Understanding Viscosity and Non-Newtonian Fluids Will Help Your Ketchup Game

While ketchup may not be the king on condiments in the US (that crown actually belongs to hot sauce now), this other red sauce is still pretty darn popular across the country. I mean, what else are you going to eat with your hotdog? Mayonnaise?

If you keep a bottle at home (In your fridge? Your cupboard?), there’s a good chance that it’s a plastic squeeze bottle. Super convenient, right? Especially the ones that sit with the cap down. But once you leave your kitchen, who knows what kind of package you’ll find your ketchup in (or, even worse — what brand. Gasp!)

Your favorite fast food joint probably has convenient little packets for when you’re on the run. There’s really no problem with these (you know… unless you want a reasonable amount of ketchup for your fries). But if you’re out at a restaurant, you might just encounter the worst: the glass bottle of ketchup.

Sure, it looks vintage and prevents plastic ketchup bottle farts (nobody wants ketchup bottle farts on a first date), but at least with plastic bottle farts you actually get some ketchup action.

But with glass? That’s a completely different story, my friend.

When you’re presented with a glass ketchup bottle you have a couple of different options.

You can…

A. Thwack the end of the bottle until 5 minutes later a glob bursts forth onto both your plate and the tablecloth.

B. Use a knife like your grandma does whenever she goes to the local diner and orders her steak cooked well beyond well-done.

C. Give up all hope of ever tasting that sweet, acidic tang from a nice dollop of ketchup.

D. None of the above.

The answer is D: none of the above.

All you need to solve your ketchup quandary is a rudimentary understanding of viscosity and non-Newtonian fluids.

So, what the heck is viscosity?

If you’ve ever changed your own motor oil (or have ever had to buy industrial lubricants), you probably know a thing or two about viscosity. Simply put, viscosity is the measure of a substance’s resistance to flow (under certain conditions). When talking about a fluid, such as water, oil, or in this case ketchup, viscosity corresponds to thickness.

When it comes to oil, the viscosity/thickness is advertised as weight. For motor oil, the weight will likely be defined by SAE (Society of Automotive Engineers). For industrial lubricants, it will mostly likely be defined by ISO VG (International Standards Organization Viscosity Grade). Putting that into practice, an ISO VG 32 oil is less viscous than an ISO VG 320 oil.

But what does that have to do with ketchup? Good question.

Ketchup is a very thick, viscous fluid. In fact, you can actually think of ketchup as a soft solid: a liquid with a bunch of stuff suspended in it. If you have a cup of water and a cup of ketchup and tip them over at the same time, the water is going to spill out significantly faster than the ketchup (if the latter even moves at all). It’s the ketchup’s viscosity that makes it so difficult to get out of the bottle.

Unlike water, ketchup is a non-Newtonian fluid.

Okay, now what’s a non-Newtonian fluid?

For starters, it doesn’t follow Newton’s law of viscosity.

Newton’s law of viscosity states that “the sheer stress between adjacent fluid layers is proportional to the negative value of the velocity gradient between the two layers.” When measuring a fluid’s viscosity, you are technically measuring its resistance to gradual deformation by stress (either shear stress or tensile stress).

Fluids that have a constant viscosity independent from stress are called Newtonian. Water, for example, is a Newtonian fluid. Regardless of whether you shake the cup of water, the viscosity/thickness or rate of flow doesn’t change.

Ketchup, on the other hand, is a different beast. Ketchup is a non-Newtonian fluid because, unlike water, its viscosity is dependent on shear rate. If you squeeze a plastic ketchup bottle, for example, the contents will pour out significantly faster. In fact, perhaps a little too fast — you might make a mess when it squirts out.

Ketchup explosions happen because when non-Newtonian fluids yield to pressure — squeezing, shaking, thwacking, etc. — and start to flow, the viscosity actually decreases the faster it flows. With force, a thick fluid becomes thinner.

How does this affect your ability to get ketchup?

So now you know that ketchup is non-Newtonian with a changeable viscosity. Remember when I said ketchup was a soft solid? Well, all those little suspended bits form a network that doesn’t really want to leave the bottle. Without adding shear force, ketchup is essentially a solid. And you can’t really pour a solid (especially not out of a glass bottle).

So what should you do?

Use the force.

The next time you encounter a glass bottle of ketchup, remember this: to get that shear-thinning ketchup out of the bottle, you need to apply force. But thwacking the bottom of the bottle isn’t good enough. While it will change the viscosity, you’re only actually changing the viscosity in that one spot — at the opposite end of the opening. You can hit it, and eventually get a blob of ketchup, but it’s not the most productive way.

Have you ever heard the myth that the “57” on a certain brand’s glass bottle is the sweet spot for ketchup hitting? It’s not so much a myth.

For the most productive ketchup pouring, the first step you should take is to shake it. Not only does shaking re-mix all those suspended particles and prevent too watery ketchup (eww), it also helps add that necessary force.

With a glass bottle, give it a good whack on that sweet spot and wait for that ketchup to pour.

If you’re feeling doubtful, stick to plastic bottles or packets. Or learn to live in a ketchup-less world.


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