Speaker A
Whoa. It's forming tiny little drops. What in the world? You can't even see any connection to the drops now. It's just levitating between my fingers.
Explore the fascinating physics behind saliva droplets, viscoelastic polymers, and their surprising connection to quantum liquids and spider webs.
Key Takeaways
- Surface tension drives liquids to minimize surface area, causing droplets to form from liquid cylinders.
- Viscoelastic polymers like saliva exhibit elastic stress that slows the breakup of liquid threads into droplets.
- Orb-weaving spiders exploit Rayleigh-Plateau instability to efficiently create adhesive droplets on their webs.
- Quantum liquids in Bose-Einstein condensates show similar droplet formation, linking classical and quantum physics.
- Fundamental physical principles can manifest in diverse systems, from everyday saliva to exotic quantum states.
What the video covers
- The video demonstrates how saliva and polyethylene glycol form tiny droplets due to surface tension and viscoelastic properties.
- It explains the Rayleigh-Plateau instability, which causes liquid cylinders to break into droplets to minimize surface area.
- Viscoelastic polymers like saliva resist breaking due to elastic stress from stretched molecular chains.
- The video highlights how orb-weaving spiders use this instability to create sticky droplets on their webs efficiently.
- It draws parallels between classical liquids and quantum liquids formed in Bose-Einstein condensates at near absolute zero temperatures.
- Quantum filaments also undergo Rayleigh-Plateau instability, forming regularly spaced beads with unique superfluid and solid-like properties.
- The video emphasizes overarching physical principles that apply across different scales and states of matter.
- A sponsored segment introduces Aura, a service that protects personal information by removing it from data brokers.
- The experiment encourages viewers to try the saliva droplet phenomenon themselves to observe this unusual physics firsthand.
- The video blends practical demonstrations with advanced physics concepts, making complex ideas accessible and engaging.
Chapters
- 00:00Introduction to saliva droplet phenomenon
- 00:35Polyethylene glycol and its properties
- 01:03Saliva droplet experiment demonstration
- 01:44Surface tension and liquid behavior explained
- 02:21Rayleigh-Plateau instability and droplet formation
- 03:02Viscoelastic polymers and elastic stress effects
- 04:30Sponsored segment: Protecting personal information with Aura
- 05:44Spider webs and natural use of Rayleigh-Plateau instability
- 06:19Quantum liquids and Bose-Einstein condensates
- 07:05Conclusion: Connecting saliva, spiders, and quantum physics
Full Transcript — Download SRT & Markdown
Speaker A
That's so weird. What is going on? What you're seeing is one of the strangest things that a liquid can do.
Speaker A
It spontaneously forms tiny droplets. This phenomenon is so odd that even though it had been discovered decades earlier, it wasn't until 2010 that scientists finally worked out the detailed physics of how and why these beads form. The liquid you're looking at
Speaker A
is called polyethylene glycol with a very large molecular weight. Now, if you want to try this, it's kind of hard to get your hands on this specific liquid.
Speaker A
But guess what? There's another liquid you can use that's even more common. So common that everyone on Earth has some.
Speaker A
Saliva. Okay, you have to try this. It sounds a little gross, but trust me, it turns out to be some really cool physics. You have to get some saliva on your fingers. Okay, don't worry. No one's watching you right now. So, get
Speaker A
some saliva on your fingers and then you have to string the saliva between your fingers and something really weird happens. So, watch this. Now, I know it's kind of gross, but the saliva does the same thing that the polyethylene
Speaker A
glycol does. It forms a chain of beads on a string. This is so weird. Once you get over that, it's a little disgusting what we're doing here. You should start to have a big question in your mind. Why
Speaker A
do the beads form? Both of these liquids are a homogeneous mixture. Why would this extremely nonhomogeneous pattern form though? Well, to understand this, first we have to understand one thing about liquids. They always want to reduce their surface area. This is
Speaker A
because of the surface tension at the air-liquid interface. So if I have a long string of liquid, it wants to pull together to form a sphere because it has less surface area. But now if you attach the liquid to your fingers, then it
Speaker A
can't pull together. It's forced to stay as a cylinder. But remember, this is a liquid. There are random variations on the surface. When I move my hand or the air blows, it creates tiny ripples on the surface. Now that there are ripples,
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there are areas of the cylinder that are pinched together that the fluid didn't have to make on its own. So it could use these to snap into a bunch of tiny spheres. But if you make too many tiny
Speaker A
spheres, then that total surface area is actually larger than the one large cylinder. So it doesn't snap into tiny droplets when the ripples have smaller wavelengths. But if you have larger wavelengths, then you can get a situation that suddenly triggers the
Speaker A
entire cylinder to snap into tiny droplets. This is called Rayleigh-Plateau instability. It happens when this ripple wavelength is equal to the cylinder circumference. And this is the reason that droplets form spontaneously when you have a stream of liquid. This is why
