The Physics Of Ketchup: All about Non-Newtonian Fluids

We all know the struggle of getting ketchup out of the bottle to enjoy on hot dogs, fries, or even hamburgers. But, most of us probably haven’t given it much thought.

What are Newtonian Fluids?

Think of it this way: water, milk, and oil pour as expected. The more force that is applied, the faster the flow. Sir Isaac Newton discovered this concept of linear fashion, and thus the term of fluids that follow this theory are called Newtonian fluids.

Non-Newtonian fluids, on the other hand, do not behave in a linear fashion. They may become thinner or thicker in viscosity, meaning their behavior changes as force is applied or over time (non-linear).

So, how does ketchup behave?

Ketchup is known as a Casson plastic, meaning it has a non-linear flow once the critical stress point is overcome. That’s why you have to hit the bottle hard to get the ketchup to flow. This force overcomes the stress point and allows the ketchup to flow very quickly.

The Future of Ketchup Bottles

For years, scientists have tried to solve the dilemma. One idea was ketchup packets, which debuted in 1968. But they never became that popular, except at fast-food chains. In 1983, plastic squeeze bottles were created. They released ketchup faster than glass bottles, but they tended to create "ketchup juice," the unappetizing, watery serum that comes out of a bottle when it's nearly empty. Sadly, squeeze bottles still didn't give you that much control over where and how much ketchup shot out.

The problem was solved in 1991 when the owner of a precision-molding shop named Paul Brown created a new silicone valve for liquids in plastic containers. This valve had right-angled slits cut into it that opened when you squeezed the bottle, allowing liquids to flow out neatly. In 2002, Heinz and Hunt have introduced this new valve when they debuted upside-down bottles. Kripa Varanasi, a researcher at the Massachusetts Institute of Technology, recently invented LiquiGlide, a product that makes a surface slippery. If ketchup manufacturers coat the insides of their bottles with LiquiGlide, the consumers will be able to get every last drop out of them.

Conclusion

I’m sure most of us haven’t given much thought to the physics in our food. Next time you’re enjoying some french fries, think about Newton and how ketchup doesn’t obey his rules.