# What Is The Speed of Surface Tension?

## Метаданные

- **Канал:** The Action Lab
- **YouTube:** https://www.youtube.com/watch?v=vWOWn93okm0

## Содержание

### [0:00](https://www.youtube.com/watch?v=vWOWn93okm0) Segment 1 (00:00 - 05:00)

In 2019, NASA scientists captured for the first time ever a realtime shock wave from a supersonic airplane. Normally images like this require sarin imaging with mirrors and carefully aligned optics, but they were able to do it using a new method called background oriented sharing where they used sunlight as a source, a telescope on the ground, and fluctuations in the desert background. They did this to study in more detail what happens in supersonic flow or flow conditions where the fluid is moving faster than the wave speed in the material. This creates some of the weirdest flow conditions where you get things like mock cones, shock waves, and flow that speeds up where it seems like it should slow down. In this video, I'm going to show you how you can build a benchtop model of supersonic flow using something called Maranggoni waves and see real shock waves form. Supersonic flow is a special condition that's actually called supercritical flow. Supercritical flow happens whenever a fluid is flowing faster than a wave can propagate through that fluid. For example, let's say you have a fluid flowing in a pipe and then suddenly there's an obstruction in the pipe. Normally, this would cause all of the fluid in the pipe to slow down because a pressure wave would quickly travel upstream and tell the fluid to slow down. But in supersonic flow, the signal can't move upstream. So the upstream fluid keeps its fast flow while the fluid downstream near the obstruction is moving slower. So there's this sudden transition point in between called a normal shock wave. This is the point where the superc critical fluid moving faster than the wave speed suddenly encounters the slower downstream fluid and the velocity drops abruptly. This shock zone actually appears upstream from where the obstruction is. So you see this sudden transformation where the velocity just suddenly drops. But this is kind of weird. How do you have fluids that are moving at two different velocities in the same diameter pipe? Well, the only way is if the density is changing. When this is the case, you get even more strange results. For example, with normal subcritical flow, when you reduce the diameter of a pipe, the fluid flows faster through that region to keep the mass flow constant. But with supercritical flow, the opposite happens. A smaller diameter pipe reduces the speed and a sudden widening of the pipe increases the flow speed. This one fact is why rocket nozzles are shaped the way that they are. The goal is to have a supersonic gas flying out the back. So you start with a subsonic gas entering here and it speeds up as the diameter decreases. Then it reaches sonic speed at the throat and when the nozzle widens, it accelerates even more. So these super critical flows are really fun to play with, but notice that I've only been showing simulations. That's because it's hard to work with and generate supersonic gas speeds flowing through a pipe. But I'm going to show you how to see the same results using a different type of supercritical flow with something called the Maranggoni effect. The Maranggoni effect happens whenever you have a difference in surface tension. For example, when you put alcohol next to water, there's a large difference in surface tension. So the higher surface tension water pulls on the surface of the lower surface tension alcohol. This creates actual flow and spreading and movement of the liquids. But just like every physical process, the effect doesn't spread infinitely fast. When surface tension changes somewhere in a liquid, that change has to propagate through the fluid. Since this speed has a limit, that means that if we flow liquid fast enough, then the surface tension information can't travel upstream. So, I'm going to try to create a superc critical flow. But instead of air and the speed of sound, I'm going to use water and the speed of surface tension. But before we do that, I want to thank the sponsor for this video, BetterHelp. BetterHelp is the world's largest therapy service, and it's 100% online. With BetterHelp, you can tap into a network of over 30,000 licensed and experienced therapists who can help you with a wide range of issues. To get started, you just answer a few questions about your needs and preferences in therapy. That way, BetterHelp can match you with the right therapist from their network. Then you can talk to your therapist however you feel comfortable, whether it's via messaging, phone, or video call. And you can message your therapist in the app or online anytime, and schedule a live session when it's convenient for you. And if your therapist isn't the right fit for you for any reason, you can switch to a new therapist at no additional charge. 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### [5:00](https://www.youtube.com/watch?v=vWOWn93okm0&t=300s) Segment 2 (05:00 - 09:00)

