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Condiment physics and an eighth continent

by Michael Torrice
March 13, 2017 | A version of this story appeared in Volume 95, Issue 11

The ketchup problem

Credit: Shutterstock
Pour: Ketchup in a bottle can be stubborn because it’s a yield stress fluid.
Photograph of ketchup being poured.
Credit: Shutterstock
Pour: Ketchup in a bottle can be stubborn because it’s a yield stress fluid.

We’ve all waged battle with a glass ketchup bottle at some point—fighting to get that stubborn tomato slurry to pour without splattering.

The process can be frustrating because ketchup is a yield-stress fluid, says Anthony Stickland, a chemical engineer at the University of Melbourne.

Like other yield-stress fluids, ketchup is a suspension of solid particles in liquid. “The solids are concentrated enough that the particles form an interconnected network that can withstand a load or a force,” Stickland tells Newscripts. So under small amounts of stress, the fluids behave like solids. But once that force exceeds a threshold—the yield stress—the materials start to flow like a liquid.

That’s why ketchup bottles sometimes need a firm whack to get the sauce moving.

Stickland recently outlined a three-step process to coax out the condiment. First, shake the bottle with the lid on to evenly mix the ketchup. This gets rid of any dried-out sauce or settled solids that increase the yield stress. Next, with the lid still on, invert the bottle to get the ketchup into the neck. Finally, remove the lid and slowly tilt the bottle so the weight of the condiment pushes it out.

Tilting too fast can lead to splattering because the viscosity of some yield-stress fluids decreases when forces are applied quickly, a phenomenon called sheer thinning. “If you push really hard, it becomes almost like water,” Stickland says.

So why does Stickland know so much about getting ketchup out of a bottle? His research focuses on industrial versions of the problem, such as pumping wastewater sludge or mineral tailings.

Meet Zealandia

Credit: GSA Today
Number 8: Most of Zealandia is a submerged continental shelf.
A map showing the continent of Zealandia.
Credit: GSA Today
Number 8: Most of Zealandia is a submerged continental shelf.

At your next cookout, once you’ve effortlessly extracted your ketchup, you can further impress your guests with some geological trivia: There is an eighth continent to the east of Australia, some geologists say.

It’s called Zealandia, and its 4.9 million-km2 area contains New Zealand and some French and Australian islands. But you won’t be able to see it just looking at a map or globe.

“The key reason why Zealandia has not been recognized traditionally is 94% of it is underwater,” says Vaughan M. Stagpoole of GNS Science.

Despite mostly being submerged, Zealandia has all the characteristics of the continents we know and love. Stagpoole and several colleagues lay out the case for Zealandia’s continental membership in a recent paper (GSA Today 2017, DOI: 10.1130/gsatg321a.1).

Basically, the land masses and submerged continental shelf of Zealandia are geologically distinct from the oceanic crust surrounding them. In general, continental crust is thicker and is less dense than oceanic crust.

One interesting feature of Zealandia is that a tectonic plate boundary runs through it. The northwestern portion of the continent is on the Australian plate, and the southeastern bit is on the Pacific plate. “The tectonic activity at the plate boundary is pushing New Zealand up,” Stagpoole tells Newscripts. “If it weren’t for the frequent earthquakes here, most of New Zealand would be underwater.”

Michael Torrice wrote this week’s column. Please send comments and suggestions to


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