Oxford scientists explain why ketchup splashes out of a nearly empty bottle

The answer to both questions, according to Cuttle, is a resounding yes. Together with his Oxford colleague, Chris MacMinn, he conducted a series of experiments to identify the forces at play and develop a theoretical model for ketchup splashes. Among the most interesting discoveries: squeezing the bottle more slowly and doubling the diameter of the nozzle helps to avoid splashing. There is also a critical threshold where the ketchup flow suddenly changes from no splashing to splashing. A preprint article has been published on arXiv and is currently undergoing peer review.

Isaac Newton identified the properties of what he considered an "ideal liquid". One of these properties is viscosity, loosely defined as the amount of friction/resistance to flow in a given substance. Friction occurs because a flowing liquid is essentially a series of layers sliding past each other. The faster one layer slides over another, the more resistance there is, and the slower one layer slides over another, the less resistance there is.

But not all liquids behave like Newton's ideal liquid. In Newton's ideal fluid, viscosity depends largely on temperature and pressure: water will continue to flow - i.e., act like water - regardless of other forces acting on it, such as stirring or mixing. In a non-Newtonian fluid, viscosity changes in response to applied stress or shear force, thus straddling the boundary between liquid and solid behavior. Physicists like to call this a "shear force": stirring a cup of water produces a shear force, and the water shears apart. The viscosity remains unchanged. But the viscosity of non-Newtonian fluids changes when a shear force is applied.

Ketchup is a non-Newtonian fluid. Blood, yogurt, gravy, mud, pudding, and thickened pie fillings are other examples, as well as hagfish slime. They are not all identical in behavior, but none of them adhere to Newton's definition of an ideal liquid.

Enlarge / Mustard, ketchup, and mayonnaise are all examples of non-Newtonian fluids

Ketchup, for example, is made up of pulverized tomato solids suspended in a liquid, making it a "soft solid" rather than a liquid, according to Anthony Stickland of the University of Melbourne in Australia. The solids connect to create a continuous network, and one must overcome the force of this network to make the ketchup flow, usually by tapping or hitting...