First- and third-order shear nonlinearities across the structural relaxation peak of the deeply supercooled pharmaceutical liquid indomethacin

Nonlinear rheological properties of viscous indomethacin are studied in the frequency range of its structural relaxation, that is, in a range so far inaccessible to standard techniques involving medium-amplitude oscillatory shear amplitudes. The first- and third-order nonlinearity parameters thus recorded using a sequence of small and large shear excitations in a time efficient manner are compared with predictions from rheological models. By properly phase cycling the shear amplitudes, build-up and decay transients are recorded. Analogous to electrical-field experiments, these transients yield direct access to the structural relaxation times under linear and nonlinear shearing conditions. To demonstrate the broader applicability of the present approach, transient analyses are also carried out for the glass formers glycerol, ortho-terphenyl, and acetaminophen.

Automated summary

The nonlinear rheological properties of viscous indomethacin were studied in the frequency range of its structural relaxation using a sequence of small and large shear excitations. The first- and third-order nonlinearity parameters were compared with predictions from rheological models, and structural relaxation times were accessed under linear and nonlinear shearing conditions. This approach was also applied to other glass formers, such as glycerol, ortho-terphenyl, and acetaminophen, to demonstrate its broader applicability.