals.

The actual chemical engineering principals governing film thickness and chemical potential are documented by Bird, Stewart and Lightfoot of the University of Wisconsin-Madison in their classic work (chef-d’oeuvre in French) entitled “Transport Phenomena.” This last reference is of such importance and value, it is one of the few textbooks from graduate school that remains in my library. It is a work that will never be outdone.

I'll never look at a glass of wine the same.

Tom Bezigian holds a B.S. in Plastics Engineering from the University of Massachusetts–Lowell. He has been affiliated with the converting industry for more than 30 years and writes a column, Poly Ploys, for PFFC. It focuses on the field of polymers, laminations, and coatings with emphasis on R&D, quality assurance, manufacturing, marketing, operations, finance, and expert witness experience in the blown film, cast film, orienting, extrusion coating, and converting industries. He is owner of PLC Technologies consultancy with over 30 years experience. Contact him at This email address is being protected from spambots. You need JavaScript enabled to view it..

Subscribe to PFFC's EClips Newsletter