Bottle battle
Question: When I fill the kitchen sink to wash plastic baby bottles, the
bottles persistently float towards the stream of water, causing hot water to
splash around my kitchen as it hits the bottles. Why do the bottles do this? I鈥檇
have assumed they would drift away.
Answer: There are many phenomena affecting objects floating near a vertical
plunging jet of liquid. Their relative importance depends on the velocity and
diameter of the jet; the shape, depth and size of the water container; the
shape, size and buoyancy of the floating objects; and the surface properties of
the liquid.
For a deep container into which the water is pouring, the jet can set up a
large circulation flow within it, tending to draw floating objects across the
surface towards the jet. Conversely for a shallow container, a radial surface
flow is generated that pushes everything away from the jet. For high-speed jets
running into deep containers, air is entrained by the jet. Depending on the
amount of this air and its trajectory through the liquid this can help draw the
floating object towards the jet or push it away.
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When an object is almost submerged and close to the jet, the jet flows around
the side of the object, producing a Coanda force that pulls the object towards
the centre of the jet. The Coanda effect occurs when a fluid jet on a convex
surface (such as a water jet from a tap curving round the back of a spoon)
generates internal pressure forces that suck the jet towards the surface.
In this instance, the symmetric alignment of the object with the jet of water
tends to be unstable, so the object oscillates from side to side almost
randomly, generating the observed kitchen mess. When the jet hits the object
directly (more mess), the bulk circulation is interrupted and surface flows can
be set up that push all but the central object away from the jet.
For very buoyant objects drawn towards the jet by the bulk circulation, the
direct impact with the jet forces the object away, temporarily, only for it to
return again. The plunging jet can also generate small droplets that can impact
and drive away small floating objects. The jet itself or an object trapped and
oscillating in the jet can generate waves that propagate radially, tending to
push objects away.
Finally, a jet of clean water entering a container of dilute surfactant
(washing-up liquid) generates a surface tension gradient (known as a Marangoni
effect) which tends to push very small particles radially outwards. Provided the
sink isn鈥檛 overflowing, a clear circular patch is generated, the perimeter of
which defines the limit of the Marangoni effect.
The last three phenomena are subtle and the radial outwards motion is
unlikely to be observed in a sink full of baby bottles. Bulk circulation, Coanda
effect and jet impacts are the likely causes. This could be avoided by using
another dishwasher (human or mechanical), but the science would be different and
more difficult to observe.
Gary Oddie
Senior Research 杏吧原创
Schlumberger Cambridge Research
This week鈥檚 questions
Dem bones: Fluffy, my pet guinea pig, died recently. We buried him in a
cardboard shoebox about 75 centimetres down. I am driving my mom mad by asking
if Fluffy is just bones yet. Is he? And if he isn鈥檛 when will he become just
bones? And if he doesn鈥檛, what will happen to him鈥擠imitri Maxwell
Fluffy was buried 4 weeks ago in the San Francisco Bay Area, which has a
temperate climate. I am constantly being asked by Dimitri 鈥渋s Fluffy bones now鈥�.
I would appreciate some detailed information on decomposition and the time
involved in order to help me answer the question. The topic may sound a bit
gruesome for an 8-year old but my son thrives on science!鈥擬om
Dimitri Maxwell (aged 8) and Kathleen Wentworth
Burlingame, California
Tank trap: On 22 July we posed a question about a curious optical illusion,
but unfortunately we printed one of the pictures the wrong way round. The two
photographs below, and the left-hand photograph given earlier, show the illusion
created by an ancient 鈥減uzzle pond鈥� seen in a temple near Canton, China.
The water tank is about four metres long and just under a metre across. If
you stand at the right-hand end, the water appears to be about 1 metre deep,
sloping up to the left end where it appears to be about 10 centimetres deep. If
you move to the left end the appearance is reversed. If you stand in the middle,
the bottom appears to be U shaped, with the deepest point exactly in front of
you, sloping away to become shallow at both ends. What is the real shape of the
bottom of the tank that it can create this illusion? The sign above the pond
says: 鈥淚f you walk around the pond and look you will discover the depth of the
pond changes all the time in an unpredictable way鈥�.