Nonlinear Rheology And Local Flow Behavior Of Typical Yield Stress Fluids

Yield stress fluids(YSF) are very common in everyday life. They arebroadly defined as materials that behave like solids below a criticalapplied stress and flow like simple liquids beyond a critical stress. Forhandling and using these fluids, it is crucial to know the solid-liquidtransition, thixotropy and some other complex rheological behavior ofthese fluids. In this work, the rheological behavior of a thixotropic yieldstress fluid (3.0wt%Laponite gel) and a simple yield stress fluid (0.25wt%Carbopol gel) was investigated and compared.The nonlinear behavior of many kinds of simple and thixotropicyield stress fluid in different rheological test mode have been widelystudied, but previous research is not systematic enough. In this work,their different responses in thixotropic loop test, shear rate ramp, shearstress ramp, ceep test and start-up shear test were systematicallycompared, which can reflect their different aging and shear rejuvenationmechanisms.Then the nonlinear rheological behavior of these tow fluidsunder large amplitude oscillatory shear (Large amplitude oscillatory shear,LAOS) was investigated. It was found that the apparent dynamic modulus is insufficient to describe the difference of the transformation process andthe mechanism of solid-liquid transition, while the relative intensity andrelative phase angle of higher harmonics obtained by the Fouriertransform can clearly define the differences in transition behavior of thesetwo fluids under LAOS. In addition, a new framework called StrainDecomposition is suggested in this work to account for the stress-strainand stress-strain rate relationship based on the concept of mean stress andmean strain (rate) in the Lissajous curves. The intracycle nonlinearmodulus and viscosity are defined from the slope of the stress-straincurve and the stress-strain rate curve, respectively. The intercyclenonlinear behaviors are obtained from the strain (rate) amplitudedependence of the zero-strain (rate) modulus or viscosity. It is found thatthe modulus (viscosity) defined from the stress-mean strain (rate) curvesis the most physical reasonable quantity. And we apply the new analysismethod to these two yield stress fluids, which reveals critical balancebetween aging and shear rejuvenation.In order to investigate the effect of different local flow behavior ofthese two yield stress fluids on the macroscopic nonlinear rheologicalbehavior, the method of particle tracking velocimetry (PTV) wasemployed to get a preliminary study of the local flow behavior.