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Salt in your beer; A most streamlined shape 

Q: Why does beer fizzle when you add salt?? Lety, Cicero, Illinois

Letting the foam settle. Photo courtesy of John White and Wikipedia

A: Sprinkle a goodly amount of salt on your beer, and watch beer bubbles fizzle.  Why?  Beer contains dissolved carbon dioxide gas (the carbonated part of the drink).  The dissolved gas forms bubbles around the salt grains, thereby releasing gas.  The bubbles fizz up.

Moreover, it isn't the salt chemical that causes the carbon dioxide to come out of solution.  It's bubbles.  A salt grain is pitted, cracked and full of little nicks that trap microscopic bubbles.  The dissolved carbon dioxide gas gloms onto a salt grain's bubbles and enlarges the pre-existing bubbles.  That's easier than creating a bubble from scratch.  Why?  Craig Bohren explains in Clouds in a glass of beer:

"A bubble consists of gas surrounded by liquid, the two phases separated by a definite surface.  It takes energy to form surfaces.  For example, when you break a piece of chalk, two new surfaces are created.  This takes energy, not much, but still a finite amount.  The chalk could break spontaneously [like the dissolved carbon dioxide could make a bubble spontaneously] but this is highly unlikely.  We would have to wait a long time for this to happen, longer than the age of the universe."

By the way, if I drop a raisin in a glass of soda pop, like Mountain Dew (the sugary, non-diet kind), bubbles will form on the raisin (much like on the salt crystals), and buoy it to the top.  Moreover, as bubbles on the raisin at the top escape to the atmosphere, the raisin will often fall back to the bottom of the glass, only to rise again, as more bubbles form on the raisin.  It is fun watching a raisin rise and fall repeatedly this way. 

Further Reading:

Clouds in a glass of beer by Craig Bohren

Catch a burp from soft drink by Anni Matsick, Highlights for kids

Why salt causes carbonated drinks to release carbon dioxide MadSci network

Q: What is the most aerodynamic shape? I've heard it is the shape of a falling raindrop but wonder why jet fighters have pointed noses.  Garrett, Surrey, Canada

A:  You said the "shape of a falling raindrop", but I bet you meant a teardrop shape.  Small raindrops fall like tiny spheres, not teardrops, and a sphere is a poor aerodynamic shape for objects moving at aircraft speeds.  See figure.  In fact, a sphere disrupts air flow, and has about ten times the drag resistance of a teardrop-shaped airfoil.

The top figure shows streamlined flow over an airfoil.  The bottom figure shows non-streamlined flow over a sphere, where air manages to flow around the sphere on the leading edge but then separates and swirls on the trailing edge.   Top image courtesy of Rod Nave and Georgia State University; bottom image courtesy of Harry A. Dwyer and the University of California at Davis.

The best aerodynamic shape for subsonic aircraft flight is a teardrop, because that shape interferes least with the surrounding air stream.  The Eclipse 500 (a light business jet), for example, flies at about 64% of the speed of sound (Mach 0.64), and slips through the air ocean with teardrop-shaped wings and fuselage

Streamlined beauties:  the supersonic F-104 (top image) and the subsonic Eclipse 500. Photos courtesy of NASA (top image) and Eclipse Aviation (bottom), used with permission.

When an aircraft goes supersonic, however, pressure waves build up in front of the craft, and form a shock wave, which creates a bow wave (much like that formed by a speeding boat).  So, aircraft designers narrow the fighter's nose to a point to push air to the sides and beneath the wings — much as the pointed bow of a ship pushes water to the sides. 

"A relatively blunt nose will generate a 'normal' shock [perpendicular to the flow direction], thus inducing higher drag," emails mechanical engineer Chiang Shih of Florida State University.  "A pointed nose can alleviate the impact by generating an 'oblique' [bow-shaped] shock. Furthermore, it's important to steer the shock away from the wing since shock can create massive flow separation on the wing and severely reduce the lift and increase the drag."

Thus, fighters have pointed noses to fly better at supersonic speeds.

The F-104 (the first fighter to achieve sustained Mach 2 flight) is a good example of supersonic design.  By the way, the long needle in front of its sharply pointed nose is a Pitot tube probing the relatively undisturbed air to measure pressure and, thereby, airspeed.   

At very low speeds, such as blood flow in our arteries, the aerodynamic/fluid-dynamic picture changes again, and we're back to the raindrop spherical shape.

"The minimum aerodynamic drag for very low speed flow is the sphere," says mechanical engineer Harry A. Dwyer of the University of California at Davis, "since it has the minimum area for a given volume."

Further Reading:

Airfoil by Rod Nave, HyperPhysics

Aerodynamics and pressure drag by Ray Preston

80,000 ft. SKY HIGH: My climb to the top in the F-104 by George J. Marrett

How a wing stalls, NASA

Pitot tubes, NASA

(Answered Jan. 29, 2007)

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