Fourier Transform Controlled Stress Parallel Superposition (FT-CSPS): Validation and application in processing printable functional materials

In this paper, the development of a multifrequency form of controlled stress parallel superposition rheometry is reported, along with the technique’s validation and use in model gelling systems and high-value particulate suspensions. The novel technique reported herein, termed Fourier transform controlled stress parallel superposition (FT-CSPS), facilitates measurements of the superposition shear moduli and their response to an imposed unidirectional shear stress. FT-CSPS measurements are reported in applications involving (i) the determination of the relaxation properties of incipient gel networks formed in rapidly gelling samples under bulk flow conditions and (ii) measurements of the parallel dynamic moduli of non-gelling samples that experience high rates of solvent loss. By probing the rheological properties of these rapidly evolving materials using a composite waveform comprising multiple harmonic frequencies, sample mutation artefacts (which limit the use of CSPS for such materials) have been minimised. Validation of FT-CSPS has been achieved by (i) showing coincidence of data obtained using CSPS and FT-CSPS for slowly gelling systems and (ii) continuation of the expected relation between gel strength and stress relaxation exponent beyond the range of data accessible to CSPS (limited by sample mutation considerations). This work demonstrates that the rapid acquisition of parallel superposition shear moduli is feasible and facilitates the use of CSPS-based techniques for tests involving rapidly changing materials (such as those undergoing rapid gelation or relatively rapid solvent loss)