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July 5, 2019

How soap bubbles freeze

From the New York Times:

You may have seen the viral videos of photographers freezing soap bubbles during a recent Snowmageddon.

They're magical, turning into ethereal globes filled with ice crystals when the surrounding temperature is just right.

Unlike water droplets, puddles or other liquid surfaces, the thin, rounded shape of bubbles makes them poor heat conductors.

So when soap bubbles freeze, lacy crystals break off from cooler points on the surface and swirl around on currents of warmer liquid.

Recently, scientists set out to explain the transfer of heat that affects how bubbles freeze.

"We've seen the unique freezing dynamics of bubbles in nature, but we've never understood the physics behind it," said Jonathan Boreyko, who studies condensation and frost phenomena at Virginia Tech.

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Dr. Boreyko and colleagues' results, published last month in Nature Communications, make for fascinating viewing.

The study could also have applications for flash freezing food, creating tastier ice cream or even developing antifreeze materials.

How bubbles behave on ice poses many questions, Dr. Boreyko said, but "You can only study that by looking at bubbles in carefully controlled situations in the lab."

The team started by pipetting tiny soap bubbles onto an ice block that they chilled to -4°F and kept in a walk-in freezer.

Using high-speed cameras, the team then filmed the soap bubbles as they froze from the bottom up.

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They noticed that as soon as the bubble came into contact with the icy surface, water that was sandwiched inside the soap began flowing upward from the base.

As the bottom of the bubble cooled and solidified, ice crystals also formed a boundary between the frozen and unfrozen part of the bubble — a freeze front —that inched up the surface.

But within milliseconds, chunks of ice crystals started breaking off from the freeze front.

These were swept up by tiny water streams, known as Marangoni currents, until hundreds of ice crystals danced across the bubble.

After about 10 seconds, the Marangoni flow dissipated and died out as the floating ice grew and crystals became interlocked.

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At that point, the rest of the bubble froze over.

The phenomenon of the Marangoni current also explains why food dyes disperse in a bowl of milk and create a tie-dye effect when you add soap, or why wine appears to leave "tears" behind in your glass as you drink: Liquids move from areas with low surface tension to areas with high surface tension.

In a bubble that's slowly freezing, the changes in temperature change the surface tension, too.

That results in Marangoni currents that produce swirls of liquid and ice crystals, resulting in a dramatic snow globe effect.

The researchers wondered how this phenomenon affected bubble behavior at different temperatures.

Could they learn to tweak Marangoni currents to speed up or prevent liquids from freezing?

Dr. Boreyko's team placed soap bubbles on an icy stage outside their walk-in freezer.

They found that the Marangoni currents were weaker when the surroundings were at room temperature.

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The soap bubbles already had an externally imposed temperature difference.

They were cold at the point of contact with the stage and warm on top.

So the researchers didn't see any ice crystals break off from the freeze front or whirl around the bubble.

Poor conduction stopped the freeze front from climbing more than halfway up the bubble.

"Bubbles stayed in this partially frozen state for a while until the liquid dome collapsed 20 to 30 minutes later," said Christian Kingett, an undergraduate student who helped conduct the study.

Anyone can see this physics in action, Mr. Kingett said. 

You may not have a walk-in freezer at home, but as long as you've got soap, water, and a surface that's cold enough, you should be able to watch soap bubbles freeze, form snow globes, and fall apart, too.

"We hope this gets people thinking about fun home experiments they can do to see how beautiful nature can be when you have just the right conditions," Dr. Boreyko said.

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More?

Read the full paper, published last month in Nature Communications.

July 5, 2019 at 04:01 PM | Permalink | Comments (0)

Elizabeth Taylor, 23, during the filming of "Giant" in 1955

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The movie, directed by George Stevens, was shot in Marfa, Texas.

Yes, that Marfa.

Photo by Richard C. Miller.

July 5, 2019 at 02:01 PM | Permalink | Comments (0)

2,300-year-old Iron Age bark shield discovered in Leicester, England

From the Guardian:

An "astonishing and unparalleled" 2,300-year-old shield made of tree bark has been discovered in Leicestershire, the only example of its kind ever found in Europe.

