ABSTRACT

We consider the buoyancy-driven motion of a single or a pair of tandem bubbles that are initially stationary in an elasto-visco-plastic (EVP) material. Our finite element software is used to solve the usual conservation equations along with the Saramito-Herschel-Bulkley constitutive model. The bubble radii are in the range (3𝑚𝑚<𝑅<15𝑚𝑚). In a single bubble the fore-aft symmetry is lost even under creeping flow, and a negative wake is formed in very good agreement with experiments particularly for the smaller or the larger bubbles among those examined. The smaller bubbles have an inverted teardrop shape, which is impossible to predict without including elasticity in the constitutive model. The larger ones have either an oblate or a spherical cap shape. Elastic and plastic forces are dominant in the smaller bubbles, whereas in the larger bubbles significant shear and extensional thinning make inertia forces dominant.  In bubble pairs, the solid-like behavior of the material preserves stresses generated by the passage of the leading bubble making the material “softer” for the trailing one. This shear thins the material and generates a “bridge” of shear stresses, which decrease the drag force on the trailing bubble and initiate their approach. Moreover, the axial normal stresses extend the backside of trailing bubble mainly, which adopts a hydrodynamically favorable shape, further promoting the approach. Bubbles of equal size always approach each other, but conditions exist under which bubbles of unequal size retain their initial distance, approach or depart from each other.  The open-source software “Basilisk” is used to address the corresponding problems of one or two Newtonian drops falling in EVP materials. For a specific unequal pair of drops, we capture a critical initial separation distance that determines whether the pair coalesces or separates consistently with previous experimental findings. Our parametric analysis examines how interaction dynamics are affected by geometric parameters and material properties. These factors determine whether droplets approach or repel each other or maintain a stable separation distance. Our findings have important implications for the stability of emulsions.

BRIEF ACADEMIC/EMPLOYMENT HISTORY:

John Tsamopoulos received his Diploma (1979) in Chemical Engineering from the National Technical University of Athens (NTUA) and his MS (1981) and PhD (1985) degrees in Chemical Engineering from MIT in the areas of Process Dynamics and Fluid Mechanics, respectively. Subsequently, he taught at the State University of New York at Buffalo for 8 years, reaching the rank of Full Professor. He returned to Greece and the Department of Chemical Engineering at the University of Patras in 1993. There, he continued working on theoretical modeling and numerical simulations of problems related to Fluid Mechanics, Rheology, Biological flows, Particulate flows, Microfluidics, and Material processing. He has developed along with his research group algorithms based on a variety of numerical methods and particularly the Finite Element Method to solve these problems. He has authored and co-authored over 150 papers in International Journals. His papers and presentations have received various awards, such as the Walters Award (twice) from JNNFM and the Bingham fluid medal during the 6th International Conference on Viscoplastic Fluids. He is a founding member and past president of the Hellenic Society of Rheology. He has been elected fellow of the American Physical Society (Division of Fluid Dynamics), the Society of Rheology (USA) and the European Mechanics Society (EUROMECH).

MOST RECENT RESEARCH INTERESTS:

Constitutive modeling and flow of elastoviscoplastic materials, linear stability and nonlinear dynamics of viscoelastic flows, two-phase flows, flows of drops and bubbles, etc.