Large amplitude oscillation shear has been an important method to investigate the yielding and flow behavior of yield stress materials. However, there are great uncertainties in determination of the yield stress from the shear stress (or shear strain) dependence of the apparent dynamic moduli or the relative harmonic intensity using Fourier transform rheology. The yield stress from these dynamic methods is also inconsistent with the steady shear and transient shear measurements. We propose 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 in large amplitude oscillatory shear flow. The results prove that typical yield stress fluids such as concentrated emulsions, polymer nanocomposites, microgels, and particulate gels all exhibit stress bifurcations, both inter and intra cyclically, in large amplitude oscillatory shear experiments. Such stress bifurcation phenomena between the average stress-strain (or strain rate) curves are independent of the type of input signal, i.e., stresscontrolled versus strain-controlled. A start yield stress (strain) (related to strain) and an end yield stress (strain rate) (related to strain rate), instead of a single critical variable, were suggested to characterize yielding transitions. 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, the Kelvin-Voigt-Herschel-Bulkley model, was also adopted to understand the stress bifurcation and frequency dependencies of critical variables in large oscillatory shear flow. (C) 2017 The Society of Rheology.

Wall slip occurs under large amplitude oscillation shear (LAOS) for yield stress fluids. In this work, 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. 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 intercycle maximum slip strain rate follows the generalized Navier's law, while the intracycle slip behavior can be well described by a Maxwell-like dynamic slip model, which helps to determine the slip relaxation time. It is also found that the slip Deborah number is independent of the angular frequency and is a monotonically decreasing function of the reduced stress. Moreover, the slip Deborah number depends on the reduced stress through a power law, and there is an evident transition of the power law exponent at the yield stress.

Large amplitude oscillation shear (LAOS) is used to investigate the yielding and flow behavior of yield stress materials. 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.

A transient sessile droplet evaporation numerical model based on Arbitrary-Lagrangian–Eulerian (ALE) formulation is developed, taking into account of the coupled transport processes in solid, liquid and gas phases as well as the evolution of free surface. The ALE formulation can trace the sharp two-phase interface and therefore can calculate evaporative flux and surface tension force accurately. The numerical simulations of axisymmetric sessile droplets evaporation at fixed contact line mode are conducted to verify the numerical model by comparing with the previous quasi-steady-state numerical simulation results and experimental data. It is shown that the present numerical model can predict the evaporation rate at quasi-steady-state, the droplet surface temperature distribution and the droplet volume variation with time correctly. The importance of evaporative cooling is emphasized and the influence of Marangoni flow on evaporation rate and surface temperature distribution are analyzed under different contact angles. The numerical model developed in this study can easily be extended to simulate more complicated droplet evaporation situations, such as contact line dynamics during droplet evaporation, droplet impingement cooling and 3-D droplet movement driven by a thermocapillary force on a substrate with surface temperature gradients.

The nonlinear viscoelasticity of a thixotropic yield stress fluid (3.0 wt% Laponite hydrogel) and a non-thixotropic one (0.25 wt% Carbopol hydrogel) under large amplitude oscillatory shear (LAOS) was investigated by Fourier-transform rheology (FTR). The relative intensities of the 3rd harmonic (I-3/I-1) and the relative phase angle of the 3rd harmonic (Phi(3)) of these two hydrogels under different stress amplitudes were compared. It is found that both the nonlinear viscoelasticity and the solid-liquid transition behavior of these two hydrogels differ greatly because of their different microscopic structures. For the Carbopol hydrogel, the nonlinear intensities change gradually with the increase of applied stress, and the transition behavior shows the correlations between the deformation, relaxation as well as relative slip of the microgel particles and the oscillatory frequency. While for the Laponite hydrogel, the nonlinear viscoelasticity exhibits a non-monotonic change, which indicates several times of shear rupture of local house-of-card structure before the full solid-liquid transition.