# Talk:Shear thinning

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## Pseudoplasticity

After seeing it in an article I created pseudoplastic to redirect here. But how does pseudoplasticity differ from regular plasticity? —Ben FrantzDale 23:48, 8 August 2006 (UTC)

Power-law fluid may have more on this. —Ben FrantzDale 23:50, 8 August 2006 (UTC)

Yes it does. Actually pseudoplasticity (a property of fluids) has very few in common with plasticity (of solids) 18.80.4.155 16:05, 5 July 2007 (UTC)

## Merge Discussion

Should this article be merged with Thixotropy? They seem to be describing the same property. --DJsunkid 04:06, 9 August 2007 (UTC)

I agree. I added a merge request to the page. ---- BAxelrod 12:27, 25 October 2007 (UTC)

I was reading a couple of pages here on WikiPedia and they say contradictory things. http://en.wikipedia.org/wiki/Pseudoplastic says, "It is important to note the distinction between a shear thinning fluid and a thixotropic fluid. The former displays decreasing viscosity with increasing shear rate, while the latter displays a decrease in viscosity over time at a constant shear rate." http://en.wikipedia.org/wiki/Thixotropic says, "It is important to note the distinction between a thixotropic fluid and a shear thinning fluid. The former displays a decrease in viscosity over time at a constant shear rate, while the latter displays decreasing viscosity with increasing shear rate." Which is it? —Preceding unsigned comment added by 205.197.200.13 (talk) 06:30, 31 January 2008 (UTC)

The answer to your question ("Which is it?") is that both statements are true and are expressing the same thing, but in opposite order. That's where you became confused. But both of those terse explanations are slightly deficient, which may cause misunderstanding by other readers.--Zymatik (talk) 20:57, 26 January 2013 (UTC)

As someone with almost 40 years working on paints and adhesives, I can make some claim to know what I am talking about. (original research, if you must). Academically, thixotropy is a change (decrease) in viscosity with time. This is clear. Industrially, thixotropy is a change (decrease) in viscosity with increasing shear. The industrial usage is common and rarely admits time as a variable. In a common man's article, you must include BOTH definitions. This is also clear. In point of fact viscosity almost *always* demonstrates a hysteresis (usually logarithmic) but the time scale may be milliseconds or less and hence irrelevant in many situations. By the way, most polymer solutions are Newtonian, I think citing polymer solutions as examples of non-Newtonian fluids is, in general, wrong. (I know there are many exceptions to this generality, also). Since the literature almost never admits the very common industrial definition, citation is difficult. "Thix Index" - the ratio of two viscosities measured on a fluid with the imposed shear rate is 10x for the numerator is recognized academically, for what its worth. So, the question is, should Wikipedia side with the Academics or include the much more common (in my opinion) definition of thixotropy? I vote for inclusion rather than exclusion, especially if done through ignorance.69.40.241.198 (talk) 01:02, 20 March 2011 (UTC) For an example see this page http://www.kitpackers.com/PDFs/TT-UnderstandingRheology.pdf note the last paragraph on the page.69.40.241.198 (talk) 01:09, 20 March 2011 (UTC)

Yes, polymer solutions are not generally non-Newtonian fluids; only those that dynamically lose molecular junctions or occur along with a flocculated phase could possibly be non-Newtonian. I will edit the article Non-Newtonian fluid to reflect the categorization of flocculated suspensions as thixotropic.--Zymatik (talk) 20:57, 26 January 2013 (UTC)

It seems there is at least a little support for a merge above. Yes, there are two different attributes, but as the article currently suggests they are closely related (the rationale for believing all shear-thinning substances to also be thixotropic certainly sounds good to me), and there is a lot of ambiguity in common usage of the two terms (as the ip user above points out). It seems to me that a single article that discusses both attributes and their relationship to each other would make more sense than individual ones. 212.159.69.4 (talk) 15:25, 5 April 2012 (UTC)

Yes, it's true that all shear thinning fluids are actually time dependent, but if the root of the confusion is a lack of appreciation for the difference between instantaneous and continuous viscosity change, then the articles should not be merged, as that diminishes importance between the two distinctions. Fluids having two significantly different characteristics usually cannot be used interchangeably for the same practical applications. Perhaps what is really needed is a unique and less confusing term for a shear-thinning fluid that has an instantaneous response. How about "instant-tropic" ;-) because the slower responding thixotropic is also shear-thinning.--Zymatik (talk) 00:10, 30 January 2013 (UTC)

