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    Microscopic inter-particle friction and shear rheology of particulate suspensions
    Kusuma, Tiara Enggar ( 2017)
    The study of the shear rheology of strongly-flocculated particulate suspensions is important for a range of industrial applications, such as start-up, pipeline flow and slumping. Examples include the improvement of industrial processes, equipment design and product development, such as gels and polymers. It can be achieved by thorough understanding of suspension behaviour and constructing a rheological model to enable an accurate prediction of the behaviour. Above a critical solid concentration, known as the gel point, these suspensions are able to withstand an applied force and show a solid-like characteristic before they yield and flow. Previous studies modelled this behaviour as viscoplastic, which described the sharp transition of no-flow and flow behaviour at a critical point termed the yield stress. Many experimental studies however, contradicted this idea and showed the non-linear elasticity below yielding, time-dependent yield and rate-dependent yielding. Understanding these phenomena requires assimilation of the bulk rheology to the microscopic features of the suspensions. The work presented in this project aims to correlate the micro- and macro- features of suspension behaviour. Constant stress (creep), stepped stress and constant rate experiments were performed on two strongly particulate suspensions, alumina and calcite, at different solids volume fractions using controlled-stress and controlled-strain rheometers with a vane in a large cup technique. In addition, to better understand the stress transfer between particles in the suspension network during shear, a novel experimental Atomic Force Microscope technique for frictional study was developed. This technique allows one to measure friction force between two microsphere particles sliding over each other. The shear rheological data for alumina and calcite were obtained from creep (constant stress), stepped stress and constant rate experiments. The suspensions exhibited non-linear viscoelasticity prior to yielding, non-monotonic shear softening flow, and time- and rate-dependent yielding phenomena. The non-linear viscoelasticity prior to yielding was studied from the creep testing data. The instantaneous and steady-state moduli were extracted. They exhibited softening behaviour with increasing strain and modelled using a modified Cross model. This model indicated that the moduli were constant at very low strain and then eventually softens to a power-law. In addition, the effects of solids concentrations were investigated. Using power-law scaled with the gel point model, the increasing magnitude of the properties from the gel point was predicted. From the stepped stress and constant rate tests, the non-monotonic behaviour of both suspensions was investigated. The steady state stress value from the constant rate test and the data from the stepped stress tests at intermediate shear rate were used to construct the non-monotonic flow curves of both suspensions. The data were modelled using the modified Herschel-Bulkey model. The curves showed shear softening of solids network at lower rates and the increasing of viscous stress with higher shear rates. As the solids concentration increased, the curve shifted up, indicating the increase of solids network and viscous stress of the suspensions. The solids concentration dependency of the extracted properties was also studied. The data showed an exponential increase with higher solids concentrations. The time- and rate-dependent yielding phenomena were studied using the creep testing and constant rate data. From the creep test, the breakage time of suspension at different stress values can be extracted. The decreasing trend with higher stresses in breakage time suggests the time-dependent yielding of particulate suspensions. The rate-dependent yielding was observed from the peak stresses at various rates from the constant rate experiment. The peak stress data, modelled using the modified Herschel-Bulkey model, showed decreasing trend at lower rates and eventually increased with higher rates. The extracted properties were also found to be solids concentration dependence and could be predicted with an exponential function. The study of particle-particle interactions in this work focused on the friction force between two spherical micrometer-sized particles. A novel experimental setup for friction force measurements using Atomic Force Microscope (AFM) was developed. A particle probe attached on the cantilever was pushed against an immobilized particle on a microscope slide. The topography of the scanned particle was obtained while measuring the normal and lateral deflections of the cantilever. Given calibration constants, these deflections were converted to forces and analysed. The results show a non-linear friction behaviour and the breakdown of Amontons’ law, which suggests a linear correlation between normal and friction forces. The friction results implied that friction between two particles are more complicated than a constant sliding friction. This work extends our fundamental understanding of the relationship between particle-particle forces and shear rheology of particulate suspensions. The results from AFM study suggest a more complicated mechanism of friction between two microsphere particles. The non-linear behaviour of friction between two particles was one of the underlying causes of the shear rheological phenomena in particulate suspension.