Micro/Bio/Nanofluidics (Shen) Unit would like to invite you to the seminar by Dr. Konstantinos Zinelis on September 3 (Tuesday).
 
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Date:   September 3, 2024
Time:  15:00-16:00
Venue: C700, OIST
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Speaker:

Dr. Konstantinos Zinelis
Postdoctoral Associate
1 Department of Chemical Engineering, Massachusetts Institute of Technology, USA
2 Department of Chemical Engineering, Imperial College London, UK

Title:

Numerical Simulations of Viscoelastic Interfacial Flows: Jets, Filaments and Drop Impact

Abstract:

In this talk, we present a Computational Rheology approach to study the jet formation, the filament thinning dynamics and the droplet generation in viscoelastic fluids, i.e dissolved polymers. We use the open-source Eulerian Volume-of-Fluid (VoF) code Basilisk to capture the dynamically-deforming liquid-gas interface, exploiting its Adaptive Mesh Refinement (AMR) capabilities that allows for computationally “friendly” and accurate solutions. We use the FENE-P constitutive equation to describe the viscoelasticity of the liquid and employ the log-conformation transformation, which provides stable solutions for the evolution of the conformation tensor as the jet or the filament thins down under the action of interfacial tension. We begin with an impulsively-started, axisymmetric viscoelastic jet exiting a nozzle. We study the effect of the flow inside the nozzle on the thinning dynamics of the viscoelastic jet and on the spatio-temporal evolution of the polymeric stresses in order to systematically explore the dependence of the filament thinning and breakup characteristics on the initial axial momentum of the jet and the extensibility of the dissolved polymer chains. We then investigate the fluid mechanics behind the operation of a Dripping-onto-Substrate (DoS) Rheometry, which is a conceptually-simple, but dynamically-complex, probe of the extensional rheology of low-viscosity, non-Newtonian fluids. This will enable experimentalists to optimise and extend the performance of this protocol. Here, we focus on understanding the roles of surface tension, elasticity and finite chain extensibility in controlling the Elasto-Capillary (EC) regime, as well as the perturbative effects that gravity and substrate wettability play in setting the evolution of the self-similar thinning and pinch-off dynamics. To illustrate the interplay of these different forces, we construct a simple one-dimensional model that captures the initial rate of thinning when the dynamics are dominated by a balance between inertia and capillarity. This model also captures the structure of the transition region to the nonlinear EC regime in which the rapidly growing elastic stresses in the thread balance the capillary pressure as the filament thins towards breakup. We also propose a fitting methodology based on the analytical solution for FENE-P fluids to improve the accuracy in determining the effective relaxation time of an unknown fluid. Finally, we study the formation of a Worthington jet resulting from the retraction of an impacting droplet on a non-wettable substrate and from bubbles that burst. In the first case, we examine the critical role of viscoelasticity in a characteristic transition from an inertiocapillary to an elastocapillary regime in the stretching Worthington jet. We also confirm the pinning conditions on the substrate are key for the development of the characteristic beads-on-the-string structures in dissolved polymers, as first captured by experiments. In the case of viscoelastic jets and droplets produced by bursting bubbles, we reveal three distinct flow regimes: droplet production, jet formation without droplets, and complete jet suppression, by mapping the two-dimensional phase spanned by the elastic modulus and the polymer relaxation time of the polymer.

Host:
Prof. Amy Shen