Speaker A
when you watch water drops fall in slow motion that there are tiny little drops that form in the middle when the water stretches out. There's inevitably wavelengths that arise that cause these droplets to pinch off from the main
Speaker A
cylinder. But this isn't the whole story with polyethylene glycol and saliva. These are special liquids called viscoelastic polymers. For example, watch this video of water compared to a polyethylene glycol solution. Notice that the drop that forms from the Rayleigh
Speaker A
Plateau instability stays attached for a while on the polyethylene glycol. So for saliva and polyethylene glycol and other viscoelastic fluids, in addition to surface tension, they also have internal forces at play. So now as the liquid inside tries to pinch off to form a
Speaker A
sphere, the long molecular chain gets stretched out. Those stretched out molecules want to contract back to their original coiled shape. So they create a strong tension along the length of the string. That elastic stress fights against the surface tension forces that
Speaker A
are trying to make the string thinner. The more surface tension tries to thin the string, the more the polymers are stretched and the greater their elastic stress becomes. So instead of the neck rapidly collapsing all the way to zero
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and breaking, its thinning can slow down dramatically. But before we continue, one thing that's always surprised me is just how much information about you can exist online that you never intentionally put there. This is a report showing what data brokers claim
Speaker A
to know about me. It has things like my addresses, phone numbers, relatives, and property information. I've seen with both myself and close friends and family just how easy it is for people to find personal information online. That's why
Speaker A
we need a product like Aura, who sponsored this video. Aura finds data brokers selling your information, automatically removes it for you, and keeps checking so your information stays off. There are several services that do this, but I like that Aura combines it
Speaker A
with a lot of other security features for about the same price. One feature that stood out to me is that Aura says it delivers fraud alerts 650 times faster than its competitors. Aura is also a US-based company whose business
Speaker A
is protecting your information, not monetizing it. The reality is that your personal information could be bought and sold while you're watching this video.
Speaker A
So, I'm not leaving myself or my family vulnerable, and I don't think you should either. To see what Aura finds out about you, go to aura.com/actionlab or click the link in the description or pinned comment, and you'll get a 14-day
Speaker A
free trial, and Aura can start removing your information automatically. And thanks to Aura for sponsoring this video. Now, let's get back to our experiment. So overall, it forms beads on a string because that's the lowest energy state when you take into account
Speaker A
the stretching of the polymers in the liquid along with the surface tension of the liquid. In researching this, I found out that orb-weaving spiders actually use the same instability to make the sticky parts of their webs. When they
Speaker A
build the spiral capture threads that are meant to catch the insects, they lay down silk fibers coated in a continuous layer of watery glue. As the spider begins to lay its silk, almost immediately the Rayleigh-Plateau instability causes the cylinder of
Speaker A
liquid to break itself up into regularly spaced rows of tiny droplets along the silk. So instead of the spider somehow having to place thousands of microscopic droplets of glue one at a time, it can basically just let surface tension do
Speaker A
all the work of organizing that liquid into beads. And what's cool is this phenomenon isn't only seen with spiders.
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Recently, scientists have discovered that something very similar can happen in Bose-Einstein condensates. So, these form when certain atoms are cooled to incredibly low temperatures near absolute zero. At these temperatures, the atoms can stop behaving like completely independent particles and
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instead collectively occupy the same quantum state. Now, these ultra-cold atoms can act like a quantum liquid. And just like real liquid, scientists recently confirmed that these quantum liquids can form long thin filaments that undergo the same Rayleigh-Plateau
Speaker A
instability that causes an ordinary system to break up into tiny droplets. So tiny variations in the thickness of the quantum filament grow and cause the narrow regions to get even narrower and the wider regions to get wider until the
Speaker A
filament breaks into regularly spaced rows of beads. Now, this is a super weird state of matter that can have both a repeating solid-like structure and superfluid properties. So, from saliva to spider webs to quantum liquids, it's amazing when we find these overarching
Speaker A
principles that stay constant. And thanks for watching another episode of the Action Lab, and we'll see you next time.
Topics:salivaquantum physicsRayleigh-Plateau instabilityviscoelastic polymerspolyethylene glycolBose-Einstein condensatesurface tensionspider websquantum liquidsAction Lab
