Rheological characterization of complex fluids under dynamic helical squeeze flow

Abstract

The measurements of rheological properties are often carried out in the linear or nonlinear regime under simple shear flow. Even though these rheometric protocols provide useful information on the rheological properties of complex fluids, they are not enough in the sense that the flow fields are still too simple compared to real processes. It is necessary to take the rheological measurements with more complex flow than the simple viscometric flow. The measurement of rheological properties even under a little bit more complex flow field is not straightforward and still remains one of the challenging subjects of rheometry. The objective of this thesis is to investigate the rheological behavior of complex fluids in both oscillatory squeeze flow (OSQ) and dynamic helical squeeze flow (DHSQ) and to provide a platform for the analysis of nonsymmetric stress signals. In the oscillatory squeeze flow, the fluid experiences nonsymmetric stress history. This nonsymmetric stress response is a unique feature of oscillatory squeeze flow (OSQ), but has rarely been investigated. It was reported a robust framework for the analysis of nonlinear and nonsymmetric stress signals at larger strain amplitude under oscillatory squeeze flow, and the information obtainable from this approach is more rich and useful than that has been reported in the past. The normal stress was found to be nonsymmetric in both magnitude and shape at large strain amplitude, which leads to the appearance of even harmonics in Fourier transformation. Dynamic helical squeeze flow of both oscillatory squeeze and oscillatory shear flow provides information for microstructural changes of material in superimposed flow field by means of mechanical spectroscopy. Although the realistic flow field is more complicated than dynamic helical squeeze flow, it is useful in understanding the flow behavior of complex fluids in well-defined complicated flow field, and enables to overcome the limitation of conventional rheometry, which has been confined mostly to simple shear flow. The stress analysis of both stress shape and Lissajous plot showed dramatic change as the strain amplitude increases. Both shear and normal stress show nonsymmetric characteristics which mean the different response during compression and extension. In dynamic helical squeeze flow, the onset of material nonlinearity in the shear stress was faster than simple shear flow. This work was undertaken to further establish the use of dynamic helical squeeze flow in order to measure the rheological properties of complex fluids under more realistic flow circumstance.