What makes wine cry? Let's explore from first principle the "Tears of wine" - A case of Marangoni effect

Marangoni may sound like some spell from the “Legend of the seeker” or “merlin” series or maybe not, but anyhow, you sure will wonder at what I’m about to reveal to you. Now, before you think I’m here to teach some magic tricks like an illusionist, Marangoni is an Italian name and the name of the man who researched and explained the Marangoni effect.

Carlo Marangoni is his full name and he was the physicist who studied this effect for his Ph.D. work at the University of Pavia and published his findings in 1865. However, it was James Thomson in 1855 who first observed this effect in a glass of wine albeit he doesn’t know that it is an instance of a general effect that will later be called the Marangoni effect.

I presume you are filled with curiosity at wine crying and the strange term “Marangoni effect”. Well, that’s why I have struck my computer keyboard severally to explain to you how the wine cries and what makes it cry. Without tarrying further, let’s delve in.

^{CC0 Creative Commons: Tears of wine}

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Prolusion

Humans (even the lazy ones) are constantly hustling to make ends meet. Everyone strives to get what they desire, but we wouldn’t do any of this if we have what we need. As a result, it is no doubt that there’s a driving force behind our hustling.

Some will say they are driven by passion, while others are in for the money. Party to the former will look at the latter with disapproval and the latter will say to the former, “Let me see how far you can go”.

Nonetheless, the debate of which is right or wrong is not germane in the course. Whichever league you belong to, you surely have one thing in common with your counterpart – A driving force. This might not be apparent to onlookers, but surely as a governor of your life you know what moves you, be it passion, money or maybe a touch of both, but what is most important is that you have a motivation.

Summarily, there is no smoke without fire and there is no effect without some cause. Thus speaking of driving force, every life’s entity has one form or the other making sure such entity exist as it is. Basically, what we observe in every matter on the macroscopic level is a manifestation of what happens on the microscopic level. That is, the effect of this covert microscopic interaction is what we perceive.

Every matter is composed of molecules which are interacting which each other, and exerting forces on one another – the repulsive force which is a short range force and the attractive force which is a long range force, collectively known as intermolecular forces. However, of interest to this course is the interaction of molecules in liquids.

The repulsive force is as a result of electron cloud of the molecules. Recall that molecules form as a result of bonding between atoms and this bonds are formed by virtue of the need to attain stability. One species, the most electronegative takes electron from another species, which is more electropositive. The core of this molecules is thus surrounded by shield of electrons – the electron cloud.

It is well established that electrons carry negative charges and likewise, the principle of like-pole repulsion is renowned. Consequently, bringing two molecules closer results in some repulsive action as a result of their electron clouds. Obviously, the closer the molecules get, the stronger the repulsion and hence, short-range.

What about the attractive forces?

The force of attraction on the other hand is as a result of partial polarity at the ends of each atom present in the molecule - this is denoted by small delta sign. We expect a molecule to be neutral, well that’s not completely true as each atom present within the molecule have different affinity for electron and thus pulls differently on the electrons shared compared to its counterpart.

Water for instance is composed of two hydrogen atoms and one oxygen atom. Like, I said earlier, the aim of each participant in the resulting molecule is to achieve a stable electronic configuration. Oxygen requires two electrons to achieve this and hydrogen requires only one electron to become stable. Hence, it takes two hydrogen atoms to satisfy the need of the oxygen atom and vice versa.

The electron affinity of oxygen exceeds that of hydrogen so that the oxygen atom will pull more on the shared electrons than the hydrogen atoms. It’s like two people, say brothers having equal rights to a thing, say a piece of cake, the eldest will want to take a larger share of the cake all in the name of being older.

The oxygen atom in water pulls stronger on the shared electron and thus acquires a partial negative charge. It’s like taking a larger share of the cake. On the other ends, the hydrogen atoms are inferior and cannot match the strength of the oxygen atom, thus they give away some of their rights, that is, settling for the lean share of the cake. Hence they inherit a partial positive charge.

The interaction of this partial positive and negative charges at the ends of various molecules in the bulk of the water results in attraction between the molecules. This attractive force spans over a long range than the repulsion resulting from the interaction of electron cloud. This long-range attractive force is known as cohesion.

Similar to cohesion is Adhesion. Both are attractive forces, however, the former exists between molecules of the same species, while the latter exist between molecules of the different kinds. Cohesion and Adhesion play important roles in the nature of liquids and water is a perfect case to review.

