i-Rheo-Indent: Broadband rheological characterisation of soft solids by macroscale indentation.

ABSTRACT: We introduce i-Rheo-Indent, a transformation-based methodology for determining the linear viscoelastic properties of soft solids directly from macroscale indentation experiments. By combining force-relaxation and indentation-depth measurements with direct time-to-frequency transformations, the method recovers the complex shear modulus, [Formula: see text], over a broad frequency range without requiring oscillatory excitation or predefined constitutive models. The methodology addresses key limitations of conventional rotational rheometry, where compressional forces required to maintain sample-tool contact may alter the measured response through stress stiffening and interfacial artefacts. Stress-relaxation indentation experiments were performed using a rigid truncated-conical indenter, with the geometry explicitly incorporated into the constitutive formulation. The approach was validated across hydrogels, polydimethylsiloxane elastomers, anatomical modelling materials, and industrial soap bar formulations. Good agreement with oscillatory rheometry was obtained over a broad frequency range, while improved consistency with capillary rheometry was observed for highly structured soft solids under Cox-Merz comparison. These results establish i-Rheo-Indent as a simple, rapid, and robust methodology for broadband rheological characterisation of soft solids.