Here is an image of oobleck, taken from Dounas-Frazer et al 2012. I can address one class of non-Newtonian fluids consisting of solid particles dispersed in a liquid medium, such as the cornstarch and water mixture commonly called "oobleck." In more scientific language, I am talking about concentrated colloidal suspensions of particles. This is why the chap in the white shirt could run on the oobleck, but when he stood still he gradually sank. If the apply a small force the water/starch grains move slowly and this gives time for the starch grains to slide around between each other so they will flow. Hence you can stand on the suspension for a moment. The water in the suspension now has to flow through the small pores in the starch grain "framework" and this requires a lot of force. At this point, when you try apply a large force to suspension the starch grains bump into each other and lock together to form a framework. As you increase the amount of starch the spacing between the grains decreases, until at some point the spacing between the grains becomes less than the size of a grain. In this case the spacing between the starch grains is large so the grains can flow around without hitting each other, and the suspension just behaves like water. Suppose you had a very dilute suspension i.e. Oobleck is a suspension of solid (starch) particles in water. See Are there good home experiments to get a feel for the behavior of yield-stress liquids? for a related question.Īnyhow, kleingordon has explained why the dilatant effect occurs, but let me try a slightly different approach to the explanation. However there are lots of other non-Newtonian fluids such as tomato ketchup and shampoo that behave in different ways. The fluid you describe is what we colloid scientists call "dilatant", and it is certainly non-Newtonian. there isn't a single constant viscosity coefficient. The description "non-Newtonian" just means the stress/flow rate graph is not linear i.e.
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