Bang But No Splash

BishopBerkeley writes "When a drop of ethanol is dropped on a surface at low pressures (1/5 atmosphere or less), it makes no splash. Science offers a brief synopsis and fascinating pictures of the phenomenon. The results seem to confirm the (perhaps counterintuitive) prediction that more viscous liquids are more likely to splash, not less likely . Links to the researchers' home page at U of Chicago (as of now, the site is timing out) and pdf version of the article on arxiv can be found on the Science page also."

Comments

We know quarks, but not this...

[Psychic Burrito]

Isn't it amazing that we're investigating quarks but haven't yet fully understood the properties of athmosphere and vacuum? We could have found those phenomena 400 years ago, but no...

Makes one wonder what else the laws of physics are hiding from us yet... and whether we have really tried to analyse physics systematically enough.

Re:We know quarks, but not this...

[efatapo]

This doesn't seem that counter-intuitive though...High viscosity liquids have a greater molecular attraction to one another than low viscosity liquids. They would therefore show a resistance to spreading out on the glass. This would give them more solid-like properties and therefore would be more like a ball hitting a wall, where energy is transfered in a rebound. The lower viscosity liquids would not be held tightly together and would therefore spread out easier.

To test this it seems like you could perform the experiment at higher temperatures. The hypothesis would be that the higher temps overcome the molecular interactions and decrease the viscosity.

I just looked at the pictures and am a biochemist so take this analysis, like everything else on /., with a grain of salt. But this seems to make sense to me.

---
Daniel Coughlin's Photographs

Re:We know quarks, but not this...

[Hognoxious]

Isn't it amazing that we're investigating quarks but haven't yet fully understood the properties of athmosphere and vacuum? We could have found those phenomena 400 years ago, but no...

I'm not sure this is new. A housemate (who worked in a dairy) told me many years ago that milk is transported in vaccuum tankers to avoid it arriving as butter.

Re:We know quarks, but not this...

[fanblade]

I don't like the other replies to this experiment. They talk about equal distribution of pressures and such, but I think that increasing inside pressure and decreasing outside pressure should create the same effect in that regard. I also don't buy the explanation that it pops "everywhere at once." I would guess that there is always a single starting point for the break. I mean, even if we say that it breaks at TWO points at once, one break probably happens a nanosecond or so before the other. I'm betting that the "everywhere at once" analysis is actually describing a super fast ripple effect that can't be observed in real time.

The pressure differential between the inside of the balloon and the outside is probably identical in both cases when the balloon pops. So the net force acting on the surface of the ballon at the moment of the pop should be the same. The only difference is the absolute pressure. I think this is the key.

The pressure in the jar is so low that when the balloon breaks, there is no force pushing inward on the ballon. The net force is basically equal to the force pushing outward. At regular atmospheric pressure there is more force on the inside but also resistance from the outside. I think the outside pressure affects the rubber after the instant of the pop by slowing it down. Maybe this keeps the rubber stable enough to only break into a few pieces.

As Dave Barry pointed out....

[MemeRot]

We invented nuclear bombs before we invented intermittent wipers for cars. Progress is never a smooth line.

Re:Air pressure is critical

[Idarubicin]

It's rather intuitive in a way. All of the momentum is downward, then converted to radially outward. What makes it go up?

How about a partially elastic collision with the surface (it bounces)?

How about collision with the leading edge of the spreading droplet (there is drag on the spreading drop as it extends across the surface--fast liquid building up behind could still splash over that barrier, even in the complete absence of atmosphere)?

Always be afraid of "intuitive" reasoning in physics when you're dealing with very slow or very fast processes that operate on very small or very large scales. :)

Re:Real world..

[jmichaelg]

We don't know ahead of time what information will turn out to be useful and what will turn out to be arcane so we just gather what knowledge what we can and plod along. It's a strategy that's worked quite well so far.

Some examples..Transistors arose from some guys shooting the breeze 20-30 years earlier as to how electrons moved around. What they were saying made no sense at all but it paid off big time. A guy sitting in a patent office speculates that light is comprised of particles and uses it to explain why electrons stream out of certain metals. Same guy speculates about what it's like to sit on a photon as it screams along and draws a few conclusions that 35 years later, rock the world. Another guy grows 1000s of peas, counts, by hand, how many of eight different traits show up in subsequent generations and figures out that wrinkled peas require wrinkled parents. Thirty years later, some other guys pick up on that idea and study fruit flys and come up with an arithmetic argument based on percentages that some traits are based on discrete loci. Weird stuff in 1911 that blossomed into billion dollar corporations 70 years later. A pair of mathematically gifted brothers figure out some equations about how fluids move over surfaces. That knowledge sits around for more than a 100 years before a different pair of brothers in a bike shop put the knowledge to an interesting use.

You just never know what's worth knowing so we gather what we can.