Cocktail Party Physics

Cocktail Party Physics

Physics With a Twist

From the Archives: Frost Flowers and Hot Capillary Action


Most science history buffs are familiar with William Herschel, the famed astronomer who discovered the planet Uranus in the 18th century. His son, John, is less well known, perhaps because his scientific interests ranged more broadly than his father's. He loved the stars, it's true, but he also found wonder much closer to home.

Evidence of that can be found in a January 12, 1833, letter printed in Philosophical Magazine, in which John Herschel describes going for an early morning walk several winters before and noticing "a remarkable deposition of ice around the decaying stems of vegetables." A few days later, he found a similar strange ice formation, this one seeming "to emanate in a kind of riband- or frill-shaped wavy excrescence."

Herschel's letter is one of the earliest recorded observations of the phenomenon of "frost flowers" (sometimes called ice flowers or ice ribbons), in which thin layers of ice curl out from long-stemmed plants in the wee morning hours of late autumn or early winter. Those ice layers often form intricate curling patterns, looking for all the world like flower petals. Herschel could only hypothesize about the cause of these formations, but he intuited that they correlated with specific atmospheric conditions and particular kinds of plants, although he couldn't explain why that might be the case, concluding, "It is for botanists to decide."

Well, botanists and physicists, perhaps; the demarcations between scientific disciplines weren't nearly so rigid in Herschel's day. Many others were inspired by Herschel's letter to relate their own discoveries of frost flowers. In 1850, a physician name John LeConte of the University of Georgia described his observed ice flowers thusly: "At a distance they present an appearance resembling locks of cotton-wool, varying from four to five inches in diameter, placed around the roots of plants, and when numerous the effect is striking and beautiful."

Thirty years later, the Duke of Argyll described similar ice formations in the January 22 issue of Nature, requesting a scientific explanation, prompted a lively exchange offering various possibilities.

In March 1884, Nature reported that one Professor Schwalbe, at a meeting of the Physical Society in Berlin, had succeeded in producing his own ice flowers from withered and rotten twigs he'd brought with him to the conference from the Harz Mountains. He simply moistened the twig thoroughly so that no water dropped off, then let it cool slowly in what's described rather vaguely as "a cold preparation."

Schwalbe was the only one on record to systematically grow ice ribbons/frost flowers until 1914, when a physicist at the National Bureau of Standards named William Coblentz observed frost flowers while strolling in Washington, DC's Rock Creek Park. (He's actually buried in Rock Creek Cemetery.) Coblentz was best known for his research on spectroscopy and infrared radiometry; for instance, he made measurements of the infrared radiation emitting from over 100 stars and the planets of Mars, Venus and Jupiter.

But like Herschel, Coblentz's curiosity ranged further afield: he held a patent for an early solar cell, and also dabbled in bioluminescence. So it's not surprising that when he observed his first frost flowers, he started experimenting to understand the physical mechanisms behind their formation.

He cut off stems, inserted them in moist soil and test tubes, recorded how quickly water moved up the dry stems, and figured out how to grow ice ribbons in the lab. Among other findings, he conclusively demonstrated that the roots of plants aren't necessary for frost flowers to form, and that the water that makes the ice comes from within the stem, rather than being deposited from moisture in the air.

(For those sufficiently intrigued to want more comprehensive details about frost flowers -- including more history and where and when you're most likely to spot them -- James Carter, a professor emeritus of geography and geology at Illinois State University, has an entire Website devoted to the history and science and his own personal sightings of frost flowers and ice ribbons. A Google search will turn up many more sites by nature enthusiasts.)

We know much more today about frost flowers thanks to the efforts of men like Coblentz, although they're still a little mysterious. They tend to happen in early winter, when the ground is not already frozen: the "first freeze." The ground temperature has to be warm enough so that the plants' root systems are still active, and the air temperature has to be cold enough to freeze water.

Plants hold water in their stems, and water expands when frozen, so long thin cracks can form along the stem. Water is drawn through those cracks and freezes upon contact with the air. Water continues to flow out, past that first layer, freezing and forming a second layer, and so on, until the telltale thin "frozen petal" shape emerges.

