  There is no universal model for thixotropy, and comparing thixotropic effects
between different fluids is a subtle yet challenging problem. We introduce a
generalized (model-insensitive) framework for comparing thixotropic properties
based on thixotropic spectra. A superposition of exponential stress modes
distributed over thixotropic timescales is used to quantify buildup and
breakdown times and mode strengths in response to step-change input. This
mathematical framework is tested with several experimental step-shear rate data
on colloidal suspensions. Low-dimensional metrics based on moments of the
distribution reveal characteristic average thixotropic properties which are
visualized on Ashby-style diagrams. This method outlines a framework for
describing thixotropy across a diverse range of microstructures, supporting
scientific studies as well as material selection for engineering design
applications.