[from a polymer engineer] Polymers and most polymer solutions are definitely non-Newtonian. A Newtonian fluid is one which has a constant viscosity, whereas polymers and polymer solutions exhibit a large drop in viscosity, as many as several orders of magnitude, with increasing shear rate or deformation rate (shear thinning) [Osswald, Materials Science of Polymers for Engineers, Hanser Publishers, 2003]. Thixotropy is also a decrease in viscosity as a result of deformation, but the mechanisms must be fundamentally different. Polymers experience shear thinning as a result of a decrease in the number of entanglements - allowing them to flow more easily. In the molten state, the polymer recovers its viscosity very quickly, within a few seconds. Paint, on the other hand, takes minutes or hours to recover its viscosity. Also, the examples provided (ketchup, mayonnaise) are better described as Bingham fluids (http://en.wikipedia.org/wiki/Bingham_plastic), or fluids that only flow once a minimum stress is provided. So, they might not actually be thixotropic (shear thinning), but that would definitely need some research. Again, molecularly they are very different than plastics. I suppose perhaps the terms are technically the same, but have very distinct usages. See this paper for a discussion on the origin and definition of the term thixotropy. http://www.dfi.uchile.cl/~rsoto/docencia/FluidosNoNewton2008/trixotropia.pdf Tomman513 (talk) 20:34, 19 February 2013 (UTC)

As a graduate student of polymer chemistry I will add my opinion: Thixotropy is a reduction in viscosity over time at a given shear rate. To be clear: a fluid may be thixotropic at a CONSTANT shear rate. Thixotropic and shear-thinning fluids can be observed exclusively or together but they simply are NOT the same property. Shear-thinning fluids are those that show a reduction in viscosity over a change in shear rate. Thixotropic behavior does not need any CHANGE in shear rate (as long as some minimum shear stress is applied) to observe the reduction in viscosity. — Preceding unsigned comment added by 129.65.100.36 (talk) 01:52, 14 May 2013 (UTC)

Okay after thinking about it: All thixotropic fluids are shear-thinning but only some shear-thinning fluids are thixotropic. So in a way thixotropic is a subset of behaviors exhibited by shear-thinning fluids. Personally I think either merge ALL rheological fluid categories or leave these apart. — Preceding unsigned comment added by 129.65.100.36 (talk) 01:58, 14 May 2013 (UTC)

To my knowledge:

• 0. Newtonian fluids are those which obey Newton's law of fluids: the shear stress (Δp/ΔA) is proportional to the velocity gradient (Δv/Δy), the proportionality constant is the viscosity (η): Δp/ΔA = η · Δv/Δy. This is equivalent to the statement, that the viscosity of Newtonian fluids does not depend on the velocity gradient. But this law does not exclude the dependence on other factors, e.g. temperature or even time.
• 1. Pseudoplastic and (pseudo)dilatant characteristics describe fluids with decreasing or increasing viscosity as a function of velocity gradient (so in a graphic representation, the dependent variable would be viscosity, the independent variable would be the velocity gradient). So these are two examples of non-Newtonian fluids.
• 2. Thixotropic and rheopectic fluids have decreasing or increasing viscosity as a function of exposition time to shear stress (so in a graphic representation, the dependent variable would be viscosity again, but the independent variable would be [exposition] time). (Erroneously, some also consider fluids thixotropic or rheopectic if their viscosity changes over time without being exposed to shear stress.)

In my opinion, the reason of confusion between these concepts is practical: it is hard to separate the two independent variables, I mean, would it be possible to prove that a fluid has exposition time independent but velocity gradient dependent viscosity or vice versa? In other words, can we change one of the variables while keeping the other constant?

• 2. Probing the time dependence of the viscosity of a fluid while keeping the velocity gradient constant: it is possible to "stir" them at a constant velocity gradient for different time periods. If the shear stress required to maintain the constant velocity gradient decreases over time, the thixotropic characteristic is proven, if it increases the fluid is rheopectic. If it does not change the fluid is neither thixotropic nor rheopectic, i.e. the viscosity is time independent. But this does not necessary mean that the fluid is Newtonian.
• 1. Probing the velocity gradient dependence of the viscosity of a fluid while keeping time constant: the above experiment has to be repeated at different velocity gradients. If the initial (i.e. measured at or extrapolated to zero time) shear stress is proportional to the velocity gradient the fluid is Newtonian, if not it is one of the non-Newtonian types.
• What causes the confusion? Let us consider a "practical" measurement where one simply stirs a certain fluid with increasing force to reach higher and higher velocity gradient. If the viscosity decreases, it can be of two reasons: either due to the increasing velocity gradient (the fluid is pseudoplastic), due to the time of exposition (the fluid is thixotropic), or both (the fluid is both pseudoplastic and thixotropic). However, if we make the two-step experiment described above, both the time and velocity gradient dependence of viscosity can be described. Timur lenk (talk) 11:34, 17 July 2013 (UTC)

