Jan 21, 2012
Viscous fluid on a moving belt
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.
Okay…so I was right in the first place! Thanks for correcting my correction.
The shape of the nozzle does not have much effect because the fluid is pulled into a circular cross section pretty soon after exiting the nozzle. It “forgets” most of the details on the inlet conditions by the time it gets to the belt.
The fluid in the experiment in NEWTONIAN. The coiling effect does not require any viscoelastic on non-Newtonian properties. The fluid properties are discussed in detail in the papers about this, which may be found here:
http://www.physics.utoronto.ca/nonlinear/papers_thread.html
The behavior is in quantitative agreement with theory and simulation which contain no non-newtonian effects.
Of course if the fluid WERE non-Newtonian (which is easy to arrange by using a different fluid), different things would happen.
Oh, and Happy New Year Andrew!
Glad an expert stepped up to the mark to comment. It’s fascinating stuff. Would the shape of the nozzle also affect the repeating patterns seen?