Thixotropy

A property of a fluid—such as printing ink—that describes its "false body," or a consistency, thickness, and viscosity that decreases with the application of stress or other forces. Some inks, such as offset inks, are stiff and thick while in their containers, but upon being worked become thinner and more fluid, typically due to an unstable structure formed by the solid particles within the ink that is broken down when force is applied. If thixotropic ink is left undisturbed, it will thicken again. Thixotropy is one reason why offset presses require so many ink rollers, to apply enough stress to the ink to make it less viscous and more easily transferred to the plate, blanket, and ultimately the substrate. Thixotropy is also called shear thinning. Some inks also exhibit the opposite property, dilatancy, in which the viscosity in'creases when stresses are applied.

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