According to my chemical engineering class, pseudoplastic fluids are different than thixotropic fluids. Thixotropic fluids act psuedoplastic when first "pushed" and then act like a dilatant (sheer-thickening) fluid when that stress is released. Oppositely, Rheopectic fluids do the opposite: they act like a dilatant fluid when they are "pushed" and then act like a psuedoplastic fluid when that stress is released. Also, ketchup is a Bingham Plastic, as it needs a "push" to get it going, and that "push" has to be greater than its yield stress. For example, in the old glass bottles you would need to give the bottle a hard enough "kick" to get the ketchup flowing, and then it would flow a bit too well. If you didn't kick it enough it wouldn't flow at all (except for the water that sometimes might sit at the top). (Please forgive me if this information is not in the right format for wikipedia, as this is the first time I have contributed to an article.) — Preceding unsigned comment added by 163.118.10.94 (talk) 19:58, 3 December 2013 (UTC)

OK, another chemical engineer with experience in rheometry chiming in here. The merge proposal is completely misguided.

Part of the confusion comes from the fact that real fluids usually present a mixture of different properties, and real fluids can seldom be exactly described by a simple model such as shear-thinning, Newtonian, purely viscoelastic or or purely thixotropic behaviour, in the same way that no real piece of wire will act as a pure Hooke's law spring or exhibit pure plastic deformation. The concepts, however, are clearly defined and clearly different. I'm going to remove the merge proposal. --Slashme (talk) 19:38, 21 May 2015 (UTC)

## Ketchup

This article contradicts: where ketchup is referenced as being Thixotropic. http://en.wikipedia.org/wiki/Non-newtonian_fluid — Preceding unsigned comment added by Crypter (talkcontribs) 18:07, 7 January 2011 (UTC)

Thank-you. I just did a quick experiment on refrigerated ShopRite® ketchup. With a tiny wire wisk I briskly stirred a 2 fl oz quantity of it in a 3 fl oz capacity condiment serving cup. The result was that, immediately afterward, I could not simply pour it out of the cup. And to the contrary, peaks and valleys remained on its surface. Thus, it is proven not to be time dependent, as its shear thinning ceases as soon as I cease shearing it. So it is merely shear thinning, but not thixotropic. I will soon edit the articles to make corrections, if/where needed.--Zymatik (talk) 21:08, 26 January 2013 (UTC)

It's probably not shear thinning, although you'd need a rheometer to prove that. It's more accurately a Bingham Fluid (see my comment in the above section). — Preceding unsigned comment added by Tomman513 (talkcontribs) 18:22, 18 February 2013 (UTC)

It is a matter of time scales. When waiting long enough ketchup will most likely flow but it might have gone bad by that time. It is supposed to have a yield stress of approximately 15 Pa (eg http://www.brookfieldengineering.com/education/rheology_papers_yield_stress.asp). — Preceding unsigned comment added by 78.148.109.142 (talk) 23:37, 7 March 2013 (UTC)

## Second / Third sentence: "This property"

What does "This property" refer to in the third sentence, "This property is found in certain complex solutions ..." of the opening paragraph?

Does it refer to shear thinning , pseudoplastic or thixotropic? From the section "Everyday examples" I gather it is not "thixotropic". But I find it a bit confusing.

Maybe the second sentence should be changed / dropped / moved.

First and third link are broken — Preceding unsigned comment added by Griva89 (talkcontribs) 09:20, 27 August 2013 (UTC)

## Should shear thinning be hyphenated?

Should shear thinning be hyphenated to shear-thinning? Zedshort (talk) 18:18, 19 July 2014 (UTC) Dear Sir: I think that the explanation is wrong. Shear thinning is specially defined as the viscosity decreases with the shear time. It is son called time -dependant. — Preceding unsigned comment added by 183.157.162.47 (talk) 00:19, 29 April 2015 (UTC)

## Accumulated shear strain or increasing shear rate?

There seems to be a possible contradiction in this page. The text says that "shear thinning is the non-Newtonian behavior of fluids whose viscosity decreases under shear strain" (which is also my understanding), but the figure shown to the immediate right says: "Pseudoplastic, Bingham and Bingham pseudoplastic all show reduction in apparent viscosity with increasing shear rate." Or does one imply the other, i.e., decrease in viscosity with shear strain results in also decrease in viscosity with increase in shear rate? I take it that shear rate is also the shear strain rate. Efischer80 (talk) 13:47, 16 March 2017 (UTC)

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