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Why does liquid poor in gulps?


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Sand, being put through an hourglass or funnel, will fall through the hole relatively consistently. Liquid, however, will not.

 

To test this, take an empty milk jug, fill it with water from your kitchen sink (you could also use milk, but kitchen sink water is expendable), and then, turn the jug upside down. It will fall into the sink in gulps, making a "BloopBloopBloopBloopBloopBloop" sound.

 

Why does liquid do this? Why does it not pour consistently like sand does?

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Sand, being put through an hourglass or funnel, will fall through the hole relatively consistently. Liquid, however, will not.

 

To test this, take an empty milk jug, fill it with water from your kitchen sink (you could also use milk, but kitchen sink water is expendable), and then, turn the jug upside down. It will fall into the sink in gulps, making a "BloopBloopBloopBloopBloopBloop" sound.

 

Why does liquid do this? Why does it not pour consistently like sand does?

 

Air pressure. Sand has gaps through which air may flow, but liquid does not. When you pour a liquid, the reservoir above it in the container has air in it. As the liquid leaves, that reservoir must expand. As it expands, the pressure drops (PV will be ~a constant). You reach a point where the air pressure outside is enough to keep the liquid from pouring. But, being a liquid, it can deform and let in a bubble if air, which raises the pressure (that's your bloop) and more liquid can pour. The cycle continues.

 

If your container is not rigid, you might observe it contracting as the pressure drops and expanding again when air is let in. Another hole in the container, or pouring so the liquid does not completely block the hole, will keep it from "blooping". Also, if the hole is small enough, surface tension will be great enough that a bubble cannot be let in, as with a straw with one end blocked.

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Even better, try it with a large soda bottle with a narrow opening. Do it right and the water won't come out at all!

 

The answer is a combination of atmospheric pressure, surface tension, viscosity, and whether there's a free surface. The water will poor smoothly if you tilt the container only slightly. That's because there's a free surface for air to flow into the bottle above the water at the same time that water is flowing out. When you fully invert the bottle there is no free surface. The air inside the bottom will be at less than atmospheric pressure after a glug of water comes out. The pressure at the opening will keep the water from coming out. A glug of air can go into the bottle, but only if the opening is wide enough so that surface tension doesn't come much into play. Now the pressure is more or less equalized, so a glug of water can come out. The cycle repeats. If you watch carefully you'll see that the glugs get bigger and bigger as the bottle empties.

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Also, if the hole is small enough, surface tension will be great enough that a bubble cannot be let in, as with a straw with one end blocked.

 

IIRC, the Greeks had some sort of ladle that worked on this principle. You dunked the bowl with small holes in it into a water source, covered the hole on the end of the long neck with your thumb and lifted out a bowl full of water.

 

I'll have to see if I can find it.

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You can use the above scientific information to pour yourself a nice glass of beer from a bottle without getting too much froth on the top.

Start by holding the glass in one hand in an almost horizontal position and the bottle in the other hand in an almost vertical position. Now slowly rotate both glass and bottle so that beer flows from one to the other. Once the glass is at about 45 degrees stop rotating it but continue to slowly rotate the bottle until it is completely empty. As the last small amount of beer comes out of the bottle it will be necessary to further rotate the glass to a vertical position to avoid wasting beer through spillage. (and we wouldn't want that!)

This takes a lot less time than glug glugging the beer and waiting for half a glass of froth to dissipate.

Hope you find this "appliance of science" useful. biggrin.gif

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If you watch carefully you'll see that the glugs get bigger and bigger as the bottle empties.

... and faster and faster, if I remember correctly.

 

(This kind of question is great. We've all experienced it, yet only the courageous and very curious really notice it and ask why.)

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Even better, try it with a large soda bottle with a narrow opening. Do it right and the water won't come out at all!

 

The answer is a combination of atmospheric pressure, surface tension, viscosity, and whether there's a free surface. The water will poor smoothly if you tilt the container only slightly. That's because there's a free surface for air to flow into the bottle above the water at the same time that water is flowing out. When you fully invert the bottle there is no free surface. The air inside the bottom will be at less than atmospheric pressure after a glug of water comes out. The pressure at the opening will keep the water from coming out. A glug of air can go into the bottle, but only if the opening is wide enough so that surface tension doesn't come much into play.

This is the principal I use for getting ketchup out of the bottle.

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Sand, being put through an hourglass or funnel, will fall through the hole relatively consistently. Liquid, however, will not.

To test this, take an empty milk jug, fill it with water from your kitchen sink (you could also use milk, but kitchen sink water is expendable), and then, turn the jug upside down. It will fall into the sink in gulps, making a "BloopBloopBloopBloopBloopBloop" sound. Why does liquid do this? Why does it not pour consistently like sand does?

 

It is not merely the difference between granular materials and liquids. Granular materials will jam as they encounter a converging nozzle, such as a bottle neck. The Miller Lite vortex bottle for beer has vanes in the neck to guide liquid flow in a vortex. Along the vortex axis, which is a low pressure gradient, air flows up and in to fill the bottle continuously as the beer flows down and out. Foaming is less, therefore chugging is maximally efficient. Even without the vanes, you can get the same result by swirling the bottle vigorously. The hurricane is a good example of axial counterflow and the low pressure sink created by the swirling descending liquid (rain).

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vortex bottle for beer has vanes in the neck

 

How about this one?

 

 

You can use the above scientific information to pour yourself a nice glass of beer

 

Or the more fun pouring it down your neck.

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It's worth remembering that ketchup is thixotropic and flows more easily after being well shaken. However you proceed give it a good shake first!

 

http://blog.enthough...rd-thixotropic/

 

 

 

 

The future of ketchup lovers will not need to worry about this problem, because only a few months back scientists at a university (my memory cannot remember which) designed a bottle which was coated with a special lubricant which will help tomato ketchup flow easily from the bottle.

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