#thixotropic

Q&A: Research improves understanding of particle interactions for better material design

Toothpaste has a hard job. Not only does it have to perform essential functions like scrubbing that coffee residue from your pearly whites, but it also must stay in the tube, exit smoothly under pressure and balance on a bunch of bristles without running all over the bathroom sink. And every user, everywhere, appreciates it when any excess paste extruded, but not needed on the toothbrush, slides seamlessly back into the tube. This is an example of a thixotropic elastoplastic suspension made from many different particles. Now that's a mouthful. According to Norman Wagner, Unidel Robert L. Pigford Chair in Chemical and Biomolecular Engineering at the University of Delaware, these types of suspensions are found in everything from lotions to 3D-printing inks to building block materials for items, including rubber tires, clays, cements and more.

Non-Newtonian Fluids: Rheopectic vs. Thixotropic

The difference between rheopectic and thixotropic fluids is very similar to the difference between dilatant and pseudoplastic fluids: both are Non Newtonian fluids but while one, pseudoplastic, is shear thinning (viscosity decreases with increased stress) the other, dilatant, is shear thickening (viscosity increases with increased stress). 

But while thixotropic fluids are occasionally mistaken for pseudoplastic fluids, and rheopectic fluids are occasionally mistaken for dilatant fluids, these two types of fluids differ in one crucial way: time dependence. The change in viscosity with respect to stress for dilatant and pseduoplastic fluids is independent of time. But for thixotropic fluids, the viscosity decreases with increased stress the longer the stress is applied. The same goes for rheopectic fluids, the viscosity increases with increased stress the longer said stress is applied. 

As such, thixotropic behavior can be defined as time-dependent pseduoplastic behavior. An example of such behavior can be found in many gels that are thixotropic fluids, almost solid at rest but much more fluid and less viscous when agitated or shaken. Thixotropic fluids return to their original, more viscous state after the stress is no longer applied and when this changes happens nearly instantaneously, the material is essentially considered to be pseudoplastic. 

A more specific example of this is in the so-called 'astronaut pen' developed by Paul Fisher that can write in no gravity. The pen contains a pressurized thixotropic ink that is too thick to flow unless a force is applied - such as the rotation of the ballpoint when writing. 

Rheopectic fluids, which can then be described as exhibiting time-dependent dilatant behavior, are much less common. Examples of materials that exhibit this behavior are printer ink or cream, fluid at first, but becoming thicker when shaken, stirred, or agitated. Much like with dilatant fluids, research is ongoing regarding the use rheopectic fluids in body armor.