Wine can cry, too!
No feelings, just alcohol
If you are not a natural-born people person, social gatherings can be a stressful situation. Today’s story is about a topic that you can use as a conversation starter or party entertainment. All you need is someone holding a glass of wine or stronger liquor and then you can step in with a question “Do you know that your drink can cry?”. OK, I admit, there is a risk you might sound like a weirdo, but you never know who will find this interesting.
I talk about this because the tears of wine phenomenon is a great STEM experiment for kids (by “experiment” I mean the behavior of liquid, not people at a party). When you see it for the first time, it looks like a magic – a liquid climbing up, defying gravity. And even though the basic explanation was given in the mid-19th century, it is still explored by scientists.
The original story of the tears of wine is, of course, based on the behavior of wine. Sometimes you can notice that a wine in a glass will keep sliding down the glass for a long time after you take a sip. To recreate this, swirl the wine in a glass and keep watching how the wine is climbing up the glass and then creates droplets that slide down like tears (it is easily visible in the shadow from a flashlight). This can go for a long time, and it works better with stronger liquors and/or warmer liquids.
Before we jump to the explanation, let’s put together an experiment that will significantly enhance the tears of wine effect. We will introduce two major changes: isopropyl alcohol instead of ethanol, and a glass plate instead of an ordinary glass. You can do it with ordinary ethanol, too, but then I suggest you warm a bit the water-alcohol liquid to enhance alcohol evaporation.
The experiment is quite simple:
Prepare a flashlight, isopropyl alcohol (or warm ethanol), water (warm if you use ethanol), a food coloring, and a glass plate (e.g., a watch glass or oil warmer dish).
In a cup, mix 2/3 water and 1/3 alcohol. Add a few drops of food coloring to make the liquid more visible.
Put the liquid on a glass plate and then swirl the liquid to wet the plate sides.
Point the flashlight down toward the plate and observe the shadows. Notice how the liquid climbs up and forms a rim. The rim quickly becomes too big to hold all the upcoming liquid and it starts to break into tears that slide down.
Gently blow above the plate and observe the changes. Also, repeat the experiment using a warm plate and/or warm liquid.
The effect of liquid climbing up the glass is now quite dramatic. You can see waves of liquid moving up. The driving force is the surface tension force where the mutual attraction of molecules in water-alcohol mixture is smaller than the attraction force in water only. Since evaporation is reducing the amount of alcohol in a thin layer of liquid on the glass surface, this liquid pulls on its molecules much stronger than the liquid below where the alcohol content is higher. This creates an excess surface force moving the liquid up. Such a flow driven by surface tension is called the Marangoni flow, and we talked about it in our newsletter. A beautiful example of it was presented in a STEM experiment on the phenomenon of Marangoni bursting.
The flow is stalling at the height were the surface tension engine stops because the alcohol is gone, or the weight of accumulated liquid is too big. The accumulated liquid creates a ridge that grows until the gravity wins and pulls down the liquid. This downward flow takes the shape of a tear. Since both the upward and the downward flow are happening at the same time, they keep going for a long time.
You can see now why stronger alcoholic liquors, warmer liquids, and isopropyl instead of ethanol make the effect more pronounced – all these choices enhance alcohol evaporation. Also, a plate reduces the gravity component along the glass surface compared to an ordinary glass with a vertical wall.
This story was described for the first time in 1855 by the British physicist James Thomson (elder brother to Lord Kelvin). One would think that after so long time, nothing interesting remains to be explored today about that. It is quite typical for STEM experiments to be described as complete stories that lack the excitement of undiscovered features.
However, in 2015 a study was published with a new twist to the story – cooling. Notice that your body sweats to cool down when you’re hot. Sweating is using the trick of extracting heat form the body to transform liquid (your sweat, made up of mostly water) into vapor. This keeps the body cooler than what it would be without evaporation. The same is happening in the tears of wine. The ridge can be more than 1°C cooler than the liquid below. The temperature difference drives a flow of liquid and adds to the Marangoni flow that we described above.
They also noticed that the tears tend to form a geometric pattern. This is a hydrodynamic instability on the ridge, where the balance between the gravity pulling down and the forces pulling up must be broken somewhere. It tends to be broken in a geometric fashion because the surface tension of tears tries to make them circular (Marangoni bursting is the same). This breaks the ridge into a periodic pattern of tears, where each tear takes about the same volume of liquid as it starts to move down. This is known as the Rayleigh-Plateau instability.
After that, another group of researchers looked more deeply into the role of gravity. If you do the experiment as described in the instruction above, you will see that the liquid creates “waves” when moving up. It turns out that this is not a trivial feature. As the surface tension increases upward, it also sucks more and more liquid along the way. This creates a flow where the liquid film increases its thickness while climbing up, while opposing the gravity at the same time. This situation is unstable, and they describe it as a “reverse undercompressive shock wave”. Such a wave breaks into tears when it encounters small inhomogeneities along the wave.
As you can see, it pays off to be creative with seemingly simple STEM projects. Just keep noticing details and ask “why?”, as these are the most important skills that your kid might get out of the home STEM experiments.