Have you observed that water clings to the wall of a glass tube? That's adhesion in action. Water molecules have a stronger affinity for glass molecules than they do towards each other. As a result of this, the surface of water when inside a glass tube will not be level. Portions nearer to the walls of the glass will be higher forming a concave shape known as a meniscus. Furthermore, this adhesion is responsible for the rise of water in a narrow glass tube - a phenomenon known as capillarity.

There's more to Cohesion

Let’s go back to the molecular level. Picture some water in a small container, say a tea cup. In the bulk of the water, molecules are interacting with one another. Each molecule experiences an attractive force from every other molecule surrounding it – remember the cohesive force. For the sake of clarity, each molecule can be assumed to contribute equally to the force felt by the molecule in question and also the system is isotropic and thus all these forces cancel out, leaving no net attraction on the molecule in question.

However, we cannot say the same for the molecules at the surface of the water, they have molecules of their kind below them, but they are interacting with another species just above the water surface. Consequently, the cohesive force felt by these surface molecules is from those below and beside them. Now, since there are less contributors to the force felt by such molecules, each molecule pull stronger than those interacting with a molecule in the bulk and there’s a net force on such molecule in a downward direction.

To better understand this, imagine if ten people are required to carry an object and a day came where there are only five men available to carry out this same task. It is obvious that each of the five available people will have to improve with their effort, most likely double it.

The molecules at the surface of water (and any liquid) pull each other stronger than molecules in the bulk do. Each molecule experiences a strong pull, which tends to reduce the surface area to the possible minimum. Consequently, interfacial tension is created at the water surface, and this is called surface tension.

It’s like trying to stretch a piece of fabric from every point on its edge although this will result in an increase in surface area but, you can get a better glimpse into surface tension with this. However, the forces resulting in this tension are always trying to contract to liquid surface, thus making the liquid surface denser and at the same time under tension, say an inward stretch instead of that with the fabric.

I believe the concept of surface tension shouldn’t be too strange to anyone of science background but the driving force is probably unusual. Well, that I believe has been dealt with here. But there’s still more, so let’s move on.

Surface Tension as a barrier
Surface tension is a barrier on the surface of a liquid. It’s like when the surface of water freezes in the artic, preventing assess into the pool of water. Although this barrier presented by surface tension offers an infinitesimal resistance to intrusion in to the bulk of the liquid. In a more technical term, surface tension is the force acting on a film of liquid of unit length. It has a unit equivalent to the ratio of force to distance, i.e. N/m.

If you wonder what the physical meaning of this definition is, then worry no more. What this definition is pointing out is that to break the barrier offered by a unit length of smallest possible sheet of a liquid, you require a particular amount of force. Like in the fabric analogy I gave earlier, if you are to tear through a portion of the fabric while it is stretched, you need specific amount of force.

Due to the minute nature of surface tension, it is measured using a lower order unit of force called dyne. One dyne is the force required to accelerate a one gram mass at a rate of one centimeter per seconds. To save you some technical jargon and the need for analysis, 1 dyne is equal to 10 micronewtons. Surface tension is measured in dyne per centimeter (dyn/cm).

With respect to air, that is if air is the fluid above the liquid, water has the highest surface tension after mercury. Additionally, surface tension is a temperature dependent phenomenon. The surface tension of water is about 72 dyn/cm at 25oC, with mercury having a value of 485 dyn/cm at the same temperature.

Surface tension is responsible for some fascinating events observed in water. For instance, a paper clip carefully place on what will float on water, that is, it is supported by surface tension. In a technical sense, the weight of the clip is not enough to break the barrier offered by surface tension. In addition, insects such as water striders are able to walk on water. The spherical shape of water droplets is another effect of surface tension.

The aforementioned events are quite fascinating and wowing but I bet the “tears of wine” will even wow you more. Perhaps you have seen a tear-like pattern on the inner wall of a wine glass containing wine and wondered how this happens or maybe you are totally unaware and what comes to your mind is - why the wine cries, is that even reasonable? Here, we go then.

The Marangoni effect and Tears of wine

Without lingering, the Marangoni effect is a surface tension gradient induced flow and this theory concludes that molecules of a liquid will drift to a region of higher surface tension. The rationale behind this is the fact that regions of higher surface tension pulls better on molecules. It’s like a tug of war where the stronger team wins.

So, if there occurs variation of surface tension in a pool of a liquid, the region with higher surface tension will pull better on the molecules at the surface and the liquid will flow towards such direction.

This said surface tension gradient or variation may be as a result of changes in temperature or concentration of the liquid.

Let's see Marangoni effect in action