Negative thixotropy vs. rheopexy?

[PCS_Exponent]

Hello all,

 

Actually joined this forum only to ask this question. I'm not too optimistic, but hey, maybe there's some rheology gurus in the house?!?!

 

The title says it all, really. I need a good explanation of the difference between ngative thixotropy and rheopexy. I couldn't find any source on the net that discusses the difference between the 2. The only viable explanation I have is from the book Foundations of Colloid Science by CJ Hunter, so let me put down in script what I think he was trying to say, and if anyone can confirm this understanding of mine, or correct me, then please do.

According to Hunter, rheopexy implies that a material being sheared has had some internal structure broken or interrupted the moment the shearing begun, and the structure is being rebuilt at a certain rate. Measuring the viscosity at different times after an impact will give a rise in viscosity, but if the material is sheared slowly then this increase in viscosity will go faster. Thus rheopexy deals only with an acceleration of a reconstruction that should take place without the shearing, albeit slower.

Negative thixotropy, if i understand correctly, is the construction of otherwise non-present molecular structure, and takes place at a specific shear rate (or rather specific range of shear rate). When the shear is removed, the newer structure breaks down and the viscosity goes back to its original value. Thus negative thixotropy refers to an increase in viscosity with time only during shearing, and this increase is completely reversible.

 

Is any of this correct? Please, if anyone has better understanding of this than I do, chime in and hit me with the book of facts across the head.

 

Also, I'm not sure whether you do this on this forum, but- if mods/admins feel the need to move this to the chemistry forum (I felt the physics forum was slightly more appropriate for this subject), feel free to do so.

 

TIA!!

[C1ay]

It basically boils down to work softening vs work hardening. A thixotropic fluid gets thinner or less viscous as you apply force to it over increased periods of time. Imagine a paint that gets thinner and thinner the longer you stir it. A rheopectic fluid is the opposite. It increases in viscosity as you work the fluid. Such a fluid would get thicker and thicker as you stir it. You could refer to these fluids as being negatively thixotropic.

[PCS_Exponent]

Thanks for your answer, C1ay, but it is my understanding that these concepts are actually used for different phenomena. From Hunter's Foundantions of Colloid Science: "Note that this [rheopexy] is not the same as negative thixotropy though the two can be confused if only a limited number of measurements is made at one (low) shear rate".

I also found this ppt representaion, which obviously discusses these two concepts as two different things- I gathered that much though I do not speak Korean: phypha.chonnam.ac.kr/lesson/disp/disp9.ppt#270,15,Negative thixotropy and rheopexy (This should go directly to the slide discussing negative thixotropy vs. rheopexy).

[eric l]

One problem is that of the definitons in rheology. Some handbooks (mainly those translated wholely or in part from other languages) mix up notions like thixotropy, plasticity and pseudo-plasticity.

 

There is an online forum on trheology that may be of some help :

http://www.malvern.co.uk/LabEng/support/rheology_forum/rheology_focus.htm

 

There are also glossaries such as :

http://www.rheologyschool.com/rheology_glossary.html

 

but according to this one anti-thixotropy is just an other word for rheopexy.

 

There is also a more or less official "Rheologiy terminology", but that is almost 50 years old by now (I have often used it myself, because it also provides some translations);there may be more recent additions to it, but I am not active in the field any longer.

Reiner, M., and Scott Blair, Rheology terminology, in Rheology, Vol. 4 pp. 461, (New York: Achedemic Press, 1967)

[C1ay]

I think Eric is onto something. When we covered this is fluid dynamics my instructor simply implied that they were opposites and that they should not be confused with pseudoplastic (shear thinning) fluids or dilatent (shear thickening) fluids.

[PCS_Exponent]

Thanks for your input, guys, expecially for those links you provided, Eric. Eric, I guess I'm going to have to take your answer to mean that the conventional definitions of thixotropy and rheopexy are as antonyms. It is, however, obvious that others (such as CJ Hunter and the Korean guy I referred to above) use the terminology differently.

I found through the Malvern link the entire "Rheological Methods in Food Process Engineering" book's second edition. The author refers to thixotropy and rheopexy as antonyms, but makes a point of adding that both processes can be reversible, irreversible or partially reversible. So, while some authors take rheopexy to mean the opposite of thixotropy (and then it's necessary to give more details as to the change's reversibility), others refer to it only as the behaviour of thixotropic materials during their recovery phase, when the recovery and increase in viscosity is faster at very low shear rates than at complete rest.

[PCS_Exponent]

I'll audaciously bump this very old thread- I have found another rheology manual which makes a distionction between anti-thixotropy and rheopexy. This manual basically says the same thing I said in the first post in this thread, namely:

anti-thixotrpoy = build-up of an otherwise non-existent structure occurs under shear. Structure disappears with removal of shearing.

rheopexy- just like thixotropic materials, materials showing rheopexy break down during (strong enough) shearing, but their rebuild is faster at very low shear rates than at complete rest (thixotropic materials will regain their structure faster at rest than at low shearing rates, or at least as fast).