London: Have you ever wondered why tomato ketchup seem solidly stuck to the bottom of a bottle, then squirt copiously on to your chips as soon as you give it a squeeze? Well, researchers claim to have cracked the mystery.
A team at Cornell University says it has uncovered the ‘single-particle dynamics’ that leads to so-called ‘non- Newtonian behaviour’ -- squirting -- as exhibited by ketchup, custard and other liquids.
Normal, Newtonian liquids, like water, stay the same viscosity no matter what force is applied. But non-Newtonian liquids, like tomato ketchup, change character under stress.
Tomato ketchup stays almost solid in the bottom of a plastic bottle until it is squeezed, at which point it drenches your hotdog in sweet goo. In the language of physicists, it is "shear thinning".
Custard, meanwhile, has the pouring, flowing characteristics of a fluid until you tap it with a spoon, when it turns near solid -- it is "shear thickening".
It was earlier believed that the peculiar non-Newtonian behaviour of some liquids had been caused by particles moving in layers, like cars moving on a motorway. Now, the team, led by Xiang Cheng, has successfully performed experiments which seem to disprove this ‘lanes of traffic’ thesis.
Using a confocal microscope and a viscosity meter, the researchers watched the flow of silica spheres suspended in a mixture of water and glycerin as it transitioned from thinning to Newtonian and, finally thickening behaviour.

From their observations, the Cornell team concluded the amount of layering does not change the liquids enough to be the cause of so-called non-Newtonian behaviour.
Instead, they now believe, shear thinning happens when the stress overrides the thermally induced ‘Brownian motion’ that would otherwise make particles randomly disperses.
Conversely, they concluded, shear thickening happens when the particles move too quickly past each other for the fluid to get out of the way, causing particles to lock together and form clumps making the fluid more viscous.
Cheng believes his results could be important in developing industrial machinery dealing with non-Newtonian fluids. "We've to understand the shear thinning or thickening properties of these fluids to better control the flow rate”
"Imagine if a fluid shear thickens -- it may suddenly clog (a machine) when the flow rate is increased above certain threshold. This may be a disaster," he said.