(woman) That's the most amazing thing to me, is this-- the little bounce of the droplet.
You don't usually notice that when you watch wine.
It's so cool.
Oh my gosh.
Hey, I'm Dianna and I am having a bad hair day.
So there's this really cool thing that wine does.
I'm personally not much of a drinker, but I'm told this is well-known.
If you swirl your wine and it forms legs or tears on the glass, it is a fine wine.
And the explanation seems self-explanatory.
You swirl the wine, the legs form, they look like tears falling down-- end of story.
But not if you look closely.
If you look really closely at the tears, things start to not make sense.
Let's dig into the weird reason behind this phenomenon and whether it can actually tell you anything about the quality of wine, and then we're gonna look at how the force behind this phenomenon helped us build the space shuttle.
[giggling] The first weird thing that you notice is that, as the tears fall, they bounce as they hit the bulk of the wine.
They dance around.
That's a little odd.
Maybe it's a hydrophobic thing?
I don't know.
But then you sit here for a minute, and another minute, and another minute, and you realize that the tears have been falling for over five minutes.
That seems excessive.
Maybe they just fall slowly?
I don't know.
Who can gauge time anyways?
And then you realize you've never seen this in a wine bottle, only a wine glass.
And then someone shows you a special glass filled with whiskey instead of wine, where the tears start without even swirling first, which means the liquid flowed up the glass first, by itself, no human intervention, and the tears still fell.
There's definitely something weird going on.
This falls into the category of everyday mysteries.
And I'm already wearing my detective hat.
But this time, there is legitimate research behind this phenomenon.
There's a whole new paper out about it.
We went to UCLA to talk to the researchers in the applied mathematics department who worked on the mathematical model describing wine tears.
People always ask me when I show them-- "Why are you researching this?
Why is this important?"
Outside of the cool, interesting mathematics behind it, which we were really excited about, this is-- anything that can use Marangoni flow will benefit from the mathematic tools behind such models.
(Dianna) At the lab, Claudia poured some wine into a cocktail glass and then put it on a projector so we could see the details of the tears close-up.
And then we saw even weirder things.
(Claudia) I have it covered.
I'm trying to minimize evaporation.
(Dianna) Okay.
(Claudia) I'm gonna swirl it.
You see this ridge right here?
This fluid that's draining down, these tears, they're not going to look like the regular tears of wine that I'm going to show you in a second once I uncover it.
It just looks like paint going down a wall.
I'm going to uncover it.
And then evaporation is going to kick in and you're going to see this ripple... (Dianna) So cool.
...coming out of the bulk of the wine, going up the glass, and then forming and enhancing that edge.
That ridge of fluid.
Now the tears flowing, notice how these tears look very different than the other ones.
(Dianna) The tears didn't start until you uncovered the glass.
And then you saw this really subtle shockwave moving up the glass from the liquid, and only then did a ridge form and the tears started falling.
So what's going on?
Clue number one, it's not called "tears of orange juice."
It doesn't work with orange juice.
I tried.
Oh, oh-- ooh!
Okay, you ready for this-- tears, tears, no tears, no tears, no tears.
So alcohol content must be important.
Keep that in mind.
Have you tried it with grape juice?
Technically, I've tried it with grape juice because I've left this-- Andrea came in and I was like, "Let me show you this really cool thing that we found."
And nothing was happening.
I didn't think it would go away that fast.
If there's no alcohol, it won't happen.
Tears of wine form because of something called... the Marangoni effect, which has to do with surface tension.
We're going to do a demo here to help explain it.
Take some water-- do this at home, by the way.
Really, you should.
Take water and put it on a plate.
Then sprinkle some nutmeg or pepper or whatever you want onto the water.
Then take a little bit of soap and put it right in the middle of the nutmeg.
Ahh!
I love this one.
A lot of people will tell you that the soap is breaking the surface tension of the water, which is not true.
When you've got liquid with a strong surface tension, the water molecules in the liquid are really strongly attracted to each other.
That creates this strong net of surface tension on top.
Water has a higher surface tension than soap.
