Thixotropic fluids are fluids that become thinner over time under the continuous influence of mechanical forces (shear stress). This effect is based on the destruction of the structure of the fluid, due to which small interactions occur between the particles.

Mechanical forces can be, for example, stirring, kneading, shaking or pumping. These fluids only regain their original viscosity (i.e. viscosity) when the forces acting on them are removed. The longer the fluid has been exposed to the forces, the longer it will take for the original viscosity to be restored. Fluids that do not fully regain their original viscosity are called partially thixotropic.

Practical examples of thixotropic fluids include ketchup, toothpaste, margarine and shaving cream.

Incidentally, the opposite of thixotropic fluids are rheopex fluids. Their viscosity increases over time when forces are applied. After the forces are released, it decreases again.