Jan 21, 2012
A stream of very viscous syrup falling from a nozzle on to a moving belt. At first, the speed of the belt is enough that the thread of syrup is just pulled out straight. However, as the belt is slowed down, the thread at first bifurcates to a meandering state, producing a sine wave and then to a “figure of eight” state and finally, it shifts to a coiling motion similar to what you would observe when drizzing syrup on to your pancakes, for instance. A wonderfully visual example of a Newtonian fluid in action.
There was a little confusion over whether or not the behaviour of the fluid is Newtonian or non-Newtonian. By definition, a Newtonian fluid is one in which the ratio of stress to strain rate is linear. Its graph would pass through the origin and the constant of proportionality is the fluid’s viscosity. A non-Newtonian fluid is any fluid that doesn’t follow that rule, by contrast. Examples include many salt solutions and molten polymers, ketchup (who knew?), custard, toothpaste, starch suspensions (check out cornstarch videos), paint, blood, and shampoo.