Measuring Cyclic Tensile Properties of Fluids with Composite Harmonic Exponential Waveforms (CHEW)

Abstract

Building off recent advances on how to practically use exponential shear in a torsional rheometer to compute transient planar extensional viscosity (Kroo et al. 2025a), we extend the technique to cyclic tensile measurements in complex fluids and soft solids. An novel input strain waveform provides a unifying approach that smoothly interpolates between exponential shear (ES) and oscillatory shear (SAOS/MAOS/LAOS) as a flow type parameter is varied. Analogous to cyclic tensile fatigue tests in solids, or the process of chewing in the oral cavity, this complex strain history is used to quantify the evolution of extensional material properties at large strains over sequential cycles of stretch. In the limit of large Hencky strain rates, the waveform locally increases exponentially and generates a period of strong material stretching. This allows for the direct computation of a transient planar extensional viscosity within specific domains of the periodic function. We demonstrate this technique on a set of model fluids, and then apply it to complex multiphase materials that mutate. These latter fluids exhibit progressive evolution in their rheological properties over repeated cycles of extensional deformation. Here we focus on two examples: a delicate foodstuff material (melted provolone cheese) which systematically decreases its extensional response over successive stretching cycles, mutating at a rate that is directly dependent on the effective Hencky strain rate. We contrast this with a PVA-borax solution which exhibits precisely the opposite effect during successive stretching cycles: increasing its planar extensional response over successive cycles, as interchain associative interactions (controlled via stretching) build structure within the fluid. These results highlight a promising new approach to study bulk extensional properties during cyclical stretching of complex fluids.