Category Archives: Fluids and rheology

From bending flames to flying cars

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In January 1800 Thomas Young wrote to the secretary of the Royal Society, Edward Whitaker Gray, to outline his recent “experiments and inquiries respecting sound and light,” as he titled the letter.

Among the findings that Young reported was the generic deflection of fluids near a boundary. Just as a stream of air blowing over water will raise a dimple, he wrote, a candle’s flame will be drawn toward a stream of air. In both cases, and in others, proximity to a surface reduces the local pressure, leading to a net force on the stream.

The effect is not named after Young, however. That honor was bestowed by the aeronautical engineer Theodore von Kármán on his fellow engineer and near contemporary, Henri Coandă. Born in 1886 in Bucharest, Romania, Coandă started off as an artillery officer in the Romanian army. But after six years of military service, during which he was able to pursue his passion for aeronautics, he left Romania for Paris in 1909 to enroll at the École Nationale Supérieure d’Ingenieurs de Construction Aéronautique.

TwoStamps

Coandă’s first job after graduation was with Gianni Caproni’s aircraft factory in Milan, Italy. There, he designed and built an aircraft, the Coandă-1910, that made use of his namesake effect. Depicted on the two stamps above, the Coandă-1910 resembled other aircraft of the time, but with a crucial difference: Instead of spinning a propeller, its four-cylinder piston engine powered a fan that expelled air back along the fuselage. Whether the Coandă-1910 ever flew or was even capable of flight has not been established.

Coandă’s effect has had a more successful life. Its natural manifestations include the diversion of air streams over mountainous terrain and the squirting of blood from the heart’s left ventricle into the left atrium during a disorder known as mitral regurgitation. Among the effect’s practical applications are boundary-layer control systems, which blow engine exhaust over wings to boost lift at low air speeds, and HVAC diffusers, which extend the reach of cooled or heated air.

My favorite application of the Coandă effect is the Avro Canada VZ-9 Avrocar, a flying car developed in the 1950s for the US Air Force and US Army. The VZ-9 looked more like a flying saucer than a car with wings. A central, upward-facing intake drew air into a centrifugal compressor, which directed the flow into three jet engines that were arranged symmetrically around the outside of the compressor like the fireworks in a Catherine wheel. Ducts directed the exhaust down (for lift) and out (for control).

As you can tell from the video, the VZ-9 could actually fly, albeit in somewhat wobbly way. It flew at a maximum speed of 119 mph, but never lifted more than a few feet off the ground. The pitching evident in the video was never eliminated. The project was canceled in December 1961.

By contrast, the Coandă effect remains in good health. Using Google Scholar, I came across several recent papers that evoked it, including

I also found a paper posted last year to the arXiv e-print server by Teresa López-Arias of the University of Trento in Italy. Having discovered Young’s letter to Gray, she posed the question whether the Coandă effect should really be called the Young effect.

Physics and biology between the sheets

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My spam filter is set at the most Cerberean level, so I was surprised to receive an e-mail last week bearing the title “The secret of life may be between the sheets.”

The saucy subject line turned out to be the work of the National Science Foundation. The foundation’s eager press office was touting a new paper by one of its grantees, Helen Hansma of the University of California, Santa Barbara. Her paper, which will appear in the next issue of the Journal of Theoretical Biology, proposes that life began in tiny cavities that form between the sheets of mica, a shiny, flaky mineral. Mica is chemically benevolent toward the protomolecules of life, but it’s the confinement afforded by the cell-like cavities that could have boosted the chance that biomolecules, once formed, later evolved.

Hansma’s paper caught my eye because it represents a trend I’ve been following for a few years: the growing role of physics in explaining the origin of life on Earth. Our home planet formed 4.5 billion years ago; the first living things showed up about a billion years later under conditions we can no longer observe. Because life began just once, either those conditions were rare or the probability of forming life from them was low—or both.

Forming life’s most basic building blocks is not a problem. In a famous experiment conducted at the University of Chicago in 1952, Stanley Miller and Harold Urey enclosed an aqueous solution of ammonia, hydrogen, and methane in a flask and zapped it with lightning-like discharges. After a week, the mixture contained amino acids, the building blocks of proteins. Last year, Matthew Powner, Béatrice Gerland, and John Sutherland at the University of Manchester found that ribonucleotides, the building blocks of RNA and DNA, could form under what they termed “plausible prebiotic conditions.”

But there’s a long way from amino acids and nucleotides to life. For one thing, amino acids are all left-handed isomers and the sugars in nucleotides are all right-handed isomers. Something broke the left–right symmetry, although it appears that mere stirring could do the trick. And while it’s plausible that large organic molecules could form spontaneously from chiral building blocks, what’s needed is a way to make sure that the ones that can catalyze and replicate have the chance to win out over the ones that can’t. That’s where physics comes in.

Hansma’s proposal is one of several in which physical processes help chemistry become biochemistry. The first such proposal I encountered came in 2002 from Dieter Braun, then a postdoc in Albert Libchaber’s lab at Rockefeller University in New York City. Quite by accident, Braun found that convection in tight spaces can concentrate large molecules by factors of 1000. Now at the University of Munich, Braun continues to work on and test his idea. In May, he and Christoph Mast showed that convective flow within a glass capillary could sustain the unzipping and replication of DNA.

When I tell nonphysicists at parties that I work for a physics magazine, they sometimes ask, “What’s new in physics?” Having gotten good mileage from my previous answer, topological insulators, I’ll now tell civilians that life on Earth could have sprung from physical activity between the sheets.

Charles Day