“Slow as molasses in January” is an old saying you won’t hear much in Australia.
Not because we don’t have a lot of use for sticky syrup in its raw form; more because the phrase refers to the middle of winter in the US.
Yet at that time of year when you’d expect molasses to be at its thickest, a giant, 7-metre wave of it swept down the streets of Boston, Massachusetts.
On January 15, 1919, 21 people died horrible, suffocating deaths as the molasses flattened buildings and coated several blocks, making a roaring sound as it moved at speeds of up to 56km/h.
It happened around midday when a 15-metre tall tank holding nearly nine million litres of molasses, destined to be turned into booze, erupted.
Those unfortunate enough to be caught in its wake realised too late that the more they struggled, the more they sank into the sticky mire. Another 150 people were injured.
Until now, nobody could understand why the molasses moved so fast. At the time, the owners of the tank tried to blame the disaster on a bomb planted by anarchists.
Nicole Sharp, a former aerospace engineer who now runs a blog on fluid dynamics, was determined to find out. Earlier this week, she presented a paper at the annual meeting of the American Physical Society Division of Fluid Dynamics in Portland, Oregon.
Working with Jordan Kennedy at Harvard University, Sharp explored as many historical maps, news and weather reports as she could find, and combined them with what we now know about fluid dynamics to come up with some initial results.
For starters, the pair say gravity currents came into play – the forces which help a dense fluid spread into a less dense fluid. In this case, molasses into air.
Here’s an example of gravity currents pushing dense salt water through fresh water:
They also know that on that particular day, the molasses in the tank was slightly warmer than the air outside, which was about 5C at the time the tank exploded.
Warmer molasses had been pumped into the bottom of the tank just before the eruption. Sharp wants to further explore how that might have affected the molasses’ viscosity.
And the possible reason the tank exploded was the outside temperature had risen from a low of -16C, which may have caused the molasses to ferment and release CO2 beyond the level at which the poorly constructed tank could contain it.
Because the molasses escaped into colder air outside, that cooling effect made it even more sticky and deadly as it rolled through the streets.
Sharp plans to release a video soon on her YouTube channel to show how it all might have played out.
You can watch some of her preliminary work here: