The Ketchup Explosion

Had any of your snacks got spoiled by your sibling, who has put extra condiment on your dish? If yes, ever wondered who is the culprit? Most of the times we can obviously blame our siblings for every bad situation, but this time the ketchup is at fault.

Fluids, we come across in everyday life, like water, flows out easily from the bottle.

Then why does we have to thwack the Ketchup bottle (glass one) the bottom, for 5 minutes? It is super convenient if you have a plastic squeeze bottle (especially the ones that sit with the cap down) or packets. 

Why does ketchup behave so weird?

Good question. Let's delve deeper into this. 

To understand this behaviour, rudimentary understanding of viscosity might help.

What is Viscosity?

Viscosity is basically a fluid's internal resistance to flow. Think of water and honey, if we start pouring water and honey from a point, which will flow faster? Water ofcourse, why is it so? Because water has less viscosity than honey i.e. fluids with less viscosity move faster and more viscous fluids move slower. 

Most fluids like water, most oils, alcohol respond to force linearly i.e. if you push them twice as hard, they will move twice as fast. This behaviour was first observed by Sir Issac Newton, hence such fluids are known as Newtonian fluids. In the case of Newtonian fluids, the viscosity remains constant as the force applied changes. Unlike water, Ketchup is a Non-Newtonian fluid or soft solid i.e., the viscosity is not constant with the applied force. Toothpaste, chocolate syrups, ooblek, whipping cream are some other examples of Non Newtonian fluids. 

Why does Ketchup Explosion happen?

Ketchup is actually a suspension of particles of pulverized tomatoes in liquid. The tomato solids, touching each other, create a continuous network which gives physical strength that resists motion. When we apply shear stress (force) on the bottle of ketchup, the thicker fluid becomes thinner, as the force applied increases, the viscosity decreases and hence the ketchup can be easily released from the glassy prison. Hence, a tap on the bottle is necessary for its flow. 

How can we get the desired amount of ketchup without creating a mess?

• Shake: Shake the bottle with the lid on. A strong shake helps to re-homoginize suspended particles and also helps add the force required for its flow.

• Invest more time: There is another way also to get the shear thinning ketchup out of the bottle, by applying series of slow languid shakes for a long time. It will eventually come out.

• Hit that sweet spot: On the glass bottles of certain brand, Heinz, there is written a number '57' which is the sweet spot for ketchup hitting. Give a good whack at the sweet spot and then wait for the ketchup to flow.

Now without dousing your favourite snacks, you can beautifully pour ketchup and enjoy.  If doubtful, hop on to plastic bottles or packets.