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Critical Consideration of the Definition of Thixotropy



Author(s)

Dimiter Hadjistamov

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DOI:10.17265/2161-6213/2022.7-9.003

DECE GmbH, Riehen 4125, Switzerland

Barnes et al. defined thixotropy as “gradual decrease of viscosity under shear followed by a gradual recovery of the structure when the shear rate is removed”. The investigated pigment dispersions, with a gel-sol-gel conversion, show a decrease in viscosity under shear. After the shear rate is removed, a gradual recovery of the structure occurs with the rest time. It was found that the pigment dispersions follow the thixotropy definition. The systems with shear thinning flow behavior show start-up curves with maximum in experiments with shear rates from the shear thinning region, as a characteristic for thixotropy. After cessation of the shear, a gradual recovery of the structure is not observed, or the second part of the thixotropic definition is not fulfilled. The structure of the systems with plastic flow behavior as well does not recover. We presented a new, second, definition of thixotropy for non-gel-sol-gel systems— when a constant shear deformation is applied, the shear stress or viscosity passes through a maximum and then achieves a steady state value. After the shear deformation is stopped, the stress relaxation begins. The shear stress comes to a steady state value—the residual shear stress. The structure does not recover after cessation of the shear flow.

Thixotropy, new second definition.