A physics experiment shows that ambient air pressure determines whether a falling water drop splashes when it hits a surface: at full atmospheric pressure (100 kilopascals) drops splash, but as the surrounding air pressure is lowered, splashing stops. The explanation lies not in the fluid alone but in the pressure difference at the interface between the drop and the surrounding air, a factor not captured by simply knowing a fluid's viscosity and Navier-Stokes behavior. The research, discussed in connection with physicist Sidney Nagel's work, also identifies a threshold pressure for splashing that shifts systematically with the liquid's viscosity.
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The authors point to practical settings where this pressure-viscosity threshold matters, including fuel sprays, spray drying, inkjet printing, and coating processes.
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The hosts frame the finding as adding a new engineering variable, ambient pressure, that was not previously considered relevant to splash behavior.
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1:55:40how it's about how the air decides whether a drop splashes or not. This was a little bit non-intuitive to me actually. >> Can I just say he's on a heater? He's on a heater. Yeah. The the the the ju just in terms of like the questions. >> Yeah. And and they're like fundamental questions. >> The drip, the stain, and now air. And like these are >> these are these are like fundamental questions that people ask like what causes a splash and then you just move on. >> Right. This guy's like, "No, no, really. like I'm going to >> and and so here's an experiment that they do where they have drops of water splash onto let's say a petri dish or a
1:56:20table or something but in different air pressures so the ambient air pressure is different and what they notice is that at atmosphere that's 100 kilopascals at the top you get a lot of splashing those are all the drips that you're seeing like in the third column but as you lower the pressure no more splashing this is very non-intuitive to me because I would have thought from our Navier Stokes episode and from what I know from fluid mechanics that you know >> the the Navier Stokes equations tell me everything about what the fluid is going to do >> right I've know the viscosity I know the pressure around but the interface pressure right the the the difference in
1:57:00pressure between the the ambient air outside and that interface is actually what's causing this >> right you >> this is a reworking of some fundamental mental physics. >> You know exactly what my brain is going to when I see this. This is how the UAPs do their trans medium travel. >> Yeah. Ah, there it is. There. Just like I how I didn't have a thingy. You didn't have a UAP in a while. >> I haven't had a UAP in a while. I had to squeeze it in there. >> There it is. Yeah. But, uh, it's I think it I think it's pretty cool because like >> I wouldn't have thought of that. I wouldn't have thought that like pressure had something to do with splashing. >> Yeah. Yeah. >> But it turns out it does. And the authors themselves they point to
1:57:42applications of this. They show that like you know fuel sprays, spray drying, inkjet printing, washing and coating there settings where splash behavior matters. >> Yeah. >> And to say that you know putting all printing lines under a vacuum, maybe that's not the idea, but the experiment gives an engineer a variable that they previously did not account for. Right? No one was thinking about ambient pressure when it comes to splashing. And again the the paper is not just oh there's a threshold pressure right he goes into the physics and he talks about why exactly the pressure matters and there's a scaling argument there's a threshold of
1:58:22pressure for different viscosities like for a certain viscosity a certain pressure is the threshold pressure where it goes splash no splash I reduce the viscosity the pressure threshold changes right and there's a systematic relationship there we the the cookbook is being written written piece by piece and engineers hope have hopefully picked up on uh a lot of because I I again I I can immediately as has already happened and will continue to happen see the practical applications to these fundamental understandings. >> Yeah. Yeah. And um that brings us to our
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