Rheological properties describe how materials flow and deform under stress. Yield stress is the minimum stress required to cause a material to permanently deform, while shear stress and shear stress rate refer to the force and speed that causes internal sliding and deformation without changes in volume. Materials can exhibit pseudoplastic, thixotropic, Newtonian, or dilatant behaviors depending on how their viscosity changes with applied stress over time. Thixotropy specifically means a material becomes temporarily less viscous when shaken or stirred and returns to its original viscosity when at rest.

1. RHEOLOGICAL PROPERTIES

2. SOME POINTS TO REMEMBER :
• YIELD STRESS: The value of stress beyond which the
material becomes permanently distorted, strain is not
completely recovered after the applied load is removed.
• SHEAR STRESS (S) :Force tending to cause deformation of a
material and slide material over each other.
• SHEAR STRESS RATE (E):Strain rate is the change in strain
(deformation) of a material with respect to time. It
comprises both the rate at which the material is
expanding or shrinking (expansion rate), and also the rate
at which it is being deformed by progressive shearing
without changing its volume (shear rate).

3. • Pseudoplastic behavior
• Thixotropic –Dental prophylaxis paste, Plaster
of paris, Resin cements, impression material.
• Newtonian Water, oil, alcohol.
• Dilatant Slurries, gels, suspensions

19. THIXOTROPY:
• The property of becoming less viscous when
subjected to an applied stress, shown for
example by some gels which become
temporarily fluid when shaken or stirred.
• Thixotropy is a time-dependent shear thinning
property.
• E.g
prophy paste dental etc