Thixotropy

The term thixotropy (Greek thixis= touch, trope = turn; change) describes a property of gels which are composed of inorganic framework formers but also of cellulose derivatives that they liquefy when subjected to shear or shear stress (e.g. when stirred or shaken or even when applied to the skin). The gel framework collapses due to the shear stress. The process is reversible. At the end of the exposure they solidify again. Their viscosity therefore changes. Many kneading masses (play kneading) behave thixotropically. In cosmetic formulations thioxotropy plays a role for certain creams. The effect of thixotropy is used industrially for example in paints. They become slightly liquid when applied by brush and then thick so that no drops are formed.

The best known example of a thixotropic liquid is ketchup or toothpaste. Synovial fluid in the joints is thixotropic due to the hyaluronic acid it contains, which is why one should "warm up" before sporting activities.

The opposite behaviour to thixotropy is called rheopexy, antithixotropy or negative thixotropy. In rheology, thixotropy refers to a time-dependence of the flow properties of non-Newtonian fluids, in which the viscosity decreases due to continuous external influences and only returns to the initial viscosity after stressing has ceased. For blood it is true that it is both a shear-thinning and a thixotropic fluid.

1. Loebich F (1985) Stabilizers in dermatics. In: Hornstein P, Nürnberg E Externa Therapy of skin diseases. Georg Thieme publishing house Stuttgart p.85