to use yours before they expire. So, get 10% off your first month at betterhelp. com/actionlab or just click the link in the description. And thanks to BetterHelp for sponsoring this video. Now, let's get back to our experiment. Surfactants lower the surface tension of water. And the more surfactant there is, the lower the surface tension. So let's say I make a bubble film with the soapy water. If there are any areas where the bubble is thicker, then the surfactants on the surface will spread more thinly. So the surface tension will be higher due to less surfactant on the surface. This will cause the thinner areas to pull the thicker areas toward them due to the Marangoni effect. So we can use this one fact here. So, I'm going to create a bubble film that gets thinner from top to bottom. The way I'm going to do this is feed the bubble solution from the top at a certain flow rate. This will cause the solution to flow down these two strings here. The string widens and then go straight down. So, I can connect the initial bubble film just by connecting my finger across and sliding it down to the bottom. So, now this soapy water is flowing down the strings here in this bubble film. So, it might not look like it, but there's actually fluid flowing down this bubble film. You can see it dripping off the bottom here. It starts out slow, but it speeds up as it falls. This causes the velocity to increase, but we have the same volutric flow rate of water. So, that means that the bubble film will thin as it falls. So, the curve we would expect would look like this. But let's check what we actually see. Okay, let's start it flowing here. So now we can see we have a nice bubble film here, but we can't really see the flow. So to see it better, I'm going to shine a bright point source of light. So we're actually going to be looking at the shadow of the film. So let's see if the velocity just continually increases. Okay, now this is weird. Look what we see right here near the bottom. There's a point where the flow suddenly just slows down. This is actually a normal shock wave. It's the point at which the surface tension becomes so strong that it can overcome the velocity flowing downward. So the shock occurs right where the fluid velocity exactly equals the Marangoni wave speed. You can see this with real data of the velocity profile. And if you look at the mathematics, there's a singularity right where this happens. It's when the change in velocity is infinitely steep where the two velocities are equal, it causes us to divide by zero in the equation. So, it's weird that in the mathematics our equation blows up, but in real life, there's just a bunch of turbulence and then a transition. So, somehow nature knew what to do when our equations didn't. What's really cool is now that we can create this flow, we can even recreate a rocket nozzle. I'm going to turn the camera sideways so we can see this full length. And if I pinch it together to make a rocket nozzle shape, you can see that at the inlet, the fluid is subcritical and slower. But then, as it diverges, it goes super critical. So there's a shock right at this pinch point here. Then at the outlet, there's another shock when it hits the downstream flow. Again, you can also see that once the transition happens, there's a lot of turbulence. This shows us that in shock waves, there's a lot of energy loss due to turbulence. The organized motion of the flow gets scrambled into chaotic motion and heat. But how is there this fast fluid and then suddenly slow fluid at the same length of pipe? Well, with air, when this happens, we know that the density changes, but in this case, the thickness of the film is what's changing. What's cool is that I can actually show how the thickness is increasing by using the interference of monochromatic light. This low pressure sodium vapor lamp only outputs one wavelength of light. So, we can see interference bands on the film that depends on the thickness. Areas where the fringes change rapidly correspond to regions where the thickness is changing. So at the top the fringes show a relatively thicker region. Then as we go down it becomes thinner but then suddenly it becomes thicker again right at the shock. So above the shock zone this is technically superc critical flow. That means I should be able to see the same type of wave cone that appears in supersonic flow. And sure enough if I stick a pin into the film and create a disturbance then I see a cone that spreads out at a defined angle. I found a research paper where they did the same method and were even able to produce beautiful shock structures in the film. Remember that shock waves appear whenever information can't travel fast enough through a flowing system. And sometimes even something as simple as a soap bubble can reveal the same deep physics that governs the most extreme objects in the universe. And thanks for watching another episode of the Action Lab. I hope you learned something. If you haven't subscribed yet, remember to hit that subscribe button and we'll see you next time.

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*Источник: https://ekstraktznaniy.ru/video/42405*