Getting the last bit of ketchup out of a plastic bottle can quickly become a pretty sticky affair. But why is the sweet sauce splashing like that? Researchers have now found that.
Every french fries connoisseur knows the phenomenon: If you squeeze a ketchup plastic bottle and there isn’t much sauce left in it, the rest splatters uncontrollably over the food – and with bad luck even onto the tablecloth and the people eating. Scientists from the established University of Oxford have now investigated exactly why this is the case. However, it wasn’t just about being able to enjoy their fries undamaged.
But also. It’s “annoying, potentially very embarrassing and can ruin clothes” when the ketchup splatters around, researcher Callum Cuttle said at a press conference earlier this week. The scientists therefore asked themselves what could be done about it. “And, more importantly, can understanding this phenomenon also help us to solve other everyday problems?”
In fact, a concrete understanding of the ketchup mess is more complex than you might think. In a series of experiments, Cuttle and his colleague Chris MacMinn not only tried to understand how this happens in the first place, but also how the effect can be predicted and ultimately prevented.
They found what they were looking for. Not only did they find that slow squeezing or a wider spout prevented splashing. But also that there is a critical threshold which, when exceeded, turns a clean output into a sparkling affair. “Squeezing just a little bit too hard can lead to a splash instead of a clean exit,” the scientists explain.
But behind the ketchup experiment is not the naive joy of experimentation of a children’s program, but real science. In their scientific essay on the results, Cuttle and MacMinn accordingly do not speak of a ketchup experiment. According to the article now available for peer review, they investigated the “complex replacement dynamics” of liquids by gases in a capillary tube.
The idea of the experiment is a very fundamental question: Why do some liquids, when gas is constantly added, begin to change from a clean flow to an uncontrolled splashing around. In order to answer that, one must first become aware of how the bottles work. By squeezing them, the pressure on the contents increases, the liquid, but also gas bubbles in the bottle increase in density and try to compensate with the outflow. The liquid is squeezed out. The main effect of the pressure is that it helps the liquid overcome its resistance against the walls of the bottle. Researchers speak of the “viscosity” of the liquid.
However, the two researchers discovered that this behavior changes when the substance is almost entirely gas. ‘As the liquid begins to flow, its viscosity decreases because there is less and less liquid to push out,’ explains Professor MacMinn in a press release accompanying the article. “At the same time, the fluidity gives the air inside more room to escape as well. Over time, that reduces thrust.” When the spray threshold is exceeded depends on several factors such as the air volume, the degree of compression and the width of the outlet. Incidentally, the particularly tight rubber seals used by some manufacturers exacerbate the problem: “They require a certain amount of pressure before flow can even occur.” As a result, splashing can no longer be prevented in some cases.
That’s how you do it right
The scientists therefore have some concrete advice for everyday users on how to prevent spraying. Pressing more slowly and carefully reduces the chance of accidentally crossing the critical threshold. Choosing a bottle with a wider spout also significantly reduces the risk. But most people know the best advice: “When you get to the end of the bottle, you should simply unscrew the cap and squeeze the rest out of the wide opening underneath,” according to the scientists. “That may sound like common sense. But it’s now also been rigorously proven mathematically.”
Sources: University of Oxford study, press release