Molasses creates a sticky situation

On Jan. 15, 1919, shortly after 12:40 p.m. local time, a giant storage tank collapsed in Boston's crowded North End, releasing more than 8.7 million liters of molasses. The initial wave was 7.6 meters tall, almost as tall as a goalpost in American football, moving more than 15 meters per second, about 11 miles per hour. It only took moments for the molasses to engulf the Commercial Street area, flattening buildings, damaging the elevated train, killing 21 people, and injuring 150 more. This research called for a unique approach, combining conventional lab-based research with exhaustive research into historical records.

"To gather relevant details about the flood and its aftermath, I've read hundreds of pages of historical accounts and contemporary newspaper articles, studied century-old maps of buildings in the area, and even called the National Weather Service to request historic meteorological data," said Nicole Sharp, a science communicator and aerospace engineer.

Harvard graduate student Jordan Kennedy performed a rheological survey, studying the flow properties, of blackstrap molasses to explore its viscosity and how it is affected by temperature. They conducted experiments on cold, spreading molasses, comparing results with models that include gravity from the literature. "The goal is to take our knowledge and understanding of highly viscous spreading flows and apply that to the Boston Molasses Flood. Ultimately, we want to use the Molasses Flood as a vehicle for fluid dynamics education and outreach and use it to engage students and the public with physics," said Sharp.

As a non-Newtonian fluid, the relationship between molasses viscosity and its rate of deformation is not constant, as it is for most familiar fluids. More specifically, molasses is shear-thinning which means that deforming it at a faster rate, i.e. flowing faster, reduces its viscosity, thereby allowing it to flow faster.

However, at the temperatures relevant to the Boston Molasses Flood, this effect is extremely small. It turns out that temperature has a much greater effect. Cooling molasses from 10 to 0 degrees Celsius increases its viscosity by a factor of 3, and as the molasses cools further, the increase in viscosity becomes more extreme.

At the time of the collapse, molasses stored in the tank was likely warmer than the surrounding air by about 5 degrees Celsius. A fresh shipment of molasses had arrived from the Caribbean to top off the tank only two days prior, and it hadn't fully cooled to Boston winter temperatures. Once the tank split and the molasses spilled over the waterfront, though, it would have cooled more rapidly, especially in the dropping temperatures after the sunset. The viscosity of the molasses would have then increased dramatically with this cooling, complicating attempts to rescue victims and to begin recovery and cleanup.

The physics of the Molasses Flood are relevant to other accidents that affect the public, including industrial spills or breaking levees, but the ultimate goal of this work is educational. "Once I delved into the history of the Boston Molasses Flood, I was surprised by how rich a subject it is, especially for engineering education," said Sharp. It offers the opportunity to address fluid dynamics, structural mechanics, engineering ethics, history and law all in one topic. "We hope that, by shedding some light on the physics of a fascinating and surreal historical event, we can inspire a greater appreciation for fluid dynamics among our students and the public."