Rheological study and rheo-microscopy of semi-flexible fiber suspensions

In this work we perform a comprehensive study of the effect of fiber flexibility and associated parameters on rheological properties, using well-controlled systems. We also visualize suspensions microstructure using rheo-microscopy and finally use available models in the literature to understand the observed phenomena in rheometry and rheo-microscopy experiments. The effective stiffness, which characterizes the relative importance of fiber stiffness to hydrodynamic forces acting on the fibers, has been used as a criterion for fiber flexibility. Based on this parameter, the fiber intrinsic stiffness and aspect ratio are ruling parameters involved in probing flexibility effects.;The evolution of the suspensions microstructure was monitored at different shear rates using a continuously-focusable InfiniVarRTM microscope placed under the lower plate of a rheometer. The components of the second-order orientation tensor at different times were calculated using the images obtained from the visualization experiments. The flow visualization experiments showed that fiber orientation is dependent on the shear rate and the amount of applied strain. Rheo-microscopy of the various suspensions showed that suspensions prepared with more flexible fibers were oriented less in the flow direction as compared to rigid fibers, especially at low shear rates.;The last point tackled in this work was concerned with the effect of fiber flexibility on the rheological behavior of fiber suspensions in a viscoelastic matrix (molten LDPE) under simple shear flow, and small and large amplitude oscillatory shear. Large amplitude oscillatory shear (LAOS) was used to help understanding the relationship between stress growth observations and fiber orientation. Molten LDPE suspensions containing fibers with different flexibilities were used in this study. For all composites the stress signal decreased with time in the LAOS experiments, and this behavior was more pronounced in the case of the more rigid fibers. The stress decrease was explained in terms of different levels of fiber orientation and fiber flexibility. The energy dissipated per LAOS cycle was evaluated for each composite, and these calculations showed that more energy was dissipated as fiber flexibility increased. In addition, the dissipated energy decreased with time and this was interpreted as a reduction of fiber contacts as a function of time. This phenomenon was more pronounced in the case of the more rigid fibers and the observations indicated that fiber contacts reduced faster for these suspensions. The first normal stress difference showed a non sinusoidal periodic response, and fast Fourier transform (FFT) analysis indicated the presence of a first harmonic corresponding to the applied frequency for the fiber-filled systems, in addition to the second harmonic observed for the neat LDPE. It resulted in asymmetric strain-normal force Lissajou curves, with this asymmetry being more pronounced in the case of the more rigid fibers. These results were explained in terms of time-dependent memory effects induced by fiber orientation. (Abstract shortened by UMI.);To investigate the effect of fiber aspect ratio and fiber stiffness, various fiber suspensions were prepared to examine the effect of flexibility parameters (stiffness, aspect ratio) as well as the role of interactions in the semi-dilute and semi-concentrated regimes with a high viscosity silicone oil as the matrix, which has a Newtonian behavior. Both viscosity and first normal stress differences increased with larger fiber flexibility, and the increases were more pronounced in the semi-concentrated regime. The enhancement of the rheological properties was attributed to additional fiber-fiber interactions when using more flexible fibers. By conducting stress growth experiments in the forward and reverse flow directions, shear stress and normal force overshoots were observed in the forward flow and attributed to fiber orientation under flow. Both the overshoot magnitude and width augmented with increasing fiber flexibility. Under flow reversal a delayed overshoot was detected and as fiber flexibility increased the strain at which the overshoot started got smaller. Since the reverse overshoot has been attributed to fiber tumbling, our transient results suggest that flexible fibers were less oriented in the previous forward flow than rigid fibers.