thixotropy
Thixotropy is a time-dependent thinning: a gel-like material softens and flows the longer you keep stirring it, then slowly stiffens back into a gel after you stop and let it rest. The key word is time. Shake a thixotropic lotion and it becomes fluid; set it down and over seconds to minutes it thickens again. The change is reversible and can be repeated over and over.
The mechanism is a delicate internal structure, usually a loose three-dimensional network of weakly bonded particles or polymer chains, that takes time to build up at rest. Shearing progressively breaks this network down, lowering the viscosity; once shear stops, Brownian motion gradually rebuilds it, restoring the gel state. On a rheogram, increasing then decreasing the shear rate traces two different curves that enclose a 'hysteresis loop', whose area reflects how much structure was broken and how slowly it recovers.
Thixotropy is a prized property in pharmaceutics. It is ideal for suspensions and topical products: at rest the rebuilt gel structure holds particles in place and prevents settling or sagging, yet a brief shake or rub thins it enough to pour, inject, or spread. The recovery must be slow enough to allow application but fast enough to re-suspend particles before they settle, which is a deliberate formulation balance.
A thixotropic intramuscular suspension is formulated to set into a gel in the vial so the drug particles do not settle, then to liquefy after vigorous shaking so it can be drawn up and injected through the needle.
Set-on-standing, liquefy-on-shaking behaviour keeps injectable suspensions both stable and usable.
Thixotropy (thinning over time under shear, recovering at rest) should not be confused with shear-thinning, which depends on shear rate at the instant of measurement rather than on time. The rare opposite, thickening over time under shear, is called rheopexy.