Abstract. Many simultaneous techniques are being used to follow structural changes in materials as a result of an imposed shear deformation. Orthogonal superposition, first mentioned by Philippoff in 1934 (W. Philippoff, Physik-Z. 35, 884, 900 (1934)) is a mechanical technique and uses a small amplitude oscillation shear applied orthogonal to a shear flow. In contrast to parallel superposition, the flow fields are not coupled and the orthogonal storage and loss moduli only measure the effect of shear on the microstructure. OSP has been implemented in the ARES-G2 rheometer by modifying the transducer to apply a small oscillatory displacement in normal direction, while the actuator of the rheometer performs the transient or oscillatory rotational shear deformation. The normal force transducer in this case applies the orthogonal deformation and records the force. The dynamic moduli, complex viscosity and phase are determined using the embedded instrument correlation technique. The flow cell used to apply the rotational and linear axial shear simultaneously is a modified double wall cylinder with an opening at the bottom to avoid annular pumping and windows at the top to eliminate surface tension effects. With the OSP option the rheometer is capable to perform the following additional test modes:
- Small (linear) strain orthogonal oscillation superposed on rotational steady shear
- (2D-SAOS) Small (linear) strain orthogonal oscillation superposed on rotational oscillation at the same frequency with varying amplitude ratio between axial and rotational oscillation shear
- Standalone small axial displacement oscillations
Electro-Rheological (ER) fluids are suspensions of extremely fine non-conducting particles in an electrically insulating fluid. They show dramatic and reversible rheological changes when the electric field is applied. The ARES-G2 OSP functionality is used to characterize the structural changes of ER fluids when different voltages and shearing conditions are applied. Particularly, 2D-SAOS gives more insight in the structure formation and provides a tool to quantify it.