Study On The Global Rheology And Local Flow Mechanism Of Concentrated Suspensions

As one kind of the Soft Glassy Materials (SGM), suspension always shows complicated flow behaviors between solid-like state and liquid-like state, becomes one of the most challenging problems in the Condensed Matter Physics. Although most researchers agree with the dynamic heterogeneities in the complex fluids, the mechanism and kinetics and correlations with flowing are still remaining unresolved. In fact, the local flow mechanism and the microstructure are the underlying physics of these problems. To what extent can the traditional rheology describe the complicated flowing of the complex fluids, and how to describe the underlying local flow mechanism and the microstructure behind the complicated flowing are of primary importance for the study on the rheology of complex fluids. New techniques coming recently on the local measurement undoubtedly provides a powerful approach for the rheology of the complex fluids, and has been received more and more attentions. In addition, concerning the theoretical models for the rheology, a central task of the development is how to step further to rationally describe the local and/or microscopic flow and even the microstructure based on the apparent rheology. In this thesis, the concentrated suspensions are consist of hard spherical particles and the Newtonian matrix. The global and the local flow behavior under the transient shear flow and Large Amplitude Oscillatory Shear (LAOS) flow is focused through the rotational rheometer and Fourier Transform Rheology (FTR) method and Particle Tracking Velocimetry (PTV) technique. Firstly, the global rheology under the given conditions is found to be insensitive to the different microstructures of the suspensions, but the local flow behavior is demonstrated to be able to differentiate the microstructures of the suspensions. Secondly, the determination of the true flow behavior through the global rheological behaviors is specified considering the local flow mechanism. Thirdly, the local flow mechanism and the influences to the local flow are illustrated on the local scale. Finally, a comprehensive and systematic study on the performance of the currently used thixotropic models around the center of the global rheology and the local flow behavior has been carried out. The main content and the results are as following:1. As far as the large particles dispersing in Newtonian matrix is concerned, the suspension microstructure is to a large extent determined by the particle concentration, and the concentration dependent microstructure is described by the global rheological behavior through the concentration dependent rheological parameters. Firstly, several global flowing experiments are carried out to investigate the concentration dependent transition behaviors, including the transition of flow regions, the transition of the dynamic plateau modulus, the transition between linearity and non-linearity and the thixotropic transitions. The critical concentration defined the static transition from the liquid-like state to the solid-like state was determined as 49.5±1.5%. The mean nearest neighbor surface distanced between spherical particles at the critical concentration is estimated. The nearest neighbor particle distance generally decreases with the increase in the volume fraction, with a transition atdc for critical concentration. Below the critical surface distanced^., the nearest neighbor particle distance decreases sharply with the increase in the volume fraction, and the increasing inter-particle interactions dominate the liquid-to-solid transition. Secondly, two types of well defined shear procedures are used to investigate the effects of Short Shear History (SSH) and Long Shear History (LSH) on the global rheology for various concentrations. Our purpose is to illustrate whether the shear history is weaken or enhance the description of the global rheology on the concentration dependent microstructure. The results show that the global rheology is not able to differentiate the low particle concentration suspensions (φ<φc) from the high concentration suspensions (φ>φc) under the LSH. That means the LSH can make suspensions ofφ<φc display the similar global rheology as the suspensions ofφ>φc, which is analog to the dynamic liquid-to-solid transition. According to the estimation of the concentration dependent mean nearest neighbor distance between the spherical particles, the dynamic liquid-to-solid transition is linked with the increasing number of particles that form certain network in the suspension under LSH, which exceeds the critical particle concentration that form certain network in the suspensionsφmetwork.2. If the global flow behavior under LSH is insensitive to the concentration dependent microstructure, whether the nonlinearity obtained from the LAOS flow field could be used to study the effects of shear history on the microstructure or not? Firstly, comparative analysis is made to investigate the concentration dependent nonlinearity from the global flow behavior under LSH. The results show that the microstructure of suspensions withφ<φc is totally different from the microstructure of suspensions withφ>φc with totally different local flow behaviors after the LSH Especially, the microstructure of suspensions withφ>φc is significantly affected by the LSH. For the excess of particles in the extensional quadrant will affect the oscillatory behavior greatly compared with low concentrated suspensions. In another word, step pre-shear before oscillatory flow will greatly heterogenize the suspensions withφ>φc thanφ<φc so that the high concentrations could not reach the overall shear state and directly transform from liquid-solid coexistence to flow jamming. However, for suspensions withφ<φc, no matter whether the shear history is short or long, the local flow behavior show four regimes:solid-like, liquid-solid coexistence, liquid-like and jamming, from the bottom up.3. The coincidence of the apparent flow curves do not necessarily mean that the local flow behaviors are coincided with each other, and how to obtain the true local flow from the global flow experiments? With the certain density differences between the particle and matrix, the settling velocity varies with the matrix viscosity. Meanwhile, the particle velocity and its distribution are determined online through the PTV technique. Our purpose is to investigate the differences between the local flow behaviors and the correlations between the local flow and the global flow under