Intriguingly, this has only been observed in a few species of plants: the white crownbeard (Verbesina virginica, a.k.a., frostweed), yellow ironweed (Verbesina alterifolia) and Helianthemum canadense. If we're talking about woody plants and tree branches, the seeping water freezes into long strings of ice that look like strands of hair: "frost beard."

That's cool and all, but just what is causing the water to flow through those cracks in the stems? It's kinda flowing upward, you see, which doesn't seem like it should be possible. You'd think gravity would make it flow down. We can thank a little something called capillary action or capillary force for all those pretty floral ice arrangements. It's the same thing that causes a sponge (a porous material) to soak up liquids from a surface.

You can witness capillary action for yourself with a simple vertical glass tube open at either end. Place the lower end in a glass of water, you'll notice that the water rises up to a certain point and then stops. Surface tension basically pulls the liquid column up until the mass of the liquid is large enough so that gravity can overcome the intramolecular forces. You know when a drop of water forms on the spigot of your tap and suspends there until you touch it? Capillary forces hold it there.

Plants use this as a transport mechanism for water, nutrients, and so forth, so it's not surprising that this same capillary action also gives rise to the frost flower phenomenon. Similarly, the reason groundwater moves from wet areas of soil to dryer areas is capillary action: the water molecules are attracted to soil particles and naturally seek them out; if a patch of soil gets too wet, the water molecules will move to dryer patches where the dry soil particles are more plentiful.

Capillary action is also behind a colorful bit of superstition known as the "Hindu Milk Miracle." Just before dawn on September 21, 1995, a Hindu worshiper at a temple in New Delhi made the traditional offering of milk to a statue of Ganesha. He held up a spoonful of milk from the bowl, and was astonished when the liquid disappeared, seemingly consumed by the statue. Apparently, Ganesha had a milk craving, perhaps to supplement a calcium or Vitamin D deficiency. As abruptly as it started, it stopped: by noon, Ganesha was no longer "drinking" the milk.

When other devout Hindu people heard, they offered milk to their own statues in other temples, all over the world, and lo and behold, many of those also lapped it up. The World Hindu Council declared it a miracle, and sales of milk in areas with large Hindu communities skyrocketed. (I can see the dairy ads now: a statue of Ganesha with the telltale white mustache and the caption, "Got milk?")

Finally, scientists from India's Ministry of Science and Technology came to New Delhi and determined that the "miracle" was actually due to capillary action: the surface tension of the milk pulled the liquid up and out of the spoon before gravity caused it to run down the front of the statue. Not that true believers cared about science: hordes of people still rushed to the temples with their offerings of milk in hopes that the statue would accept their offerings.

Just last month, an Indian skeptic named Sanal Edamaruku was arrested for blasphemy in Mumbai by the order of the local Catholic Church. His crime? Explaining that a weeping cross -- touted as a modern miracle drawing hundreds of pilgrims daily to witness the water drops seeping from Jesus' feet -- was really just another example of capillary action. (The cross was located near a leaky drain.)

There's so much we couldn't do without capillary forces. In chemistry, for example, there's a common technique called thin layer chromatography, in which capillary action is exploited to move a solvent vertically up a plate, usually taking dissolved solutes with it.

Bounty paper towels -- the "quicker picker upper" -- also utilize capillary action to absorb liquid; it's porous, like a sponge, and those pores act like small capillaries, much like the tube-like stems of plants, so that fluid on a surface is transferred to the paper towel. And much of my workout gear employ "wicking fabrics," which use capillary action to "wick" sweat away from the skin, thereby avoiding undue chafing during strenuous workouts.

More importantly, our eyes wouldn't be able to drain away tear fluid efficiently without capillary action. (Our eyes produce tears constantly via the lacrymal ducts in the inner corner of the eyes.) Capillary action is rather miraculous in that respect. It's just not magic.

Images: (top) Frost flower in the Ozark Mountains. Credit: Marvin Smith, via Wikimedia Commons. (bottom) Detailed view of hair ice aka frost beard taken at Mount Maxwell, Salt Spring Island, British Columbia, Canada. Public domain, via Wikimedia Commons.


The views expressed are those of the author and are not necessarily those of Scientific American.

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