Archaeologists say the discovery of the shield, made between 395 and 250 BCE, has completely overturned assumptions about weapons used in the Iron Age, sparking breathless reactions among experts of the period.

"This is an absolutely phenomenal object, one of the most marvellous, internationally important finds that I have encountered in my career," said Julia Farley, curator of British and European iron age collections at the British Museum.

"So often it is gold which grabs the headlines, but this bark shield is much rarer."

The shield was discovered in 2015 by archaeologists from the University of Leicester Archaeological Service in a site close to the River Soar.

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[Above, the shield as found, facedown on the ground at excavation]

Organic objects from the period very rarely survive, but the shield was preserved in waterlogged soil and may have been deposited in a water-filled pit, according to Matt Beamish, the lead archaeologist for the service.

Bark shields of the period were entirely unknown in the northern hemisphere, he told the Guardian, and the assumption was that the material may have been too flimsy for use in war.

However experiments to remake the weapon in alder and willow showed the 3mm-thick shield would have been tough enough for battle but incredibly light.

It was likely that, contrary to assumptions, similar weapons were widespread, Beamish said.

The shield is made from green bark that has been stiffened with internal wooden laths, described by Beamish as "like a whalebone corset of split hardwood," and surrounded by a rim of hazel, with a twisted willow boss.

"This is a lost technology. It has not been seen before as far as we are aware, but presumably it is a technique that was used in many ways for making bark items."

The malleable green wood would then tighten as it dried, giving the shield its strength and forming the rounded rectangles into a slightly "waisted" shape, like a subtle figure of eight.

That was significant, said Farley, because it was exactly the shape of the ornate Battersea shield, which was dredged from the Thames in the mid-19th century and dates from the same period.

"So it is possible this incredibly rare organic object is giving us some little hints about why we see what we see when we look at the metal objects. The Battersea shield might be pretending to be a shield like this."

Because so little organic material survives from the period, she said, "we are left with the earthworks, the shiny metal work, some of the ironwork, but we don’t really see the everyday world of these people: the wooden houses they lived in with their thatched roofs, their clothing… and so really the visual world of the iron age is lost to us. But something like this is just a little tiny window into that, which for me is fabulous and so exciting."

The shield has been donated to the British Museum where Farley said she hoped it would go on display next year.

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The Independent featured a wonderfully detailed piece about the sophisticated nature of the technology used to make the shield, along with a description of the precise location where it was discovered, here.

July 5, 2019 at 12:01 PM | Permalink | Comments (1)

Seven Blunders of the World

Nn

From Wikipedia: "The Seven Blunders of the World is a list that Mohandas Karamchand Gandhi gave to his grandson Arun Gandhi, written on a piece of paper, on their final day together, shortly before his assassination."

[via Joe Peach]

July 5, 2019 at 10:01 AM | Permalink | Comments (0)

1/24 Concorde Model

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Wrote Benedict Evans: "A 1:24 scale Concorde from 1969 (the Iran Air office on Piccadilly used to have one in the window, in original 1970s Iran Air livery, which must have been worth a small fortune on this basis...)."

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From 1stdibs:

Exceptionally rare original BOAC fibreglass Concorde model at 1:24 scale, Circa 1969.

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By renowned model makers AGM (Aeronautical & General Models).

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Made at a time when BOAC was the UK's national airline and when it was assumed Concorde would be flying in these colors.

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Concorde first went in to commercial service in 1976, however in 1974 British Overseas Airways Corporation (BOAC) was amalgamated with British European Airways (BEA) and two smaller, regional airlines to form British Airways (BA).

The supersonic aircraft therefore never flew in the BOAC livery, making this a very rare item.

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A detail that was carried over to BA was that their Concordes carried registrations of G-BOAA to G-BOAG and the first Concorde delivered to British Airways was registered G-BOAC.

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$11,079.61.

July 5, 2019 at 08:01 AM | Permalink | Comments (0)

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