the LAOS flow field. The results show that the competition between the gravitational and the viscous stresses contribute to the yield stress and shear-banding. For the suspensions with non-ignorable sedimentation, there is a evident solid-to-liquid transition in the global flow curves, which corresponds to a clear shear-banding in the global transition region. At the same time, the stress dependent wall slip velocity shows a discontinuous transition during the solid-to-liquid transition. Then, the global rheology is totally determined by the local rheology from the flow regime with low shear rate, whereas the solid-to-liquid transition could not be found in the high shear band, in which only viscous flowing shows up. However, there is no solid-to-liquid transition in the suspensions with ignorable sedimentation, and the global flow is dominated by the viscous dissipation. The corresponding local flow velocity shows a continuous linear distribution, and the stress dependent wall slip velocity displays no transition behavior. If the wall slippery was subtracted from the global rheology, the global flow behaviors of the suspension with ignorable sedimentation would show the same tendency as the true local flow behaviors.4. The global rheology is able to describe the local flow behavior through the controlling on the stability and the uniformity of the materials, but how to step further to correlate the microstructure with the global flow behaviors? It should be note that the only difference between the global and the local waveform is the dynamic wall slip, i.e., the local nonlinearity represents the structures of the materials only, while the global one is the collective contributions from the structures of materials and the wall slip. Therefore, the nonlinearities from global rheology and local flow could be compared with each other based on the FTR method. The results show that wall slip indeed contributes to the second harmonic relative intensity I21 and does not affect the third harmonic relative intensity I31. On the other hand, the global rheology and the local rheology show the same stress dependent I31. It means that the third harmonic can be used to represent the structural changes in suspension, meanwhile the representation is less affected by the dynamic wall slip. The stress dependency of high harmonics can be a good indicator to the effect of yield stress, wall slippage and the shear banding behavior. However, not all the local flow mechanisms can be involved in the nonlinear intensities of macroscopic rheology.5. If the true local flow behaviors could not be determined from the global rheology, and how is the true flow happening? Our purpose is to study the local flow mechanism in the transient flow field with the help of PTV technique. The results show that the time evolution of the shear-banding depends on the strength of the flow field. The wall slip also plays an important role during the evolution of shear-banding. In specific, if the global shear rate is lower than the critical shear rate (γa<γc), there are no structural destructions in the suspensions, but the deformation and the shear-induced fracture shows up as the shearing time increases. The shear-induced fracture of the microstructure will result in the severe discontinuous local velocity distribution across the gap, and the flow regime is made up of several non-flowing bands. Moreover, the drastic fluctuation of slip velocity on the two both walls is ready to worsen the discontinuous velocity distribution. If the global shear rate is close to the critical shear rate (γa≈γc), the high shear band will be found among the several non-flowing bands. It means that the shear-induced fracture is accompanied by the local flow. If the global shear rate is higher than the critical shear rate (γα>γc) or the shear stress overcomes the yield stress (σ>σy), the structural destruction happens at the early stage of shearing. Meanwhile, the slip velocity on the moving wall is larger than that on the stationary wall, which let the high shear band due to the structural destruction localize close to the stationary wall, whereas the low shear band stays close to the moving wall. Thereafter, the low shear band disappears gradually as the time/shear rate/shear stress increases and the sample shows an overall flowing at last. In addition, if the applied shear stress was lower than the yield stress under the stress controlled mode, there is no structural destruction but only with wall slip.However, material does not flow if the stress was below the slippery stressσy'.6. From now and a fairly long period of time to come, the development of the theoretical models should focused on the performance of the prediction on the global flow and the local flow. The main thixotropic models at the present almost concentrated on the prediction of the global rheology. However, now take the global prediction, for instance, different models give obvious different results, let along the local prediction. In this thesis, a comprehensive and systematic study on the performance of the prediction of the currently used thixotropic models around the center of the global rheology and the local flow behavior has been carried out. The comparative results show that the structural kinetics models not only have more excellent prediction ability than the other models, but also show more excellent agreement with the experiment only if the models are well simplified. Among the structural kinetics models,λ-DM model andλ-C model are the most favorable models. Because of the well defined kinetic equation and the introduction of the structural-dependent elastic stress,λ-DM model shows the very powerful prediction ability for the global rheology, especially in thixotropy and viscoelasticity. However, the main limitations ofλ-DM model are unable to describe the local flow behavior. On the other hand, althoughλ-C model is simple in form, the model is capable of predicting several key phenomena, such as viscosity bifurcation, aging and shear-banding. This is because the model allow the structural parameterλtend to increase infinity as the shear rate or shear stress decreases to very low values, which leads to the structural build-up dominating the flow behavior. However, for materials without structural aging, the flow stoppage during the decreasing shear rate/stress is analog to the process of aging, but the structural aging could not be used in the other flowing processes. In addition, viscoelasticity as a result of the strong network structure should be considered in this model.