27 August 2013

Why you can't swim in molasses

Most readers here are probably at least vaguely aware of Boston's Great Molasses Flood of 1919.  This week I found an article at Scientific American discussing the relevant physics:
A wave of molasses does not behave like a wave of water. Molasses is a non-Newtonian fluid, which means that its viscosity depends on the forces applied to it, as measured by shear rate. Consider non-Newtonian fluids such as toothpaste, ketchup and whipped cream. In a stationary bottle, these fluids are thick and goopy and do not shift much if you tilt the container this way and that. When you squeeze or smack the bottle, however, applying stress and increasing the shear rate, the fluids suddenly flow. Because of this physical property, a wave of molasses is even more devastating than a typical tsunami. In 1919 the dense wall of syrup surging from its collapsed tank initially moved fast enough to sweep people up and demolish buildings, only to settle into a more gelatinous state that kept people trapped...

At least two researchers have directly investigated how people swim in a low Reynolds number environment. Their 2004 study is candidly titled "Will Humans Swim Faster or Slower in Syrup?" Brian Gettelfinger and Edward Cussler, both engineers at the University of Minnesota, asked 16 volunteers—including a few people training for the Olympics—to swim 25 yards (22.5 meters) in a swimming pool filled with plain water and in one filled with water and guar gum...

Depending on the way it is made, molasses is between 5,000 to 10,000 times more viscous than water. The Reynolds number for an adult man in water is around one million; the Reynolds number for the same man in molasses is about 130. To make matters worse, a man immersed in molasses will not get anywhere with the kinds of symmetric swimming strokes that would propel him in water. Each repetitive stroke would only undo what was done before. Pulling his arm towards himself would move molasses away from his head, but reaching up to repeat the stroke would push the molasses back where it was before. He would stay in place, like a gnat trapped in tree sap. Even burly men struggled to tread molasses in the wake of the Boston Molasses Disaster... 
The article goes on to address the physics of bacterial propulsion in a variety of fluids.


  1. When I was younger, I would swim in the pool without using my arms, simply undulating my body somewhat like a whale or dolphin. It required complete submersion to work correctly, but would that be an effective means of escape should I find myself trapped in molasses?

  2. Another Myth Busters experiment dealt with this.

  3. For the same reason, an animal as massive as a whale would not be able to swim in fresh water. It would sink.
    A cruder way of explaining the Reynolds' number is: the larger you are, the more a change in the viscosity of a fluid will affect your movement through it.

  4. "you can't swim in molasses"

    You also can't roller skate in a buffalo herd.



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