A Study On The Rheology Of Yield Stress Fluids In Large Amplitude Oscillatory Shear Flow

Large amplitude oscillation shear?LAOS?has become one of most important methods to study the yielding transition of yield stress materials.However,the determination of yield stress in LAOS becomes much more difficult,and various methods have been suggested,but in many cases the results are contradictory and lack of clear phyisal meaning.Therefore,it is not meaningful to invertigate the time dependence of yielding transition by the critical values determined by the traditional dynamic methods.Moreover,it has been found that flow curves can be rescaled onto one master curve above and below the jamming transition,which is regarded as a critical characteristic for yield stress fluids.But it is still not clear whether this scalling law is universal or not.Furthermore,wall slip occurs inevitably and evidently at low shear rate range even in the solid region of yield stress fluids.The boundary conditions have a strong influence on accurate characterization of viscoelastic behavior and yielding transition of yield stress fluids.The understanding of the slip behavior in LAOS of yield stress fluid are of great importance.In this thesis,the following aspects about yielding transition of yield stress fluids in LAOS are studied:?1?.Considering the problems in determination of the yield stress from the apparent dynamic moduli and relative harmonic intensity using Fourier Transform Rheology,we proposed a new approach based on 2D mechanical correlation spectra?2D-MCS?to quantify the yield stress.We have proved that the nonlinear synchronous self-correlation intensity as functions of stress/strain amplitude can be used to determine the yield stress unambiguously from the change of scaling exponent.The yield stresses from 2D-MCS analysis are well consistent with those from the stress ramp experiments.?2?.We proposed a new method,namely stress bifurcation,based on the geometric average of elastic and viscous Lissajous curves to study the yielding transition of different yield stress fluids LAOS.The results prove that typical yield stress fluids such as concentrated emulsion,polymer nanocomposites,microgel,and particulate gel all exhibit stress bifurcations both inter and intra cyclically in LAOS.A start yield stress?strain?and an end yield stress?strain rate?,instead of a single critical variable,were suggested to characterize yielding transition.The frequency dependences of critical stresses,critical strain and critical strain rate determined by the new method were also investigated systematically for the different kinds of yield stress fluids.A visco-elastic-plastic model,Kelvin-Voigt-Herschel-Bulkley?KVHB?model,was also adopted to understand the stress bifurcation and frequency dependencies of critical variables in LAOS.Both the inter cycle and intra cycle stress bifurcation phenomenon,frequency dependences of start/end yield stress and start yield strain rate observed experimentally are in good qualitative agreement with the prediction of KVHB model.The stress bifurcation in LAOS can be a reliable and feasible method to study the solid-liquid transition,and offers new possibilities to understand the pathways of such transition.?3?.The rescalling to one master curve is regarded as a critical and universal characteristic of yield stress fluids above and below jamming transition.To justify the universality of such character,a concentrated emulsion where the continuous phase is a kind a low mass nematic liquid crystal?8CB?and dispersed droplet?PDMS?is a kind of polymer was selected to study the critical scaling behavior above and below the jamming transition.The results show that flows curves can't be rescaled onto one master curve,implying the influences of long range interaction on the solid to liquid yielding transition.Above jamming,the yield stress shows an exponential law with the volume fraction,instead of a power law with the distance to jamming.The stability of8CB/PDMS emulsions is attributed to the Frank elastic stress of the continuous phase8CB in nematic phase.The interactions between PDMS droplets changes with the thickness of 8CB films which is varing with the increasing of concentration of PDMS.The results prove that the scaling behavior is not a generic and universal law.?4?.We investigated how the boundary conditions affect the nonlinear behavior under LAOS,and proposed a simple methodology based on the geometric average of Lissajous curves to study the dynamic wall slip behavior under oscillatory shear,which solved the issues of slip on nonlinear viscoelastic behavior and provided new approach to determine the slip strain rate and slip behavior of yield stress fluids under LAOS.The results show that the stress-mean strain curve is a good candidate to define material's functions since it is almost not influenced by the wall slip effect.Meanwhile,the stress-mean strain rate curves from smooth plates and rough plates can be used to determine the wall slip velocity.It is found that the inter-cycle maximum slip strain rate follows the generalized Navier's law,while the intra-cycle slip behavior can be well described by a Maxwell-like dynamic slip model,which helps to determine the slip relaxation time.As the stress amplitude becomes higher than the yield stress,much stronger dependence on the reduced stress is seen.Moreover,the slip Deborah number(De_slip=??_slip,where ? is angular frequency and ?_slip is slip relaxation time)depends on the reduced stress through a power law,and there is an evident transition of the power law exponent at the yield stress.Slip Deborah number is close to 1 when the stress amplitude is smaller than the yield stress and far samller than 1 after yielding,implying that slip behavior is more time dependent at solid-like region and more viscous in liquid like region.