So when you put the soap in the middle of the plate, it tends to spread out over the surface, which has to do more with chemistry, but we're not getting into that; this is not Chemistry Girl.
So you've got the soap in the middle with the lower surface tension, and you've got the water on the sides with the higher surface tension.
The high surface tension will pull and overcome the lower surface tension so that everything moves outward.
Because you had a surface tension gradient from low surface tension to high surface tension.
Remember that; it'll help us figure out what's going on with the wine tears.
So now, the wine.
Wine is mixture made of mostly water and alcohol, which have different surface tensions.
So there's the potential for a surface tension gradient, just like with the soap and the water.
But the water and the alcohol are mixed together.
Hmm.
(Claudia) So you just think of flat, flat, bulk of wine and then a little bit dip.
Over here, it's going to form a little curved surface, and this is the starting point of our climbing film.
(Dianna) And that's the meniscus?
That's the meniscus.
Then I'm gonna remove this, and evaporation is going to be enhanced.
What's happening is now at the meniscus, there's less alcohol concentration.
(Dianna) Less than where?
Than at the bulk.
So the meniscus is less 'cause it's a little bit spread out-- And there's more evaporation there.
Aha!
So alcohol evaporates more quickly than water, and at the thin meniscus, you get more evaporation effects.
So then the meniscus becomes more watery, and therefore has a higher surface tension than the bulk of the wine.
The fluid at the meniscus is pushed up the walls of the glass because it has a higher surface tension.
So this is crazy-- the fluid is actually moving up the glass.
The higher the alcohol content, the stronger the effect will be, which is why it works really well with whiskey.
But that means that whether wine forms tears or not really only tells you about the alcohol content of the wine, and not much about the quality.
Sorry.
Now, fluid moving up the glass explains the shockwave that you see.
It also explains why the tears last so long.
That ridge at the top where the tears are falling from is constantly being replenished.
And as we mentioned, the movement of fluid because of a surface-tension gradient is called the Marangoni effect.
And it's the same effect responsible for this award-winning fluid dynamics photo.
I actually got to see the lab where this image was made, where they did this, at ESPCI when I was in Paris to go on the Vomit Comet with e-penser and Veritasium.
Shout-out to that old video.
[laughs] So the difference in evaporation rates of alcohol and water combined with the surface tension gradient caused by the difference in surface tension of alcohol and water creates the tears of wine because of the Marangoni effect.
The Marangoni effect is also responsible for the way welders distribute molten steel correctly at joints.
We can form this surface tension gradient using different effects.
In this case, it's the evaporation, but in the thermal management system, it could be because of the thermal effects.
So when you change the temperature, the surface tension can also be changed.
(Dianna) In this case, a difference in temperature causes the difference in surface tension, which drives the metal to flow toward the joint.
Without the Marangoni effect, you could say goodbye to space satellites and the space shuttle.
TIG welding is used to build the spacecraft because it's the best choice for aluminum, the lightweight metal most spacecraft are built from.
It is so cool that researchers are constantly learning more about phenomena that seems so simple, that you'd think we'd already know everything there is to know.
But this whole thing started from a class by our PI-- Professor Andrea Bertozzi.
She was like, "Let's try this tears of wine experiment."
So one of the grad students got very interested in this project.
It was a really, really cool project because you're starting off from this very physical, everyday problem.
But then it also involves all the cool aspects of applied math.
You got to drink wine in class.
Yeah, it was amazing-- wine and cheese.
The best things you could do in class.
It's also a cool project, but not as tasty-- people studied how milk turns into cheese and rots.
That's also a fluid dynamic problem.
By the way, I never explained why the droplets bounce when they reach the bulk of the fluid.
It's because right there at the meniscus, you're getting this really strong flow upwards.
So the droplets are coming down, but they're meeting this upward flow, and they kind of just bounce off-- it's so cute.
This is a really fun mystery.
Another case in the bag.
Thank you so much to all the researchers at UCLA who spoke to me about this and got excited with me.
It was really fun, and thank you guys for watching.