The Science Behind Why Ketchup Is so Hard to Pour
If you are a ketchup lover, you have no doubt had to face a very real struggle trying to get it out of a bottle, especially if the bottle is glass. If you’ve ever wondered why you have to suffer from this persistent condiment, the answer is: ketchup is a non-Newtonian liquid.
Click the link below to see a brilliantly illustrated description of the physics behind this, but it boils down to the fact that ketchup doesn’t behave like “normal” liquids. Most fluids, such as water or olive oil, have a constant viscosity, which is a property that describes the resistance of a fluid to flow. These fluids are called “Newtonian fluids” and they come out nicely out of bottles. Ketchup and other non-Newtonian liquids do not work this way:
On the other hand, non-Newtonian fluids have different viscosities depending on external forces. Their ability to flow depends on the shear rate. Some of them thicken liquids on shear – they become thicker when agitated. Others, such as ketchup, are shear-thinning liquids – when shaken (thumb on a glass bottle of ketchup) they become liquid.
This is why you need an external force like a knife or a good hard punch to get it moving, but for really hassle-free dispensing of ketchup (no watery drops) you should avoid glass bottles altogether and stick to the ‘flower petal-shaped hole’ which can ‘ squeeze out the liquid and then plug it, trapping the remaining liquid and preventing leakage. “
Ketchup Physics 101 | Cook Science