Computational and experimental study of the dynamics of dripping and jetting of viscoelastic fluids from round nozzles

Abstract

The breakup and formation of droplets from a round nozzle, be it in a jetting or a dripping process, is a fundamental part of many known applications such as traditional inkjet printing. However, novel applications of these dynamics, such as additive manufacture and tissue engineering, often use complex fluids, creating challenges in producing droplets to a high degree of quality and confidence. Researchers have studied the balance of forces that controls the dynamics of ejection, filament thinning and breakup of droplets for many years, but there are still multiple areas that have not received enough, if any, attention when establishing the ideal conditions for droplet formation. In this thesis, we have developed CFD models and experimental procedures to study the impact of some of those contributions to these dynamics. We use numerical models to simulate the pulsation of Newtonian and non-Newtonian fluids through a round nozzle, in order to replicate the conditions of inkjet printing. We used different values of pulse duration and velocity and see that these have a significant impact in the observed regimes of droplet formation, as does the existence of elasticity in the fluid's constitutive model. We also use these numerical models to assess the ideal conditions of periodic pulsation and how pulse frequency and velocity can influence the regimes of jetting and droplet formation of viscoelastic fluids, finding that the elastic stresses present in the fluid can lead to complex regimes of constant over and under-ejection of fluid, as well as chaotic breakup at the nozzle. We have also studied the influence of inertia and fluid properties in the breakup of dripping Newtonian and non-Newtonian fluids, using an experimental set-up and a one-dimensional numerical approach. In the experimental set-up, solutions of Glycerol in water at various concentrations were used to assess the influence of increasing the Ohnesorge number on the thinning of these fluids. For the non-Newtonian fluids, Boger fluids were used. We find that the contribution of different fluid properties to the forces that govern the flow can significantly change the way the liquid filament thins before breakup in Newtonian fluids at different flow-rates. We have also found that inertia has a significant role in accelerating a transition to an exponential thinning regime in viscoelastic fluids, creating a longer lasting liquid filament at higher inertia flows. This conclusion was verified using both nozzles of different sizes and an analytical calculation of the thinning of fluids using a one-dimensional approximation. The results of the different studies of this thesis highlight the need of a better understanding of the balance and different impacts of each of these contributions to the dynamics of thinning and breakup of droplets, in order to better define the ideal conditions for applications that rely on those dynamics.

Date of Award	31 Dec 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Peter Quayle (Supervisor), Flor Siperstein (Supervisor) & Claudio Pereira Da Fonte (Supervisor)