Underwater Explosions (Science with Alan Sailer!) - Smarter Every Day 63

Hey, it's me, Destin. Welcome back to Smarter Every Day! So today, I'm in California, and I have the great privilege of introducing the man, Alan Sailer. Hello, Alan! Sailer is, if you don't know, one of the best high-speed photographers that currently does this art.

So, I'm here in his garden, back behind his house, and we're going to basically study explosions underwater with some firecrackers. You typically do it with the high-speed photography method, right?

Yeah, it's just an instant, the instant of that, and I'm really looking forward to seeing.

Umm, time?

Yeah, exactly. It's not just one moment. This is an M320 Miro Phantom camera. What do you hope to learn here?

I just want to be impressed. So, uh, do it! 3, 2, 1, zero!

All right, pretty cool! Let's look at the video. [Music] Oh, look at that ripple! Yeah, okay, so we decided we wanted a lower exposure time and also a higher frame rate. So, what we're going to do is we're going to change the lens. 3, 2, 1, zero!

About 20 ft? Yep, man! The, uh, I wonder if that was from impact or if the bottle itself sucks in. There's cavitation appears on top if you looked at the photograph.

Yeah, yeah. All right, let's just look at the [Music] video.

All right, I have a scientific mystery that I need your help figuring out. Let's first define what an explosion is. Right, so basically, it's a rapid formation of gas, and you have it contained in some type of mechanical device. It increases pressure, and that mechanical restraint device will eventually fail and blow outwards, correct?

But here's the question: After each shot, I would go find the bottle on the ground, and the bottle would be sucked in on the top instead of blown out, like you would think. This is counterintuitive, and we have to figure out why.

My theory is that as the bottle blows up on the bottom, it begins to shoot up like a rocket, and when it does this, it starts to wrinkle like Grandma legs, if you can see it, as it pushes the rest of the bottle forward. Now, the cap has more mass than the sidewalls of the bottle, so my theory was that the bottle was pushing the cap, but the cap weighs too much, and so the bottle is outrunning the cap.

Alan, on the other hand, disagreed with me.

My position is that after detonation, there's nothing but high pressure in the bottle, but Alan referenced me back to some of his photos from his Flickr stream. He always sees bubbles after detonation.

Always! What causes bubbles to form in a liquid?

Of course, cavitation. A phase diagram is a fancy chart that uses pressure and temperature to describe what state the matter should be in. We're located about right here, which is 23° C. Now, there's only a couple of ways we can go from liquid over to vapor.

One is we can move along the right here and increase the temperature. I don't think we're heating the water up enough in order to do that. So, I will conclude that is correct. I believe we are lowering the pressure enough that we go beyond this vapor boundary and turn into vapor, and all we have to do to turn it back into a liquid is repressurize.

Okay, let's look at the cavitation immediately after detonation. We're seeing bubbles all over the bottle. That's interesting, but 150 micros later on the next frame, all the bubbles seem to be confined in one area.

Basically, the shock waves are bouncing off the sides of the cylindrical bottle on the inside, and they're bouncing back toward each other, and then they're interfering with each other and creating a low-pressure spot.

I think that's pretty cool, because you can already see the cap beginning to suck in. On the next frame, you can see what I call the wrinkly Grandma leg effect. Now, I think it's pretty interesting that the bubble is collapsing from the left to the right, which might mean that's a compression wave going towards the front of the bottle.

But what's confusing to me is the cap of the bottle is still being sucked in.

So, I convinced Alan to do a self-portrait with me, but what I didn't realize is this self-portrait would reveal more information than any of the other videos.

At the point of detonation, clearly, we have high pressure in the bottle. Then you can see the lid suck in, and then check that out! The lid shoots off!

So that implies to me that you have low pressure and then high pressure. Once the fluid is blown out of the bottom of that bottle, it begins to pull fluid out of the top of the bottle with it. So, we have an oscillation in pressure. Where have we seen something like that before?

Sometimes when I'm drinking with a straw, I like to put my finger over the end of the straw and bring the fluid up out of the cup, and then I'll release my finger and watch what the liquid does. It oscillates!

That's pretty cool, and I think that's caused by fluid momentum or the inertia of the fluid itself. So this momentum effect of fluid applies to what we're doing, too.

As the fluid is blown out of the bottom of that bottle, you can see that it starts to pull liquid out of the top of the bottle and collapse the sidewalls of the bottle. That's pretty cool, too.

So seriously, thanks for your time! It's way more fun to think this through with you. Anyway, if you want the high-speed footage, feel free to download it, draw me some pictures, and show me what you think the shock waves are doing on the video.

This guy did it; I don't think he's exactly right, but he took a good stab at it. I'm Destin, you're getting smarter every day. Have a good one!

By the way, Alan Sailer grows very good apricots. The Russians have developed a way to use cavitation to create a bubble on the front of a torpedo. This is called supercavitation. If you fly the torpedo in that bubble, you have way less drag, and you can move that torpedo way fast. [Music]