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What common household liquids / foodstuffs show Herschel-Bulkley fluid behavior? I assume tomato soup to be one, since when sloshing it around lightly on a spoon the visible surface moves as one (as can be seen by following the dots of black pepper that don't change relative position) - is this observation basically correct?

What are (other) easy and sure ways to identify yield-stress behavior (vs. other non-Newton flows)?

How can common states of flow (through a pipe/ open channel, discharge from a small source into a large standing volume of the fluid, mixing) be observed - not necessarily quantitatively?

Are there easy to produce transparent yield stress fluids?

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A rheologist friend of mine claims there is no such thing as a yield stress. By this he means that the viscosity of the fluid never becomes infinite (or even effectively infinite), it just gets very high so you have to wait a long time to see any flow in response to low stresses.

Leaving this aside, there are lots of household liquids that show non-Newtonian behaviour. My favourite is cornflour mixed into a paste with water. This is a dilatant fluid i.e. it flows like a liquid at low stress, but at high stresses it becomes effectively solid. Experience suggests this is an excellent way of entertaining nephews and nieces for hours (though they will spill the paste everywhere so be prepared for the cleanup operation!).

To make a clear "yield stress" (pace my rheologist friend) fluid take ordinary shampoo and add calcium chloride to it. Table salt will also do but you need more of it. The electrolyte causes the sodium lauryl ether sulphate molecules to organise into macroscopic phases like cubic liquid crystals.

Re your specific questions: I don't think tomato soup is strongly non-Newtonian unless it's thick, though tomato ketchup is strongly shear thinning and also slightly thixotropic.

A good qualitative way to look at shear thinning behaviour is to watch bubbles in the fluid. Typically shear thinning liquids can suspend bubbles for prolonged periods, especially if the bubbles are small.

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thanks so far, I was looking into fluids where the memory/influence of stress over time is small/nonexistant (non thixotropic in the strictest sense). is the shampoo - salt mx thixotropic, or yield-stress? –  mart Mar 26 '12 at 15:28
    
Presumably there must be some relaxation time associated with (re)formation of the liquid crystal, but in practice this seems to be short and the shampoo-salt mixture is not noticably thixotropic. NB you ask "is the shampoo - salt mx thixotropic, or yield-stress" as they were mutually exclusive, but of course they aren't, and most thixotropic systems are shear thinning as well. –  John Rennie Mar 26 '12 at 16:19

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