Role of Micellar Entanglement Density on Kinetics of Shear Banding Flow Formation

We investigate the effects of micellar entanglement number on the kinetics of shear banding flow formation in a Taylor-Couette flow. Three sets of wormlike micellar solutions, each set with a similar fluid elasticity and zero-shear-rate viscosity, but with varying entanglement densities, are studied under start-up of steady shear. Our experiments indicate that in the set with the low fluid elasticity, the transient shear banding flow is characterized by the formation of a transient flow reversal in a range of entanglement densities. Outside of this range, the transient flow reversal is not observed. For the sets of medium and high elasticities, the transient flow reversals exist for relatively small entanglement densities, and disappear for large entanglement densities. Our analysis shows that wall slip and elastic instabilities do not affect the transient flow feature. We identify a correlation between micellar entanglement number, the width of the stress plateau, and the extent of the transient flow reversal. As the micellar entanglement number increases, the width of the stress plateau first increases, then, at a higher micellar entanglement number, plateau width decreases. Therefore, we hypothesize that the transient flow reversal is connected to the micellar entanglement number through the width of the stress plateau

Squirmer locomotion in a yield stress fluid

An axisymmetric squirmer in a Bingham viscoplastic fluid is studied numerically to determine the effect of a yield stress environment on locomotion. The nonlinearity of the governing equations necessitates numerical methods, which is accomplished by solving a variable-viscosity Stokes equation with a Finite Element approach. The effects of stroke modes, both pure and combined, are investigated and it is found that for the treadmill or "neutral" mode, the swimmer in a yield stress fluid has a lower swimming velocity and uses more power. However, the efficiency of swimming reaches its maximum at a finite yield limit. In addition, for higher yield limits, higher stroke modes can increase the swimming velocity and hydrodynamic efficiency of the treadmill swimmer. The higher-order odd-numbered squirming modes, particularly the third stroke mode, can generate propulsion by themselves that increases in strength as the viscoplastic nonlinearity increases till a specific limit. These results are closely correlated with the confinement effects induced by the viscoplastic rigid surface surrounding the swimming body, showing that swimmers in viscoplastic environments, both biological and artificial, could potentially employ other non-standard swimming strategies to optimize their locomotion

Anomalous coalescence in sheared two-dimensional foam

We report an experimental study on shearing a monolayer of monodisperse bubbles floating on liquid in a narrow-gap Couette device. The bubbles in such a "bubble raft" coalesce only if the shear rate exceeds a threshold value. This is in contrast to the conventional wisdom that bubbles and drops coalesce for gentler collisions, at shear rates below a critical value. Furthermore, the threshold shear rate increases with the bubble size and the viscosity of the suspending liquid, contravening reasoning based on capillary number. Through visualization and scaling arguments, we investigate several plausible mechanisms for the anomalous coalescence. None explains all aspects of the observations. The most promising model is one based on inertial forces that compress the bubbles radially inward and accelerate film drainage

Dynamics and rheology of sheared two-dimensional foam

Using a shear cell device, we have studied four associated problems in foam by experiments: Bubble-bubble coalescence in sheared two-dimensional foam; lateral migration of a single large bubble in an otherwise monodisperse foam; size segregation of bubbles in sheared bidisperse foam; and the effect of non-Newtonian rheology of foam on lateral migration of bubble. For bubble-bubble coalescence in sheared two-dimensional foam, we observed a threshold of shear rate beyond which coalescence of bubbles happens. The most promising explanation was the model based on the centripetal force with qualitative agreement with experimental results. Next we studied the dynamics of monodisperse foam in the presence of a single bubble whose size is different from the neighboring bubbles. We reported the lateral migration of a larger single bubble away from the wall. We also reported thresholds of shear rate and bubble size ratio beyond which migration occurs. In this study we modified the Chan-Leal model and predicted the experimental trajectories of migrating bubbles. For bidisperse foams, we reported evolution in foam structure to a size segregated structure, in which large bubbles accumulate at the middle of the gap whereas smaller ones close to walls. Then, we adopted a model based on convection-diffusion equation to account for both lateral migration and shear induced diffusion. Finally, we extended the second work by widening the gap of Couette coaxial cylinder geometry. Similar to the second work, we found that large bubble migrates laterally to an equilibrium position close to the inner wall. We believe this new mechanism is the non-Newtonian feature of foam. We characterized our foam by measuring its degree of shear thinning and also estimated its elasticity based on the literature data on foam. Then, we found out for a shear thinning fluid bubble migrated to position even closer to the inner wall than in the foam while a bubble in Boger fluid migrated to a position closer to the outer cylinder. Therefore, for a viscoselastic fluid which has the same feature one would expect to see bubble migration to a position between these two for two fluids.Applied Science, Faculty ofChemical and Biological Engineering, Department ofGraduat