Ph.D Thesis

Ph.D StudentHolenberg Yulia
SubjectDynamics of Drops and Solid Particles in
Viscoplastic Fluids
DepartmentDepartment of Chemical Engineering
Supervisors PROFESSOR EMERITUS Avinoam Nir
DR. Olga Lavrentev
Full Thesis textFull thesis text - English Version


Dynamics of drops and solid spheres in viscoplastic materials has been studied experimentally. Inclusions settle in visco-plastic material if the local stress near their interfaces exceeds a critical value. In such cases the inclusions are surrounded by a yielded region. The shape of this region and the dynamics of the flow in it during a steady sedimentation of drops and spheres under gravity have been studied. We used optical observations to establish the boundary and shape of the yielded region and the flow field within it.

Low concentrated aqueous gel of Carbopol 940 (0.07% w/w) was used as the yield stress material. Newtonian drops (R ~ 2.9mm) of various densities were used. Heavier and faster drops move with velocity of O(1mm/s) when isolated, while lighter ones move with velocity of O(0.1 mm/s). For the motion in unbounded domain, it was demonstrated that the yielded region extends more vertically than horizontally.

The axisymmetric dynamics of two equal-size drops trailing each other and eventually coalescing have been studied. When the trailing drop reaches the leading one, the drop doublet moves ensemble a relatively long time before the coalescence begins. At the beginning of the coalescence process the flow field around the drop doublet changes drastically and the flow field becomes more intensive. We have also studied axisymmetric and non-axisymmetric dynamics of non-equal size drop pairs. Based on non-axisymmetric interaction patterns, we have estimated the size of the yielded region and its shape around the drop.

When the drops settle in cylindrical tubes, their velocities decrease with increased ratio of drop to tube radius. Such behavior is consistent to what is anticipated when they settle in a Newtonian or a viscoelastic domain. However, in the proximity of a vertical glass wall the settling speed of the drops is augmented. The increase in settling speed can be attributed to the dynamic formation of a thin solvent layer providing an apparent wall slip. The formation of the layer of the solvent is also expected on the surface of the smooth solid bodies. As a result, the flow field induced by smooth solid spheres moving in yield stress material is similar to the flow field around drops. When the sphere surface is rough and slip effect is reduced, the flow field changes. The yielded region shape becomes similar to the one predicted by various numerical simulations for solid spheres.