Dynamics of particulate suspensions in complex flow: vortex formation, channel clogging, and agglomerate breakup

Abstract

In this thesis, the dynamics of particulate suspensions in complex flow, such as vortex formation, channel clogging, and agglomerate breakup, was studied. Three main goals have been pursued in this thesis. First, the effect of silica particles in poly(ethylene oxide) (PEO) solutions on both rheological properties and vortex dynamics in micro contraction channel flow was investigated. Although the materials used in industry are not merely polymeric solutions or melts but also suspensions consisting of particles as well as polymers, the researches on the interaction between particle-polymer and their vortex behaviors inside the contraction are rare. The effect of silica particle was demonstrated by comparing between PEO solutions and silica/PEO suspensions. The addition of 1.0 wt% silica particles caused the decrease in vortex size and delayed the formation of lip vortex and the proceeding to corner vortex. Then, as the silica concentration increased, the vortex size increased and the flow rate, at which the lip vortex and corner vortex formed, decreased. The vortex behavior inside micro contraction channel was related to the storage modulus than viscosity. The storage modulus also decreased at first and then increased with silica concentration like the vortex size. The decrease in both the vortex size and the storage modulus was arisen from the adsorption of polymers onto the silica particles. Even though 1.0wt% silica particles were added, the storage modulus decreased because the amount of free polymer in the medium significantly decreased. Then, the storage modulus increased because the amount of free polymer was kept nearly constant and the silica concentration increased. The same tendency was observed when the suspension was compensated with the additional polymers corresponding to the amount of adsorbed polymer. In the second part of the study, the clogging mechanism of poly(styrene) particles in the flow through a single micro-pore was investigated. Together with the microscopic observation, the pressure drop was also measured. The pressure drop fluctuated according to the amount of particles deposited inside the channel. When the particles deposited and blocked the channel, the channel was clogged and the pressure drop increased sharply. During the clogging process, the particles were often detached by the flow, and interesting behaviors, such as rolling and stick and detach, were found to be the key factors that determine whether the clogging completely occurs or not. Above a certain flow rate, the channel was not clogged and the pressure drop did not increase further. The particles deposited in the upstream had an influence on the flow path. When the particles were deposited in the upstream, the flow detoured and the vortex was formed. The effect of viscosity was examined by controlling the concentration of glycerol solution. As the viscosity and flow rate increased, the shear stress applied to the particles became larger and it was more difficult for the particles to get accumulated. When the shear stress was high enough, the particles could not withstand the flow and the channel was not clogged. As the last part of the thesis, the deformation and breakup of a single agglomerate exposed to purely planar extensional flow in a cross channel were experimentally observed and investigated. Aggregation was generated by applying shear with destabilized poly(styrene) particles, and the fractal dimension, df, of the agglomerate was 2.25. The aggregation focused on the center of the channel by the sheath flow was rotated while approaching the stagnant point. Then, the aspect ratio increased as it was deformed close to the stagnant point. The probability of breakup and the fragment distribution were dependent upon the viscosity and flow rate, and were superimposed on a master curve as a function of the applied stress. Also, with the increase in stress, the projected area of the fragment which was split by the flow decreased with the power-law relationship and the exponent was in good agreement with the model prediction. This work provides an insight on the effect of the interaction between polymer and particle on the rheological properties and flow field, and it is thought to be helpful for designing various particle suspensions. It is also expected that the mechanism of agglomerate formation and breakup in the flow field will contribute to the control of the size of agglomerate required in many processes.