Υποστήριξη Διδακτορικής Διατριβής - Giancarlo Esposito

This thesis investigates the dynamics of multiphase systems consisting of rigid or deformable particles suspended in complex fluids, with a particular focus on yield stress materials. These systems are relevant in a wide range of applications, including biomedical, food, and consumer product industries, exemplified by air bubbles in gels, oil droplets in pastes, and cells in blood. Yield stress materials are distinguished by their dual behavior: they act as solids under low stress but flow as liquids once a critical stress threshold is exceeded. Understanding the microscopic mechanisms underlying this behavior is a major challenge, as it depends on the system’s microstructure—colloidal suspensions, non-Brownian systems, or colloidal gels. Despite these microscopic differences, macroscopic constitutive models can describe their rheology. Motivated by recent experimental observations of elastic effects in these materials, this thesis employs an elastoviscoplastic (EVP) constitutive model, which captures the combined viscous, elastic, and plastic characteristics of yield stress fluids, as a framework to study the dynamics of single and multiple inclusions.

The first part of the thesis focuses on single-object dynamics. It examines the rise of a viscous drop in a polymeric fluid without yield stress, emphasizing the interaction between fluid elasticity and interfacial forces, including the formation of trailing filaments observed experimentally. The sedimentation of a drop in EVP materials is then analyzed, exploring the conditions for entrapment and how material parameters influence settling. Finally, the motion of an air bubble near walls in EVP fluids is studied through fully 3D simulations, revealing how confinement, material properties, and initial bubble shape affect migration patterns.

The second part addresses interactions between multiple particles. The sedimentation of two coaxial viscous drops in EVP materials is numerically investigated, reproducing experimental trends and elucidating the mechanisms driving drop approach or separation. A parametric study identifies master curves linking relative velocity to drop distance. Inspired by microfluidic applications, the behavior of rigid and deformable particle pairs in pressure-driven microflows of viscoelastic and EVP fluids is also examined, revealing a critical initial separation that determines attraction or repulsion and its dependence on rheological parameters. These insights provide guidance for the design of microfluidic systems for non-invasive particle manipulation and controlled assembly in soft-matter systems. Finally, extending beyond axial symmetry, a 3D numerical study of two sedimenting viscous drops in EVP fluids explores non-symmetric scenarios, offering new perspectives on the stability of previously assumed axisymmetric configurations.