thixotropy

Thixotropy is shear thinning with a memory of time. A thixotropic slurry thins when you stir it, like any shear-thinning fluid — but the key is that it does not snap back to thick the instant you stop; it stays thin for a while and then slowly, over seconds or minutes, rebuilds its structure and thickens again. Non-drip paint and freshly stirred yogurt behave this way: stir them and they flow, let them stand and they set back up. Left undisturbed long enough, a thixotropic slip can stiffen almost into a gel; shake it and it liquefies once more.

The mechanism is a flocculated network that takes time to build and time to rebuild. At rest, weakly attracting particles slowly find each other and knit into a space-filling network, so viscosity and yield stress climb with resting time. Shear tears that network apart faster than it can reform, so the slurry thins; stop shearing and Brownian motion and the attractive forces slowly re-link the particles, and the structure — and the thickness — creeps back. Because the response lags behind the shearing, a plot of stress versus shear rate ramped up and then down traces a loop rather than a single line: the area of that hysteresis loop is a common measure of how thixotropic a material is. This time lag is exactly what separates thixotropy from plain shear thinning, where the viscosity tracks the current shear rate with no memory.

In ceramics thixotropy is usually something to control rather than celebrate. It can be useful — a slip that thins while being pumped or cast and then stiffens on standing resists settling and holds shape between operations — but it is often a nuisance, because a slurry whose viscosity depends on its recent history is hard to process reproducibly: the reading you get depends on how long ago and how hard it was last stirred. That is why good practice is to shear a slip in a standard way (pre-shear) before measuring or using it, so every batch starts from the same structural state. Formulators tune thixotropy through the degree of flocculation, since it is really just the time-dependent face of the same particle-network physics behind yield stress and shear thinning.