Callum Cuttle of the University of Oxford has finally solved an old-age problem, why does ketchup splatter from a near-empty bottle? Along with his Oxford colleague, Chris MacMinn, he conducted a series of experiments to identify the forces at play and develop a theoretical model for ketchup splatter. Now we know the science, what are we going to do about it?
When there’s only a little ketchup left in the bottle, you need to whack it that much harder, thereby increasing the risk of splatter. “By the time you get to the end, much of what’s inside is air,” said Cuttle. “So when you squeeze, what you’re doing is compressing the air inside the bottle, which builds up pressure that drags the [ketchup] out.” The nozzle provides a viscous drag force that counters the viscous flow of the ketchup, and the balance between them determines the flow rate. As the bottle empties, the viscosity decreases because there is less and less ketchup to push. And the outflow of liquid means there is more and more room for the air to expand inside the bottle, decreasing the driving force over time.
“The splattering of a ketchup bottle can come down to the finest of margins: squeezing even slightly too hard will produce a splatter rather than a steady stream of liquid,” said Cuttle. One handy tip is to squeeze more slowly, thereby reducing the rate at which the air is compressed. Widening the diameter of the nozzle would help even more since the rubber valve at the spout can exacerbate the risk of splatter. Granted, the valves help avoid leads, but they also force you to build up a certain amount of pressure to get the ketchup to start flowing form the bottle. Cuttle recommends just taking the cap off the bottle when it’s nearly empty as a practical hack, squeezing the last bits of ketchup out of the broader neck.