Plastic flow: needing a push to start
Some materials behave like a solid until you push hard enough, then suddenly flow like a liquid. This is plastic flow. Toothpaste is the perfect example: it holds its shape on the brush, but squeeze the tube past a certain point and it streams out. The stress you must reach before flow begins is the yield value. Below it, the material only deforms elastically, like a soft solid; above it, it flows.
On a rheogram, plastic flow shows up clearly: the line does not pass through the origin. It starts flat along the stress axis until the yield value is reached, then rises. That intercept on the stress axis — the gap before flow begins — is the yield value made visible. This is a special form of non-Newtonian flow and one of the most useful in formulation.
Thixotropy: structure that rebuilds
Thixotropy adds the dimension of time. A thixotropic material becomes thinner the longer you shear it, and then slowly thickens again after you stop. Shake a bottle of certain calamine lotions or an antacid suspension and it pours; let it stand and within minutes its internal structure rebuilds and it stiffens. The change is reversible — break the structure, leave it alone, and it knits itself back together.
On a rheogram, thixotropy reveals itself as a hysteresis loop: the curve you trace while increasing the shear rate does not retrace itself on the way down, because the down-curve corresponds to a partly broken-down, thinner material. The area enclosed by the loop is a rough measure of how much structure was broken and how much energy that took.
Why this combination is gold
A well-designed antacid suspension (plastic + thixotropic)
On the shelf (no shear):
structure intact, high yield value
-> drug particles held up, no caking
You shake it (high shear, several seconds):
structure breaks, viscosity drops
-> pours cleanly, easy to dose
After pouring (shear removed):
structure slowly rebuilds over minutes
-> stays uniform until the next dose
Net result: easy to use AND physically stable.Notice how the two ideas team up. The yield value keeps the product stable at rest; the thixotropy lets it flow easily when shaken and then re-set so it does not separate before the patient takes the next dose. Designing for the right rheology is therefore not cosmetic — it is central to whether a suspension, ointment or gel works as a medicine at all.