OT:Filling bottles (physics ?)

Can anyone explain why occasionally, when filling a bottle with a funnel, despite the rate of pour remaining steady, you get a sort of backsplash which seems to create a barrier in the neck of the bottle, and the liquid then comes out and over the top of the bottle, spluttering.

I have tried to watch when it happens - it's *not* because the rate of flow exceeds the rate the neck can take liquid in. Even lifting the funnel slightly (so there's a gap all the way round) doesn't guarantee it won't happen.

Would this be classed as hydrostatics ?

Reply to
Jethro_uk
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Maybe the air in the bottle isn't being evacuated quickly enough by the liquid and in effect the liquid bounces back against the pressure exerted by the trapped air.

Reply to
David in Normandy

Hmmm, seems to make sense ....

Reply to
Jethro_uk

How are you assesing that rate? I'd expect not to have more than half of the cross section of the neck occupied by fluid going in, so that there's an equal cross section devoted to air coming out.

If the funnel isn't lifted, and blocks the neck of the bottle, then I'd say you could only use 50% of the cross section of the funnel spout to tip in fluid.

Some funnels have ridges on the outside of the funnel shape so that the funnel is held a little proud of the neck, so there's at least some air gap all the time. But it isn't necessarily enough for the funnel spout to be used entirely for ingoing fluid.

I think it depends on how you regard use of the funnel - if you're going to pour lots of fluid into it so that it then acts as a reservoir feeding the bottle then you have to allow for a decent enough gap. On the other hand if you're just using the funnel as a wide-necked entry for the bottle and pour stuff slowly into the funnel so it more or less immediately drains into the bottle, you might get away without the gap.

Reply to
Jeremy Nicoll - news posts

Well are you sure it does happen if you lift the funnel? I always use funnels with the groove to let the air out as the liquid goes in and have not seen this effect with hot water.

Brian

Reply to
Brian Gaff

Definitely. For a brief second, I did wonder if the liquid was forming a disc (like a soap film) under surface tension that plugged the neck.

It doesn't always happen. If you tilt funnel & bottle slightly, then you can actually pour *faster* since the liquid sticks to the side of the neck. (Not always advisable if your hands aren't too strong) It's only when you pour down the centre this happens.

Reply to
Jethro_uk

If you can (depending on what you're pouring out of), try pouring into the side of the funnel perpendicularly to the radius so the liquid spirals down to the middle. You may be able to get a "hurricane" effect so the air can come up a hole in the middle of the liquid.

Yes, or homebrewing technique. ;-)

Reply to
Adam Funk

Hot liquid? Heating the air in the bottle makes it expand rather quickly.

Reply to
Dave Liquorice

No. Air is less dense and therefore more compressible than water.

Think of the similar problem of drinking tea or coffee out of a mug from a roadside cafe, in the clip on caps the holes for drinking through and letting air in are completely different sizes, yet one can drink almost without forcibly sucking.

I'd expect a comparatively small hole to let air out to be perfectly adequate.

No again, for the same reason as before.

Then wouldn't they have just designed it to be bigger? Funnels are not new technology, and have been well tested in their design for hundreds of years. Of course, this could be a case of cheap from China, like teapots that can't pour properly, etc.

I am not clear from the OP's description what exactly is going wrong, but I suspect that somehow all possible means of air escaping from the bottle are momentarily being blocked - perhaps because there is something about the funnel that causes the liquid entering the bottle to form a curtain rather than a single stream, or something similar.

Reply to
Java Jive

Reply to
Java Jive

Nothing to do with density, all to do with the fact that air is a *gas* and may therefore be compressed. Water is a *liquid* and has little compressibility. Even at the bottom on the Marianna Trench, water is less that 1% more dense than at sea level.

Reply to
Tim Streater

Reply to
Java Jive

Nothing to do with compressibility either. Although air is compressible you need a substantial head to compress it.

One atmosphere is equivalent to a head of about 10m of water. If your funnel is 10cm high you'll be getting a compression of 1% in the air (and 2% for 20cm, etc) which isn't going to matter.

The interesting bit is that air is a lot less viscous than water.

Andy

Reply to
Andy Champ

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