Thursday, January 29, 2015

Three Physics Phenomena From a Winter Wonderland

On Tuesday the "storm of the century" rolled through the northeast of the United States and despite it not living up to the doom and gloom predictions, many places, including here in DC, received a decent amount of snow and ice.

While walking through my local winter wonderland, I made a few observations which unsurprisingly all come back to physics. Here are three physics phenomena from snow and ice.

1. Ice is slippery

This seems an exceedingly obvious place to start, but it turns out the fact that ice is slippery is still a bit of a mystery to physicists.

Physicists seem to agree that ice is slippery because a thin layer of liquid water on top causes everything from shoes to sleds to slide easily across the surface.

But the question is where does this water layer come from? The old answer is that the pressure from standing on the ice causes melting underneath. Even Richard Feynman thought this was the case while interviewing for his Fun To Imagine series for the BBC.

In this clip, Feynman indirectly addresses a journalist's question about magnetism by talking about the slipperiness of ice (starting at 2:10) and how tricky it is to answer a 'why' question. It's a fascinating chat, but I wouldn't have fancied being the journalist!

But pressure is not the slippery culprit after all. The change in pressure from a person's weight on the ice is not enough to actually melt the ice, particularly at temperatures well below freezing.

Instead physicists today are debating whether it is melting due to friction while sliding across the ice or an intrinsic layer of water that causes the slipperiness of ice.

When an ice skater glides across the ice, the friction of the blade against the ice will generate heat and temporarily melt a layer of ice under the blade, making it feel slippery.

An ice-skating scene from the early 1820s. Credit: public domain

This is a well-accepted argument today, but as Robert Rosenberg pointed out in a 2005 article for Physics Today, the friction-melting theory relies on motion to generate heat, which does not explain why ice can be slippery even when standing still.

Instead, he suggests ice might be an unusual solid that carries around a layer of liquid at all times, without requiring heat or pressure.

This is an old idea dating back to 1850 when physicist Michael Faraday suggested that ice might have an intrinsic layer of liquid water. He observed that two ice cubes brought in contact would freeze together — evidence that ice must have a layer of water on the surface which freezes when no longer exposed, according to Faraday.

Experimentalists are still actively looking for the cause of ice's slipperiness, as Rosenberg reviews in his article. It has proven surprisingly tricky to detect such a thin liquid layer, which some experiments have estimated at a few tens of nanometers thick.

For such a common substance, ice is still "a very mysterious solid", according to Rosenberg in a New York Times article.

2. Salt melts ice

If you live in a place that gets snow, you will have noticed the trucks spreading salt over the roads and sidewalks in order to prevent ice.

Sodium ion solvated by water molecules.
Credit: public domain
Salt, or other impurities, lowers the freezing point of water by preventing water molecules from solidifying into neat, compact ice crystals.

Imagine you are trying to stack a set of cubes together into a stable structure. This is easy as long as you only have cubes. But if you inject a few balls into the mix suddenly it becomes impossible to stack the cubes and the balls.

This is analogous to trying to pack together water molecules and the dissolved salt molecule (sodium and clorine ions) into neat ice crystals. Water that would have frozen solid at 0oC is unable to freeze as saltwater until it reaches much lower temperatures.

But at the coldest of temperatures, salt water can still freeze and roads are better served by spreading down a layer of high-friction grit or sand.

3. Snow melts on pavement first

Credit: author, Tamela Maciel
Even without a layer of salt, you may have noticed that snow disappears from pavement long before it melts from the grass or leaves. Why is this?

The pavement is in direct contact with the earth, which, unless you've had a deep freeze for a long time, is probably much warmer than the air temperature. This is because the ground takes a long time to adjust to sudden changes in temperature.

The warmer ground continuously heats the roads and sidewalks through direct transfer of heat, or thermal conduction, and causes the snow to melt.

But the grass, leaves, tree branches, and even bridges are separated from the ground by a layer of air. Air is an excellent insulator and prevents heat transfer from the ground. This keeps the grass and other elevated surfaces frozen and snowy for much longer, and creates a lovely winter wonderland long after the snow has stopped.

By Tamela Maciel, also known as "pendulum"
Top image credit: public domain


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